AquaLab Series 3 v5

Water Activity Meter

Operator’s ManualVersion 5

for AquaLab models Series 3 and 3TE

Decagon Devices, Inc.

Decagon Devices, Inc.2365 NE Hopkins CourtPullman WA 99163(509) 332-2756fax: (509) [email protected]

TrademarksAquaLab is a registered trademark of Decagon Devices, Inc.

© 1990-2007 Decagon Devices, Inc.

10607-05

AquaLabTable of Contents

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Contents1. Introduction . . . . . . . . . . . . . . . . . . .1

About this Manual . . . . . . . . . . . . . . . . . . . . 1Customer Service . . . . . . . . . . . . . . . . . . . . . 1

Phone: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Fax: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2E-mail: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Note to our AquaLab Users . . . . . . . . . . . . 3Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . . 3

2. About AquaLab . . . . . . . . . . . . . . . 5AquaLab Models and Options . . . . . . . . . . 5AquaLab and water activity . . . . . . . . . . . . 5How AquaLab works . . . . . . . . . . . . . . . . . . 6AquaLab and Temperature . . . . . . . . . . . . . 7

Series 3TE . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3. Getting Started . . . . . . . . . . . . . . 12Components of your AquaLab . . . . . . . . . 12Choosing a Location . . . . . . . . . . . . . . . . . . 13Preparing AquaLab for Operation . . . . . 13

4. The Menus . . . . . . . . . . . . . . . . . . .15The Main Menu . . . . . . . . . . . . . . . . . . . . . . 15Changing Languages . . . . . . . . . . . . . . . . . . 16Normal sampling mode . . . . . . . . . . . . . . . 16Continuous Mode . . . . . . . . . . . . . . . . . . . . 17Temperature Equilibration Screen . . . . . . 18


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System configuration . . . . . . . . . . . . . . . . . 18Changing the beeper . . . . . . . . . . . . . . . . . . 19EXIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Adjusting for linear offset . . . . . . . . . . . . . 20Series 3TE menu . . . . . . . . . . . . . . . . . . . . . 20

5. Linear Offset and Verification Stan-dards . . . . . . . . . . . . . . . . . . . . . . . .24

What is Linear Offset? . . . . . . . . . . . . . . .24Verification Standards . . . . . . . . . . . . . . . .24When to Verify for Linear Offset . . . . . .26How to Verify and Adjust for Linear Offset 26

Adjusting for linear offset . . . . . . . . . . . . . 27

6. Sample Preparation . . . . . . . . . . 32Preparing the Sample . . . . . . . . . . . . . . . . .32Samples Needing Special Preparation . .34

Coated and Dried Samples . . . . . . . . . . . 34Slow Water-Emitting Samples . . . . . . . . . 35Propylene Glycol . . . . . . . . . . . . . . . . . . . . 36

Low Water Activity . . . . . . . . . . . . . . . . . .37Samples not at Room Temperature . . . . . .38

7. Taking a Reading . . . . . . . . . . . .39Measurement Steps . . . . . . . . . . . . . . . . . .39How AquaLab takes Readings . . . . . . . . .40Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

8. Computer Interface . . . . . . . . . . .44AquaLink Software . . . . . . . . . . . . . . . . . . .44Using Windows Hyperterminal . . . . . . . . .45

9. Theory: Water Activity in Products 47


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Water content . . . . . . . . . . . . . . . . . . . . . . .47Water activity . . . . . . . . . . . . . . . . . . . . . . .47Effect of Temperature on water activity 49Water Potential . . . . . . . . . . . . . . . . . . . . . .50Factors in determining Water Potential . 51

Osmotic effects . . . . . . . . . . . . . . . . . . . . . . 51Matric effects . . . . . . . . . . . . . . . . . . . . . . .52

Sorption Isotherms . . . . . . . . . . . . . . . . . . .53Relating aw to water content . . . . . . . . . .53

10. Cleaning and Maintenance . . . . 55Tools Needed . . . . . . . . . . . . . . . . . . . . . . .55Cleaning the Block and Sensors . . . . . . . .56

Accessing the Block . . . . . . . . . . . . . . . . . 56Checking Calibration . . . . . . . . . . . . . . . . .59

11. Repair Instructions . . . . . . . . . . . 61Shipping Directions . . . . . . . . . . . . . . . . . . . 61Repair Costs . . . . . . . . . . . . . . . . . . . . . . . . . 63Loaner Service . . . . . . . . . . . . . . . . . . . . . . 63

12. Troubleshooting . . . . . . . . . . . . . 64Problems and Solutions . . . . . . . . . . . . . . .64Component Performance Screen . . . . . . .72

13. Further Reading . . . . . . . . . . . . . 74Water Activity Theory and Measurement 74Food Quality and Safety . . . . . . . . . . . . . .78Water Activity and Microbiology . . . . . .79Water Activity in Foods . . . . . . . . . . . . . . 83

Meat and Seafood . . . . . . . . . . . . . . . . . . . 83Dairy Products . . . . . . . . . . . . . . . . . . . . . . .85Fruits and Vegetables . . . . . . . . . . . . . . . . .86


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Baked Goods and Cereals . . . . . . . . . . . . 88Beverages/Soups/Sauces/Preserves . . 90

Pharmaceuticals/Cosmetics . . . . . . . . . . . 91Miscellaneous . . . . . . . . . . . . . . . . . . . . . . .92

Appendix A . . . . . . . . . . . . . . . . . . . 94Preparing Salt Solution . . . . . . . . . . . . . . .94

Appendix B . . . . . . . . . . . . . . . . . . . 96

Declaration of Conformity . . . . . . . 97

Certificate of Traceability . . . . . . . 98

AquaLabIntroduction

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1. Introduction

Welcome to Decagon’s AquaLab Series 3, the industrystandard for measuring water activity (aw). AquaLab is thequickest, most accurate, and most reliable instrumentavailable for measuring water activity. Whether you areresearching or working on the production line, AquaLabwill suit your needs. It is easy to use and provides accurateand timely results. We hope you find this manual informa-tive and helpful in understanding how to maximize thecapabilities of your AquaLab.

About this ManualIncluded in this manual are instructions for setting upyour AquaLab, verifying the calibration of the instrument,preparing samples, and maintaining and caring for yourinstrument. Please read these instructions before operat-ing AquaLab to ensure that the instrument performs to itsfull potential.

Customer ServiceIf you ever need assistance with your AquaLab, or if youjust have questions, there are several ways to contact us:

AquaLabIntroduction

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Phone:Our toll-free telephone number is available to our cus-tomers in the US and Canada, Monday through Friday,between 8 a.m. and 5 p.m. Pacific time at 1-800-755-2751.For our customers outside of the US and Canada, our reg-ular telephone number is (509) 332-2756.

Fax:Our fax number is (509) 332-5158. When you fax us,please include your AquaLab’s serial number, your name,address, phone and fax number along with a descriptionof your problem.

E-mail:If you need technical support, you can send us messagesvia e-mail at [email protected]. Again, pleaseinclude as part of your message your AquaLab’s serialnumber, your name, address, phone, fax number, andreturn e-mail address.

If you have a question about your application withAquaLab, please send your message with the above infor-mation to [email protected].

WarrantyAquaLab has a 30-day satisfaction guarantee and a three-year warranty on parts and labor. To validate your war-ranty, please complete and return the warranty card

AquaLabIntroduction

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included with this manual. You can return your warrantyinformation by fax, e-mail, phone or by mailing the post-age-paid card. Please include all the information requestedon the card. It is necessary for Decagon to have your cur-rent mailing address and telephone number in case weneed to send updated product information to you.

Note to our AquaLab UsersThis manual is written to aid the end user in understand-ing the basic concepts of water activity, enabling them touse our instrument with confidence. Every effort has beenmade to ensure that the content of this manual is correctand scientifically sound.

Seller’s LiabilitySeller warrants new equipment of its own manufactureagainst defective workmanship and materials for a periodof three years from date of receipt of equipment (theresults of ordinary wear and tear, neglect, misuse, accidentand excessive deterioration due to corrosion from anycause are not to be considered a defect); but Seller’s liabil-ity for defective parts shall in no event exceed the furnish-ing of replacement parts F.O.B. the factory whereoriginally manufactured. Material and equipment coveredhereby which is not manufactured by Seller shall be cov-ered only by the warranty of its manufacturer. Seller shallnot be liable to Buyer for loss, damage or injuries to per-sons (including death), or to property or things of whatso-ever kind (including, but not without limitation, loss of

AquaLabIntroduction

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anticipated profits), occasioned by or arising out of theinstallation, operation, use, misuse, nonuse, repair, orreplacement of said material and equipment, or out of theuse of any method or process for which the same may beemployed. The use of this equipment constitutes Buyer’sacceptance of the terms set forth in this warranty. Thereare no understandings, representations, or warranties ofany kind, express, implied, statutory or otherwise (includ-ing, but without limitation, the implied warranties of mer-chantability and fitness for a particular purpose), notexpressly set forth herein.

AquaLabAbout AquaLab

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2. About AquaLab

AquaLab is the fastest instrument for measuring wateractivity, giving readings in five minutes or less. Its readingsare precise, providing ±0.003 accuracy. The instrument iseasy to clean and checking calibration is simple.

AquaLab Models and OptionsAquaLab comes in two models to suit the needs of differ-ent users. Here is a brief description of each:

Series 3: Our standard model, adequate for most wateractivity needs.

Series 3TE: User-selectable internal temperature controlmodel, uses thermoelectric (Peltier) components to main-tain internal temperature.

AquaLab and water activityWater activity (aw) is a measurement of the energy statusof the water in a system. It indicates how tightly water is“bound”, structurally or chemically, within a substance.Water activity is the relative humidity of air in equilibriumwith a sample in a sealed measurement chamber. The con-cept of water activity is of particular importance in deter-


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mining product quality and safety. Water activityinfluences color, odor, flavor, texture and shelf-life ofmany products. It predicts safety and stability with respectto microbial growth, chemical and biochemical reactionrates, and physical properties. For a more detailed descrip-tion of water activity as it pertains to products, pleaserefer to Chapter 9, titled “Theory: Water Activity of Prod-ucts” of this manual.

How AquaLab worksAquaLab uses the chilled-mirror dewpoint technique tomeasure the aw of a sample. In an instrument that uses thedewpoint technique, the sample is equilibrated with theheadspace of a sealed chamber that contains a mirror anda means of detecting condensation on the mirror. At equi-librium, the relative humidity of the air in the chamber isthe same as the water activity of the sample. In theAquaLab, the mirror temperature is precisely controlledby a thermoelectric (Peltier) cooler. Detection of the exactpoint at which condensation first appears on the mirror isobserved with a photoelectric cell. A beam of light isdirected onto the mirror and reflected into a photodetec-tor cell. The photodetector senses the change in reflec-tance when condensation occurs on the mirror. Athermocouple attached to the mirror then records thetemperature at which condensation occurs. AquaLab thensignals you by flashing a green LED and/or beeping. Thefinal water activity and temperature of the sample is thendisplayed.


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In addition to the technique described above, AquaLabuses an internal fan that circulates the air within the sam-ple chamber to reduce equilibrium time. Since both dew-point and sample surface temperatures are simultaneouslymeasured, the need for complete thermal equilibrium iseliminated, which reduces measurement times to less thanfive minutes.

AquaLab and TemperatureFor the reasons noted above, temperature control isunnecessary for most applications with AquaLab. Thechange in aw due to temperature change for most materi-als is less than 0.002 per degree Celsius. Thus, AquaLab isideal for the measurement of samples at room tempera-ture. However, samples that are not at room temperatureduring the read cycle will equilibrate to the temperature ofAquaLab before the aw is displayed. Large temperaturedifferences will cause longer reading times, since a com-plete and accurate reading will not be made until the sam-ple and the instrument are within 2 degrees of each other.To better help you control the temperature differencebetween your sample and the instrument, you can access asample equilibration screen at the main menu that canshows the difference in temperature between the sampleand chamber block (see chapter 4).

Though temperature control is usually unnecessary formost applications, there are some instances where it isdesired. For this reason, Decagon offers a temperature-


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controlled AquaLab model: the Series 3TE. There are sev-eral advantages in having a temperature-controlled model.Here are a few major reasons:

1. Research purposes. To study the effects of tempera-ture on the aw of a sample, comparison of the aw ofdifferent samples independent of temperature, accel-erated shelf-life studies or other water activity studieswhere temperature control is critical. There are manyshelf-life, packaging, and isotherm studies in which theadded feature of temperature control would be verybeneficial.

2. To comply with government or internal regula-tions for specific products. Though the aw of mostproducts varies by less than ± 0.002 per °C, some reg-ulations require measurement at a specific tempera-ture. The most common specification is 25°C, though20°C is sometimes indicated.

3. To minimize extreme ambient temperature fluc-tuations. If the laboratory and AquaLab temperaturesfluctuate by as much as ± 5°C daily, water activity readingswill vary less than ± 0.01aw. Such variations in ambienttemperatures are uncommon. As stated above, this muchuncertainty in sample aw is sometimes acceptable, so theremay be no need for temperature control. However, if yourlab temperature varies to this degree and you require bet-ter than 0.01aw precision, you may want a temperature-


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controlled model.

If your application meets the criteria listed above, you maywant to use the Series 3TE. If you already own a Series 3model and wish to utilize the temperature-control optionsof the Series 3TE, contact Decagon for information onupgrading your system. Here is a description of theAquaLab Series 3TE:

Series 3TEThe AquaLab Series 3TE is an AquaLab that has thermo-electric components installed that allow the instrument tomaintain a set chamber temperature. The 3TE has an extraoption in the System configuration menu (see chapter 3)that allows the user to set the temperature.

LimitationsAquaLab’s only major limitation is its ability to accuratelymeasure samples with high concentrations of certain vola-tiles such as ethanol or propylene glycol, which can co-condense on the surface of the chilled mirror. Not all vol-atiles react this way, but it is important to note that somevolatiles can affect the performance of your instrument.The extent of the effect is both concentration- and matrix-dependent; thus, just because a product contains someethanol or proplylene glycol does not necessarily mean thereadings will be erroneous. Therefore, if your sample con-tains propylene glycol or a high concentration of othervolatiles, it is still possible to make accurate readings. Refer


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to Chapter 6’s section titled ”Propylene Glycol” or callDecagon for more details.


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Features

Front view of AquaLab

Back view of AquaLab

LED indicatorlight

LCD

Function Keys

Sample drawer

RS-232 interface

fuse wellON/OFF Switch

Power cord

Plug

Fan

Lid thumb-screw

AquaLabGetting Started

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3. Getting Started

Components of your AquaLabYour AquaLab should have been shipped with the follow-ing items:

• AquaLab water activity meter

• Power cord

• RS-232 interface cable

• 100 disposable sample cups

• Operator’s Manual

• Quick Start guide

• Cleaning Kit

• 3 vials each of the following verification solutions:0.984 aw 0.5 molal KCl0.760 aw 6.0 molal NaCl0.500 aw 8.57 molal LiCl0.250 aw 13.41 molal LiCl


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Choosing a Location To ensure that your AquaLab operates correctly and con-sistently, place it on a level surface. This reduces thechance that sample material will spill and contaminate theinside of the instrument. To protect the internal electricalcomponents, and to avoid inaccurate readings, place yourAquaLab in a location where the temperature remainsfairly stable. This location should be well away from airconditioner and heater vents, open windows, outsidedoors, refrigerator exhausts, or other items that may causerapid temperature fluctuation.

Preparing AquaLab for OperationAfter finding a good location for your AquaLab, plug thepower cord to the back of the unit. Before turning it on,pull open the sample drawer (turn the knob to the“OPEN/LOAD” position). An empty disposable samplecup is usually placed upside-down in the drawer to protectit during shipment. Remove this sample cup and turn theinstrument on. The ON/OFF switch is located on thelower right corner of the AquaLab’s back panel. The fol-lowing screens will appear on the LCD (model Series 3TEused here as an example):

AQUALAB TE Series 3B v 3.0


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(Note: the “v.3.0” shown the above illustration is an example show-ing the version of operating code used in the instrument. We periodi-cally update the code and add new features, so if your AquaLabshows a version number higher than this, don’t be alarmed.)

Then:

This is the main menu, displaying the water activity (aw)on the top left portion of the screen, and the sample tem-perature in the lower right.

In order to provide the most accurate readings, yourAquaLab should ideally be allowed a warm-up period of atleast 15 minutes after turning it on. This allows the airinside the AquaLab to equilibrate to the temperature of itssurroundings. The Series 3TE model allows you to equili-brate the sensor block and drawer assembly to a set tem-perature. For instructions on how to adjust thetemperature setpoint in the TE models, see Chapter 4.

0.000 a0.0 °C

w

AquaLabMenus

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4. The Menus

The Main Menu

Each time you turn on your AquaLab, it will come to thisscreen. If this screen doesn’t appear, refer to Chapter 13for troubleshooting instructions. As mentioned earlier, thewater activity and sample temperature are displayed on thescreen. On each side of the LCD there are two buttons.Each button performs a different function. Following is adescription of the modes and options you may use, andthe buttons used to set them.

diagram of button options from main menu

0.000 a0.0 °C

w

temperature equilibration screen system setup

continuous/normal

0.000 a0.0 °C

w

language selectionmode

AquaLabMenus

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Changing LanguagesThe AquaLab comes to you with English as the defaulton-screen user language. If you prefer not to use English,you can change it to one of a variety of other languages:German, French, Spanish, Italian, Swedish, Danish, Nor-wegian, Czech, Portuguese, Japanese, Polish or Finnish.This is done simply by pressing the upper right button ofthe instrument while the drawer knob is in the OPEN/LOAD position. You will see the following screen:

Press the upper right key again, and the next languageoption (German) will appear:

Each time you press the right button, the display will scrollto the next language option. Select the desired language,then press the lower left button to exit.

Normal sampling modeThe first time you turn on the AquaLab, it will be in nor-mal sampling mode. In this mode, the sample is measuredonce, after which the instrument notifies you that it is fin-ished. Here is a brief summary of the normal sampling

English

-exit-

Deutsch

-zurück-

AquaLabMenus

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mode features:

• single sampling (not continuous).

• instrument may or may not beep after measure-ment is made, depending on how it is set up. (Factory default is 4 beeps)

• green LED blinks until you open the sample drawer.

Continuous ModeContinuous mode reads your sample continuously untilyou turn the knob to the OPEN/LOAD position. It willread the sample, display the water activity and sample tem-perature, then begin another read cycle without turningthe knob or breaking the seal of the cup. Between samples,it will signal you with the green LED flash, accompaniedby the beeper, if it is enabled.To toggle between the normal and continuous modes,press the top left button. The display will show a small “c”to the left of the water activity readings:

main menu with continuous mode enabledIf you press the upper left button again, the “c” will disap-

0.000 a0.0 °C

w

c

“c” for continuous mode

AquaLabMenus

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pear and you will be back in normal sampling mode.

Temperature Equilibration ScreenTo see the temperature difference between your sampleand the AquaLab, press the lower right button at the mainmenu. This screen can only be accessed when the drawerknob is in the OPEN/LOAD position. The followingscreen will appear:

This screen shows the temperature difference between thesample (Ts) and the chamber block (Tb), allowing you toquickly check if the sample is too hot, which may causecondensation inside the chamber. Press the lower rightbutton to exit.

System configurationIf you press the bottom left button while in the mainmenu, it will bring you to the system configuration menu.

This menu allows you to make minor system changes. You

Ts - Tb = 0.125

Ts = 23.1

system configuration menu button

0.000 a0.0 °C

w

AquaLabMenus

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can change the beeper signal after each sample, enter thelinear offset adjustment menu, and adjust the temperaturesetpoint (TE model only) from this menu.

System configuration menu

Changing the beeperWhen you are sampling, the AquaLab has two ways ofnotifying you that the water activity reading is complete:the beeper and a flashing green LED, located on the leftfront corner of the AquaLab’s case. In normal samplingmode, when a sample is started, the LED will flash once,and when it is finished it will flash continuously until theknob is moved to the OPEN/LOAD position (if notoperating in continuous mode). You cannot turn off orchange the LED flashing functions.

There are three beeper options, represented by three

-Exit-

+-

set T

Beeper BeeperMode indicator

Linear offsetmenuIcon

(continuous shown)

temp. set (TE model only)

AquaLabMenus

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icons:

definition of beeper icons

The beeper can be turned off completely, it can beepmomentarily when the sample is finished and then stop, orit can beep continuously until the knob is turned to theOPEN/LOAD position. After you have adjusted thebeeper setting, it will remain as you have set it until youchange it again, and will not be affected by turning theinstrument on or off.

EXITYou may press the EXIT button (the lower left button) toexit back to the main menu at any time.

Adjusting for linear offsetWhen you need to adjust for linear offset, press the upperright button in the system configuration menu, and youwill be brought to the linear offset menu. For more detailson linear offset and how to verify for it, please refer to thenext chapter.

Series 3TE menuIf you have an AquaLab Series 3TE, you have the ability tomanually set your AquaLab’s sample chamber tempera-

0x

4x

No beeping.

Beeps four times, then stops.

Beeps until drawer is opened.

AquaLabMenus

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ture. To access this menu, press the lower left button inthe main menu, and then the lower right button next tothe “set T” in the system configuration menu. The fol-lowing screen will appear:

Adjusting the Setpoint TemperatureUse the buttons next to the + and - buttons to adjust thetarget setpoint temperature (displayed in the lower rightcorner). If you press either button it adjusts in incrementsof 0.1°C. Note: Holding down the button for 2 seconds will scrollthe values.

The target setpoint temperature roughly corresponds tothe temperature at which you wish the chamber to read.Therefore, adjust the setpoint to the temperature that youwant, then begin measurements to see how close yourAquaLab comes to your desired temperature (this worksbest by putting the AquaLab in continuous mode). Afterseveral samples, it should show consistent temperaturereadings. At this point, make any needed adjustment to thesetpoint index number to reach your desired temperature.

You will be able to adjust the index number between 50°and 15.1°C. If you press the - button after you reach15.1°, it will disable the temperature control function, dis-playing “off ” until you raise the index number again. Note:

0 Adjust setpoint

-Exit- 25.0

+

-

AquaLabMenus

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To make aw measurements at temperatures below 15°C, disable thetemperature control feature and place the entire instrument into atemperature-controlled chamber.

Setting Temperature Equilibration TimeIn the upper left corner, before the words “Adjust Set-point” there is a number (default is 0). This value can beone of three integers: 0, 1, or 2, and is adjusted by pressingthe upper left button. This number sets the level of tem-perature equilibration desired after the knob is turned tothe READ position and before the AquaLab begins tomake measurements. A setting of 0 begins measurementimmediately (as long as sample is not above 4°C of blocktemperature), 1 requires it to be within 1 degree, and 2requires it to be within 1/2 of a degree. Note: If the equlib-rium level is set to 0, the AquaLab will begin taking readings whenthe temperature is within 4 degrees, but you will not get a final read-ing until the temperature has stabilized enough for the instrument tomake an accurate final value.Important tips with the Series 3TE

• Before sampling, wait for approximately 30 min-utes to let the chamber’s temperature to stabilize after turning it on.

• The AquaLab may have a difficult time measur-ing the temperature of a sample that is being cooled if the ambient temperature is high.

AquaLabMenus

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• When changing samples, the opening and closing of the drawer may cause the temperature in the chamber to fluctuate. Therefore, make sure to take more than one reading per sample to ensure that the temperature and aw are stable between readings.

• Never place a hot or warm sample in a cooled chamber, because condensation will form inside the chamber, causing errors.

AquaLabLinear Offset and Verification Standards

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5. Linear Offset and VerificationStandards

What is Linear Offset?AquaLab uses the chilled mirror dewpoint technique formeasuring water activity. Because this is a primary mea-surement method of relative humidity, no calibration isnecessary; however, it is important to check for linear off-set periodically. The components that the instrument usesto measure aw are subject to changes that may affectAquaLab’s performance. These changes are usually theresult of chamber contamination. When this occurs, itchanges the accuracy of your readings. This is what iscalled a “linear offset.” Therefore, frequent linear offsetverification can assure you that your AquaLab is perform-ing correctly. Linear offset can be checked by using a saltsolution and distilled water.

Verification StandardsVerification standards are specially prepared salt solutionsthat have a specific molality and water activity that is con-stant and accurately measurable. The verification stan-dards that were sent with your initial shipment are veryaccurate and readily available from Decagon Devices.


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These particular standards are accurate and easy to use.Most importantly, they greatly reduce preparation errors.Because of these reasons, we recommend using the stan-dards provided by Decagon for the most accurate verifica-tion of your AquaLab’s performance.

Performance Verification Standards come in four wateractivity levels: 1.000, 0.984, 0.760, 0.500, and 0.250 aw.The standards are produced under a strict quality assur-ance regime. The accuracy of the standards is verified byan independent third party and are shelf stable for oneyear.

See Appendix B for water activity values for these stan-dards at different temperatures. If for some reason youcannot obtain Decagon’s verification standards and needto make a saturated salt slush for verification, refer toAppendix A.

Verification Standard@25°C

Water Activity

Distilled Water

0.5m KCl 0.984 ±0.003

6.0m NaCl 0.760 ±0.003

8.57m LiCl 0.500 ±0.003

13.41m LiCl 0.250 ±0.003

1.000 ±0.003


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When to Verify for Linear OffsetLinear offset should be checked against a known verifica-tion standard daily, either once per shift or before eachuse. Linear offset should never be verified solely againstdistilled water, since it does not give an accurate represen-tation of the linear offset. For batch processing, the instru-ment should be checked regularly against a knownstandard of similar aw. It is also a good idea to check theoffset with a standard of similar aw when the general wateractivity range of your sample is changing. Checking the aw

of a standard solution will alert you to the possibility ofcontamination of the unit or shifts in the linear offsetfrom other causes.

How to Verify and Adjust for Linear OffsetTo verify for linear offset of your AquaLab, do the follow-ing:

1. Choose a verification standard that is close to the aw

of the sample you are measuring. Make sure that yourstandard is at ambient temperature (or the instrumentset temperature for TE models) before you load it intothe sample drawer, and that your AquaLab haswarmed up long enough to make accurate readings.

2. Empty the whole vial of solution into a sample cupand place it in the AquaLab’s sample drawer.


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3. Carefully slide the drawer closed, being especially care-ful so the solution won’t splash or spill and contami-nate the chamber.

4. Turn the drawer knob to the READ position to makean aw reading. Make two readings. The aw readingsshould be within ± 0.003 of the given value for yoursalt solution. See Appendix B for the correct wateractivity value for standards at temperatures other than25°C.

5. If your AquaLab is reading within ±0.003 of the saltsolution, prepare a sample cup half full of steam-dis-tilled water or a second verification salt and make tworeadings—the first reading may be low. The second aw

reading for distilled water should be 1.000 ±0.003. Ifyour salt reading is correct and your distilled waterreading is not, it is probably due to contamination ofthe sensor chamber. For cleaning instructions, seeChapter 10. After cleaning, repeat these instructions.

6. If you consistently get readings that are outside of theaw of your salt solution by more than ±0.003, a linearoffset has probably occurred. In this case, adjust thereading on the salt solution to its correct value. This isdone by the following:

Adjusting for linear offset

1. Once you are certain that a linear offset has occurred,


28

choose a verification standard that is close to the aw ofthe sample you are measuring. Each of the verificationstandards supplied by Decagon has its aw labeled.Before you begin sampling, make sure that your stan-dard is at ambient temperature before you load it intothe sample drawer, and that your AquaLab haswarmed up long enough to make accurate readings.Refer to Appendix B for a table of correct aw values atdifferent temperatures.

2. Enter the system configuration menu by pressing thelower left button in the main menu. Press the upperright button in the system configuration menu to enterthe linear offset menu. You will be guided through thelinear offset routine. The following screen will appear:

3. If you wish to continue, press the button next to “yes.”To return to the main menu, press the button next to“no.” After selecting “yes,” the following screen willappear:

Change the offset?

yes no

place standardin drawer and read


29

4. Empty the whole vial of solution into a sample cupand place it in the AquaLab’s sample drawer.

5. Carefully slide the drawer closed, being especially care-ful so the solution won’t splash or spill and contami-nate the chamber.

6. Turn the drawer knob to the READ position to makean aw reading. Note: If you decide at this point not to continuewith the linear offset program, just return the knob to theOPEN/LOAD position and you will be returned to the mainscreen.

After it has finished sampling the verification standard, itwill bring you to the following screen:

7. At this screen, adjust the water activity value to itsproper value for the particular solution you are mea-suring. Press the upper right button to move the valueup, the lower right button to move it down. When it isat the value you want, press the Exit button. The valuewill be stored.

Note: This is the only menu where these buttons can change the lin-

0.760 adjust +exit -

use these buttons to adjust the value


30

ear offset, so you won’t hurt anything by pressing these buttons inother menus.

8. Re-measure the sample of your verification standardsagain in the normal sampling mode. It should read theproper value at a given temperature for your particularstandard (see Appendix B).

9. Prepare and read a sample of distilled water or secondverification standard. Read it twice. Distilled watershould be within ±0.003 of 1.000.

10. If the water activity value of distilled water is correct,resume sampling. If, after adjusting for linear offsetand cleaning the chamber, you still are getting incor-rect readings when reading verification standards, con-tact Decagon at 509-332-2756 (1-800-755-2751 in USand Canada) for further instructions.


31

This flowchart is a graphical representation of the directions givenabove for checking for linear offset.

Measure Verification Standard

Measure dH2O

Clean

Adjust for Offset

Correct Not Correct

Not Correct

Correct

OK to Sample

or 2nd VerificationStandard

AquaLabSample Preparation

32

6. Sample Preparation

Your AquaLab will continually provide accurate wateractivity measurements as long as its internal sensors arenot contaminated by improperly-prepared samples. Care-ful preparation and loading of samples will lengthen timebetween cleanings and will help you avoid cleaning anddowntime.

Preparing the SampleTo prepare a sample, follow these steps:

1. Make sure that the sample to be measured is homogeneous. Multi-component samples (e.g., muf-fins with raisins) or samples that have outside coatings (like deep-fried, breaded foods) can be measured, but may take longer to equilibrate. For samples like these, AquaLab may take more than five minutes to give an accurate reading, or may require multiple readings of the same sample. Measuring the aw of these types of product is discussed in detail later in this chapter (see Materials Needing Special Preparation).

2. Place the sample in a disposable sample cup, completely covering the bottom of the cup, if pos-sible. AquaLab is able to accurately measure a sample


33

that does not (or cannot) cover the bottom of the cup. For example, raisins only need to be placed in the cup and not flattened to cover the bottom. A larger sample surface area increases instrument efficiency by provid-ing more stable infrared sample temperatures. It also speeds up the reading by shortening the time needed to reach vapor equilibrium.

3. Do not fill the sample cup more than half full. Overfilled cups will contaminate the sensors in the sensor chamber! Filling the sample cup will not make the readings faster or more accurate. There only needs to be enough sample in the cup to allow the water in the sample to equilibrate with the water in the vapor phase and not change the moisture content of the sample. There is a minimum amount of sample needed; therefore, covering the bottom of the sample cup is normally enough.

4. Make sure that the rim and outside of the sample cup are clean. Wipe any excess sample material from the rim of the cup with a clean tissue. Material left on the rim or the outside of the cup will contaminate the sensor chamber and be transferred to subsequent samples. The rim of the cup forms a vapor seal with the sensor block when the drawer knob is turned to the READ position. Therefore, any sample material left on the cup rim will be transferred to the block, preventing this seal and contaminating future samples.


34

5. If a sample will be read at some other time, put the sample cup’s disposable lid on the cup to restrict water transfer. For long-term storage, seal the lid by placing tape or Parafilm completely around the cup/lid junction. It is necessary to seal the cup if it will be a long time before the measurement is made.

Samples Needing Special PreparationAquaLab reads most materials in less than five minutes,depending on which mode you are operating in. Somesamples, however, may require longer reading times, dueto the nature of the material you are sampling. Thesematerials need additional preparation to ensure quick,accurate readings. To find out whether special samplepreparation is necessary, take a reading and see how long ittakes to find the water activity. If it takes longer than sixminutes, remove the sample and take a reading of a verifi-cation standard. This will ensure that the sample itself iscausing the long read time, and that there is not a problemwith your instrument. If the verification standard alsotakes longer than six minutes to sample, refer to Chapter12 of this manual for more information.

Coated and Dried SamplesSamples with coatings such as sugar or fat often requirelonger reading times, because it takes longer for them toequilibrate. If this is the case for your samples, don’t worry


35

that something is wrong with your instrument; it simplymeans that your particular sample takes longer than mostto equilibrate water with its outside environment.

To reduce the time needed to take an aw reading forcoated or dried samples, one thing you can do is crush,slice, or grind the sample before putting it in the samplecup. This increases the surface area of the sample, thusdecreasing reading times. Keep in mind, however, thatmodifying some samples may alter their aw readings.

For example, a candy may have a soft chocolate center anda hard outer coating. The aw reading for the center and theouter coating are different, so one would need to evaluatewhich part of the sample needed to be measured beforecrushing it. When the candy is crushed, for example, theaw will represent the average water activity of the entiresample; whereas leaving the candy whole will give a read-ing for the coating, which may act as a barrier to the cen-ter.Note: if you crush, grind, or slice your sample, be consistent in themethod you use in order to obtain reproducible results.

Slow Water-Emitting SamplesSome extremely dry, dehydrated, highly viscous water-in-oil (butter), high fat, or glassy compositions may haveincreased read times, due to their moisture sorption char-acteristics. AquaLab may require up to ten minutes toreach an accurate measurement of aw, and nothing can be


36

done to decrease the reading times of these types of sam-ples.

For faster reading, it is important to have the water activityof the chamber at or below the water activity of these typeof samples. This causes the sample to release water to thevapor phase and equilibrate with the chamber. If the wateractivity of the headspace is greater than this type of sam-ple, a long period of time will be required to reach equilib-rium and the aw of the sample may be affected.

Propylene GlycolAquaLab will give accurate readings on most alcohols.However, samples with propylene glycol in concentrationshigher than 10% will give inaccurate aw values for consec-utive samples. This is because propylene glycol condenseson the mirror during the reading process, but it does notevaporate from the mirror as water does. As a result, thefirst reading will be accurate, but subsequent readings willnot unless the propylene glycol is cleared out of the cham-ber after each reading. You can clean the propylene glycolout of the chamber by running a sample of the activatedcharcoal (included as part of the AquaLab cleaning kit)after each propylene glycol-bearing sample. Once youhave finished sampling, clean the chamber to make sure allresidue is eliminated (see Chapter 10).

As another solution, Decagon offers a special sampleblock designed for measuring volatiles such as propylene


37

glycol and ethanol. The Volatiles Sensor Block attachesand operates in a similar manner as the chilled-mirrordewpoint block that is currently in your AquaLab. When itis installed, the AquaLab will sense that the Volatiles Blockis attached, and will make measurements accordingly. Theonly difference in operation is a lower accuracy of ±0.01aw. All other operations and features will be the same,including measurement times and adjusting for linear off-set. Contact Decagon for more details about this acces-sory.

Low Water ActivitySamples that have an aw of less than about 0.03 cannot beaccurately measured with the normal AquaLab Series 3model. Samples with such low aw values are rare. When asample’s aw value is below its ability to accurately measure,your AquaLab will display an error message indicating thelast reading it could make on that particular sample. Forexample, say you are measuring a dry sample and the fol-lowing screen appears:

This screen indicates that the last water activity reading theAquaLab measured on this sample was 0.031 at 24.7° C.Therefore, the actual water activity of the sample is lowerthan the instrument’s components can measure.

aw < 0.03124.7 °C


38

If your sample is not extremely dry but is still getting theerror message, refer to Chapter 12 for other possibleexplanations.

Samples not at Room TemperatureSamples that are 4 degrees colder or warmer than theinstrument (chamber) temperature will need to equilibrateto ambient temperature before a fast, accurate reading canbe made. Rapid changes in temperature over short periodsof time will cause the aw readings to rise or fall until thetemperature stabilizes. When the temperature stabilizeswithin one or two degrees of the chamber temperature,you can proceed with normal measurements.

High-water activity samples that are warmer than thechamber temperature can cause condensation inside themeasuring chamber, which will adversely affect subse-quent readings. A warning message appears (Sampletoo hot) if the sample temperature is more than 4° Cabove chamber temperature. If this message appears,immediately remove the sample from the instrument,place a lid on the cup, and allow the sample to cool towithin 4° C of the instrument before measuring.

Samples that are lower than 4° C of the instrument’s tem-perature will cause long read times. The sample tempera-ture must be within one or two degrees of the chambertemperature before fast, accurate readings can be made.Refer to Chapter 12 for additional information.

AquaLabTaking a Reading

39

7. Taking a Reading

Measurement StepsOnce you have prepared your sample, you are ready totake readings. The process is simple:

1. Turn the sample drawer knob to the OPEN/LOAD position and pull the drawer open.

2. Place your prepared sample in the drawer. Check the top lip of the cup to make sure it is free from sample residue (remember, an over-filled sample cup may contaminate the chamber’s sensors).

3. Carefully slide the drawer closed, being especially care-ful if you have a liquid sample that may splash or spill and contaminate the chamber.

4. Turn the sample drawer knob to the READ position to seal the sample cup with the chamber. The follow-ing screen will appear:

chamber sealedmeasurement started


40

This will start the read cycle. In about 40 seconds, the first aw measurement will be displayed on the LCD. Length of read times may vary depending on tempera-ture differences between the chamber and your sam-ple, and other properties of your sample.

For TE models: If you have the temperature equilibrium menu set at 1 or 2, then an equilbration screen appears:

Note: Samples that have a large difference in aw from previoussamples may need extra time to reach equilibrium, since some of theprevious sample’s atmosphere stays in the chamber after measure-ment.

How AquaLab takes ReadingsAquaLab’s reading cycle continues until the rate of changeof three consecutive readings are less than 0.0005 of eachother. The instrument crosses the dew threshold numer-ous times to ensure the accuracy of readings. When theinstrument has finished its read cycle, the aw is displayed,accompanied by the LED flash and beeper (if you havethe beeper enabled).

Ts - Tb = 0.125

Ts = 23.1


41

Cautions

• Never leave a sample in your AquaLab after a reading has been taken. The sample may spill and contaminate the instrument’s chamber if the instrument is accidentally moved or jolted.

• Never try to move your instrument after a sample has been loaded. Movement may cause the sam-ple material to spill and contaminate the sample chamber.

• Take special care not to move the sample drawer too quickly when loading or unloading liquid samples, in order to avoid spilling.

• If a sample has a temperature that is four degrees higher (or more) than the AquaLab’s chamber, the instrument will display:

alerting you to cool the sample before reading. Although the instrument will measure warmer samples, the readings may be inaccurate. Warm samples can cause condensation in the chamber if they have a high water activity.

• The physical temperature of the instrument

sample too hot


42

should be between 5°- 43°C. Between these ambi-ent temperatures, AquaLab will measure samples of similar temperature quickly and accurately. The AquaLab Series 3TE has temperature con-trol capabilities that enable it to read samples at temperatures different from ambient tempera-ture, but no higher than 43°C.

• If you are sampling and a triangular warning sym-bol appears in the top right corner, this indicates

that the mirror has become too dirty to give accu-rate measurements, and you need to clean the mirror and chamber before continuing to sample. For more details about this symbol, please refer to Chapter 12. For cleaning instructions, refer to Chapter 10.

• If a sample has an aw lower than about 0.03, AquaLab will display a message, accompanied by the flashing light, notifying you that your sample is too dry to be accurately measured by the AquaLab. Following is an example:

0.853 aw ∆24.7°C

aw < 0.03124.7 °C


43

This message will stay on the screen until you open thesample drawer. If you know that your sample’s water activ-ity is above what the screen is telling you, your instru-ment’s sensors may have been contaminated and will needto be cleaned (see Chapter 10) or serviced (see Chapter11).

AquaLabComputer Interface

44

8. Computer Interface

Your AquaLab was shipped to you with a standard RS-232interface cable. Using this, you can use your computer’sterminal program to send water activity data to your com-puter for further analysis and storage.

AquaLink SoftwareAn optional program that is available for use with yourAquaLab is AquaLink. AquaLink is a Windows-based pro-gram designed for collection and graphing of data from allAquaLab models. It logs water activity, temperature, timeof measurement, and a time and date stamp (from yourcomputer's clock). It also has sample identification andcomments fields that you can use to help annotate the datathat your AquaLab is taking. AquaLink takes the data anddisplays it in a real-time graph. You can control the incre-ments of each axis to customize the graph as you wish. Ifyou are interested in purchasing a copy of AquaLink, con-tact Decagon or your authorized distributor. Here is asample picture of the AquaLink program:


45

AquaLink screen

Using Windows Hyperterminal To use Hyperterminal with your AquaLab, follow thesesteps:

1. Press the Start button and select Programs > Accesso-ries > Hyperterminal and click on the Hyperterminal icon.

2. At the prompt, choose a name for this program (AquaLab is a good one) and choose an arbitrary icon above to represent it. In future downloads, you will be able to click on this icon in have it already set up for you to download. Click the OK button.


46

3. A pop-up menu labeled “Connect To” will appear. Click on the scroll bar on the bottom of the screen labeled “Connect Using” and select the COM Port your RS-232 cable is connected to.

4. A pop-up menu labeled “COM Properties” will appear, showing the port settings for the COM port you selected. Make sure the settings are the following: Bits per second, 9600; 8 databits, no parity, 1 stop bit, and flow control set to hardware. Click OK.

5. Plug your RS-232 cable to the COM port you selected and connect it to your AquaLab. Begin sampling. AquaLab’s data will be displayed on screen as it sam-ples. It will be output in the following format:

time (since chamber was closed), sample temperature,and aw. Here is an example:

6. When you are finished sampling, you can print the data in the terminal session, or cut and paste it to a spread-sheet or text editor. To save the data, go into the Transfers menu and select “Capture text,” and designate where it should be saved.

3.1, 24.3, 0.862

time since chamber was closed

temp(°C) aw

AquaLabTheory

47

9. Theory: Water Activity in Products

Water is a major component of foods, pharmaceuticals,and cosmetics. Water influences the texture, appearance,taste and spoilage of these products. There are two basictypes of water analysis: water content and water activity.

Water contentThe meaning of the term water content is familiar to mostpeople. It implies a quantitative analysis to determine thetotal amount of water present in a sample. The primarymethod for determining water content is by loss on dry-ing, but secondary methods such as infrared, NMR, orKarl Fisher titration are also used. Moisture content deter-mination is essential in meeting product nutritional label-ing regulations, specifying recipes and monitoringprocesses. However, water content alone is not a reliableindicator for predicting microbial responses and chemicalreactions in materials. The limitations of water contentmeasurement are attributed to differences in the intensitywith which water associates with other components.

Water activityWater activity is a measure of the energy status of the waterin a system, and thus is a far better indicator of perishabil-

AquaLabTheory

48

ity than water content. Figure 1 shows how the relativeactivity of microorganisms, lipids and enzymes relate towater activity. While other factors, such as nutrient avail-ability and temperature, can affect the relationships, wateractivity is the best single measure of how water affectsthese processes.

Fig. 1: Water Activity Diagram—adapted from Labuza

Water activity of a system is measured by equilibrating theliquid phase water in the sample with the vapor phasewater in the headspace and measuring the relative humid-ity of the headspace. In the AquaLab, a sample is placed ina sample cup which is sealed against a sensor block. Insidethe sensor block is a fan, a dew point sensor, a tempera-ture sensor, and an infrared thermometer. The dew pointsensor measures the dew point temperature of the air, andthe infrared thermometer measures the sample tempera-

AquaLabTheory

49

ture. From these measurements the relative humidity ofthe headspace is computed as the ratio of dew point tem-perature saturation vapor pressure to saturation vaporpressure at the sample temperature. When the water activ-ity of the sample and the relative humidity of the air are inequilibrium, the measurement of the headspace humiditygives the water activity of the sample. The purpose of thefan is to speed equilibrium and to control the boundarylayer conductance of the dew point sensor.

In addition to equilibrium between the liquid phase waterin the sample and the vapor phase, the internal equilib-rium of the sample is important. If a system is not at inter-nal equilibrium, one might measure a steady vaporpressure (over the period of measurement) which is notthe true water activity of the system. An example of thismight be a baked good or a multi-component food. Ini-tially out of the oven, a baked good is not at internal equi-librium; the outer surface is at a lower water activity thanthe center of the baked good. One must wait a period oftime in order for the water to migrate and the system tocome to internal equilibrium. It is important to rememberthe restriction of the definition of water activity to equilib-rium.

Effect of Temperature on water activityTemperature plays a critical role in water activity determi-nations. Most critical is the measurement of the difference

AquaLabTheory

50

between sample and dew point temperature. If this tem-perature difference were in error by 1°C, an error of up to0.06 aw could result. In order for water activity measure-ments to be accurate to 0.001, temperature differencemeasurements need to be accurate to 0.017°C. AquaLab’sinfrared thermometer measures the difference in tempera-ture between the sample and the block. It is carefully cali-brated to minimize temperature errors, but achieving0.017°C accuracy is difficult when temperature differencesare large. Best accuracy is therefore obtained when thesample is near chamber temperature.

Another effect of temperature on water activity occurswith samples are near saturation. A sample that is close to1.0 aw and is only slightly warmer than the sensor blockwill condense water within the block. This will causeerrors in the measurement, and in subsequent measure-ments until the condensation disappears. A sample at 0.75aw needs to be approximately 4°C above the chambertemperature to cause condensation. The AquaLab warnsthe user if a sample is more than 4°C above the chambertemperature, but for high water activity samples the opera-tor needs to be aware that condensation can occur if asample that is warmer than the block is put in theAquaLab.

Water PotentialSome additional information may be useful for under-standing what water activity is and why it is such a useful

AquaLabTheory

51

measure of moisture status in products. Water activity isclosely related to a thermodynamic property called thewater potential, or chemical potential (µ) of water, whichis the change in Gibbs free energy (G) when water con-centration changes. Equilibrium occurs in a system whenµ is the same everywhere in the system. Equilibriumbetween the liquid and the vapor phases implies that µ isthe same in both phases. It is this fact that allows us tomeasure the water potential of the vapor phase and usethat to determine the water potential of the liquid phase.Gradients in µ are driving forces for moisture movement.Thus, in an isothermal system, water tends to move fromregions of high water potential (high aw) to regions of lowwater potential (low aw). Water content is not a drivingforce for water movement, and therefore can not be usedto predict the direction of water movement, except inhomogeneous materials.

Factors in determining Water PotentialThe water potential of the water in a system is influencedby factors that effect the binding of water. They includeosmotic, matric, and pressure effects. Typically water activ-ity is measured at atmospheric pressure, so only theosmotic and matric effects are important.

Osmotic effectsOsmotic effects are well known from biology and physical

AquaLabTheory

52

chemistry. Water is diluted when a solute is added. If thisdiluted water is separated from pure water by a semi-per-meable membrane, water tends to move form the purewater side through the membrane to the side with theadded solute. If sufficient pressure is applied to the solute-water mixture to just stop the flow, this pressure is a mea-sure of the osmotic potential of the solution. Addition ofone mole of an ideal solute to a kilogram of water pro-duces an osmotic pressure of 22.4 atm. This lowers thewater activity of the solution from 1.0 to 0.98 aw. For agiven amount of solute, increasing the water content ofthe systems dilutes the solute, decreasing the osmoticpressure, and increasing the water activity. Since microbialcells are high concentrations of solute surrounded bysemi-permeable membranes, the osmotic effect on thefree energy of the water is important for determiningmicrobial water relations and therefore their activity.

Matric effectsThe sample matrix affects aw by physically binding waterwithin its structure through adhesive and cohesive forcesthat hold water in pores and capillaries, and to particle sur-faces. If cellulose or protein were added to water, theenergy status of the water would be reduced. Work wouldneed to be done to extract the water from this matrix. Thisreduction in energy status of the water is not osmotic,because the cellulose or protein concentrations are far toolow to produce any significant dilution of water. Thereduction in energy is the result of direct physical binding

AquaLabTheory

53

of water to the cellulose or protein matrix by hydrogenbonding and van der Waal forces. At higher water activitylevels, capillary forces and surface tension can also play arole.

Sorption Isotherms

Relating aw to water contentChanges in water content affect both the osmotic andmatric binding of water in a product. Thus a relationshipexists between the water activity and water content of aproduct. This relationship is called the sorption isotherm,and is unique for each product. Figure 1 shows a typicalisotherm. Besides being unique to each product, the iso-therm changes depending on whether it was obtained bydrying or wetting the sample. These factors need to bekept in mind if one tries to use water content to infer thestability or safety of a product. Typically, large safety mar-gins are built in to water content specifications to allowfor these uncertainties.

While the sorption isotherm is often used to infer wateractivity from water content, one could easily go the otherdirection and use the water activity to infer the water con-tent. This is particularly attractive because water activity ismuch more quickly measured than water content. Thismethod gives particularly good precision in the center ofthe isotherm. In order to infer water content from wateractivity, one needs an isotherm for the particular product;produced, ideally, using the process that brings the prod-

AquaLabTheory

54

uct to its final water content.

For example, if one were using the AquaLab to monitorthe water content of dried potato flakes, one would mea-sure the water activity and water content of potato flakesdried to varying degrees using the standard drying processfor those flakes. An isotherm would be constructed usingthose data, and the water content would be inferred usingthe measured water activity of samples and that isotherm.

The importance of the concept of water activity of foods,pharmaceuticals, and cosmetics cannot be overly empha-sized. Water activity is a measure of the energy status of thewater in a system. More importantly, the usefulness ofwater activity in relation to microbial growth, chemicalreactivity, and stability over water content has beenshown.

AquaLabCleaning and Maintenance

55

10. Cleaning and Maintenance

The accuracy of your AquaLab is vitally dependent onkeeping your instrument clean. Dust and sampling debriscan contaminate the sampling chamber and must there-fore be regularly cleaned out. To clean your instrument,carefully follow these instructions.

Tools Needed

• thin plastic rod or other non-metal implement

• Distilled Water

• Isopropyl Alcohol

• Lint-free or sizing-free tissues (Kimwipe®)

or

• AquaLab Cleaning Kit

Note: Kimwipes® are ideal because they don’t leave a lint residue,like most tissues. They also don’t have any other compounds in thetissue that may contaminate the sensors in the AquaLab’s block.Also, never use cotton swabs to clean the block sensors. Most cotton


56

swabs contain adhesives and other compounds that are released andtransferred to the mirror and other surfaces, contaminating them.

Cleaning the Block and SensorsAccessing the Block

1. Turn the power off on your AquaLab (switch in back.)

2. Remove the case lid screw located on the back panel. Carefully remove the lid by pulling the back of the lid upward and then sliding the lid back (away from the front of the case) and off.

3. Unscrew the two thumbscrews that secure the sensor block.

4. Unplug the cable with the 20-pin socket that attaches the block to the main circuit board by releasing the two locking levers that are on either side of the socket.

5. Carefully lift the block straight up from its mount. Turn the block over to expose the chamber cavity as shown in the illustration:


57

View of inside block chamberMirror:

1. Wash hands with soap and water (to prevent oils from contaminating the Kimwipe tissue and being trans-ferred to the mirror).

2. Wrap 1.5 — 2cm wide strips of Kimwipe around the plastic spatula

3. Cleaning your AquaLab sensor block is a three-step procedure. First, clean the sensors and block using an iso-propyl alcohol-moistened Kimwipe tissue. Follow this with a Kimwipe tissue moistened with either Decagon’s cleaning solution or distilled water. Finally, wrap a new, dry Kimwipe around the spatula, and use it to thoroughly

thermopile

mirror optical sensor

chamber fan


58

wipe the cleaning solution from the mirror, thermopile and chamber surfaces.

Optical Sensor: You will probably clean the optical sensor while you arecleaning the mirror, since they face each other in a verysmall gap.

Thermopile Sensor: Clean the thermopile in the same manner as describedabove for the mirror. If the thermopile is very dirty, cleanit with isopropanol, followed by multiple distilled water-moistened tissues. This sensor must be free of all dirt,smudges, and lint for the AquaLab to measure accurately.

Inside Chamber: Clean all other portions of the chamber block with a Kim-wipe moistened with distilled water.

Note: Be extremely careful not to damage the fan blades in thechamber. The fan blades are very fragile; if one of them breaks, yourinstrument won’t work properly. Therefore, take extra care whencleaning this portion.

Inside Case:

1. Clean the sample drawer and drawer base. Remove any debris that may have collected inside the case.


59

2. Check once more to make sure there is no debris in the sample chamber cavity.

3. Replace the block, and insert the ribbon cable socket into to the 20-pin plug on the block. Lock it in place with the locking levers.

4. Screw the thumb-screws on the block back in until they are hand-tight.

5. Replace the AquaLab case lid and secure the lid screw.

6. Plug the AquaLab’s power cord back in.

Checking CalibrationAfter you have cleaned the chamber and other parts ofyour AquaLab, it is important to check the instrument’sperformance in order to correct for any linear offset thatmay have occurred during cleaning procedures.

Before you check the instrument, however, we recom-mend that you run a sample of the activated charcoal pel-lets that come with your AquaLab cleaning kit. This cleansthe air inside the chamber, helping it come back to a stablesampling environment.

Check the response of your instrument by measuring thewater activity of both a verification standard and a distilledwater sample.


60

If a linear offset has occurred, refer to chapter 5 for direc-tions on how to correct for linear offset. If, after adjustingfor linear offset your instrument is still not reading sam-ples correctly, contact Decagon for technical support.

AquaLabRepair Instructions

61

11. Repair Instructions

If your AquaLab ever needs to be sent in for service orrepair*, call Decagon at (509) 332-2756 or (US and Can-ada) 1-800-755-2751, or fax us at (509) 332-5158. We willask you for your address, phone number, and serial num-ber. For non-warranty repairs, we will also ask for a pay-ment method (such as a purchase order or credit cardnumber), a repair budget, and billing address.

*Note: If you purchased your AquaLab from one of our interna-tional distributors, please contact them before contacting Decagon.They may be able to provide you with local service and help you rem-edy the problem.

Shipping Directions When you ship your instrument back to us, please includewith it a document with the complete shipping address,name and department of the person responsible for theinstrument, and (most importantly) a description of theproblem. This information will better help our techniciansand our shipping department to expedite repair on yourinstrument and ship it back to you in good time.


62

Following are steps that will help in safely shipping yourinstrument back to us:

1. If possible, ship your AquaLab back to us in its origi-nal cardboard box with foam inserts. If this is not pos-sible, use a box that has at least 4 inches of space between your instrument and each wall of the box.

2. Put your instrument in a plastic bag to avoid disfigur-ing marks from the packaging.

3. Don’t ship your AquaLab to us with the power cord; we have plenty here to use with your instrument.

4. If you aren’t using the foam inserts, pack the box mod-erately tight with packing material, like styrofoam pea-nuts.

5. Tape the box in both directions so it cannot be broken open in shipment.

6. Include necessary paperwork so your repair can be processed quickly. This should include your name, address, serial number, phone and fax numbers, pur-chase order, and a description of the problem.

Ship to:

Decagon Devices Inc.ATTN: Repair Department2365 NE Hopkins Court


63

Pullman, WA 99163

Repair CostsManufacturer’s defects and instruments within the three-year warranty will be repaired at no cost. For non-war-ranty repairs, costs for parts, labor, and shipping will bebilled to you. We have a $65 minimum repair charge. Anextra fee will be charged for rush work. Decagon will pro-vide an estimated repair cost, if requested.

Loaner ServiceWe have loaner instruments that can be provided whileyour instrument is being serviced. There is, however, alimited number of loaner instruments. They are grantedon a “first-come-first-served” basis. This service is inplace to help you if your AquaLab needs service duringcritical operations.

AquaLabTroubleshooting

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12. Troubleshooting

AquaLab is a high performance instrument, designed tohave low maintenance and few problems if used with care.Unfortunately, sometimes even the best operators usingthe best instruments encounter technical difficulties. Hereis a list of some problems that may occur. If you haveencountered a problem that isn’t addressed here, or ifthese remedies still don’t resolve your problem, contactDecagon at 1-800-755-2751 or (509) 332-2756 (for thosenot in the US or Canada).

Problems and SolutionsThe following table is a brief guide to help you quicklydefine solutions to your problems. For more detaileddescriptions of these problems and their solutions, see theexplanations below the table.

Table 1: Troubleshooting

Problem Possible Solutions

1.Won’t turn on • Power cord discon-nected

• Blown fuse


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2. Long read time • Dirty sample chamber

• Product desorbs slowly

• Propylene glycol

• Broken chamber fan blade

3. aw readings on verifi-

cation standards are too high/low to adjust

• Dirty thermopile

• Dirty mirror

• If using a saturated salt solution, it may not be in equilibrium

4. “Sample too hot” • Sample not in thermal equilibrium with AquaLab.

5. “aw < 0.0....” • Sample too dry to accu-rately measure

• Dirty chamber or mirror

6. Triangle appears in upper right corner

• Mirror is dirty




66

1. PROBLEM:AquaLab won’t turn on.

SOLUTION:

• Check to make sure your power cord is securely attached the back of the instrument, and into the power outlet.

• A power surge may have caused a fuse to blow. To change the fuses, follow these instructions:

1. Unplug the power cord from the wall and the instru-ment.

2. Locate the panel where the power cord plugs in. The fuse box is on the right side of that panel. Press in on the release tab and pull the fuse-holder out.

3. Pull the broken fuse(s) out and replace with a 1.25

7. “Block failure” appears on screen after turning on AquaLab

• Block is disconnected from motherboard

• Block memory compo-nent has failed




67

Amp 250V fuse.

Caution: Do not use any other kind of fuse or you will risk damageto your instrument as well as void your warranty.

4. Replace the fuse-holder and push it into the fuse-well until the release tab snaps in place.

5. Re-connect the power cord and turn your instrument on. If the fuse blows again, a failed component may be causing the problem. Contact Decagon to make arrangements for repairs.

2. PROBLEM:Readings are slow or inconsistent.

SOLUTION:

• The sample chamber may be dirty. Refer to Chap-ter 11 of the manual for directions on cleaning the sample chamber.

• Some products absorb or desorb moisture very slowly, causing measurements to take longer than usual, and nothing can be done to speed up the pro-cess. Refer to Chapter 6 for further explanation.

• Your sample may contain propylene glycol. This compound is known to cause unstable readings, because it condenses on the surface of the chilled mir-


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ror and alters readings. Please refer to the propylene glycol section in chapter 6 for hints on reducing diffi-culties with measuring samples with propylene glycol. If you have further questions regarding the measure-ment of propylene glycol, contact Decagon.

Note: As yet, propylene glycol is the only volatile know to act unpre-dictably. Glycerol and other alcohols used to flavor foods can usuallybe measured without problems; however, some aromatics may alsocaused unstable readings. If this seems to be a problem for you, con-tact Decagon.

• A fan blade may be broken inside the block. If even salt standards take a long time to read, and the sample chamber is clean, you may have a broken chamber fan blade. This is especially likely if you have just cleaned the chamber. If you suspect this may have happened, contact Decagon for details on replace-ment.

3. PROBLEM:Water activity readings on verification standards aretoo high/low and a linear offset adjustment cannot bemade any higher/lower.

SOLUTION:

• The thermopile in your chamber, which measures sample temperature, may have become contami-


69

nated. Refer to Chapter 11 for directions on cleaning.

• The chamber mirror may be dirty. Refer to Chapter 11 for directions on cleaning.

• If you weren’t using Decagon’s performance veri-fication standards, high readings may indicate that the salt solution you are using is not in equilibrium.

4. PROBLEM:Message on screen displays the following:

SOLUTION:

• Your sample’s temperature is too high for the instrument to equilibrate with it in a reasonable amount of time. The instrument and sample need to be in temperature equilibrium before accurate samples can be made. Therefore, very cold samples will take a very long time to measure due to the same reason. To avoid this problem, make sure to only measure sam-ples that are at the same temperature as the instru-ment.

5. PROBLEM:

sample too hot


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Message on screen displays the following (example):

SOLUTION:

• The sample is too dry for the instrument to read accurately. If your sample has a water activity that is less than below the detection limits of the instrument, this message will come up. Essentially, it means that there is not enough sample moisture to condense on the mirror and provide a reading.

• The mirror may be dirty. Try cleaning the mirror and chamber and measuring the sample again.

6. PROBLEM:A small triangle appears in the upper right cornerafter sampling:

SOLUTION:The mirror needs to be cleaned, along with the rest ofthe sample chamber, or a volatile contaminant is interfer-ing with the dewpoint determination. This triangle is a

aw < 0.02824.7 °C

0.853 aw ∆24.7°C


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mirror performance indicator. The performance of themirror is measured on a 0 to 1 scale, with 1 being thecleanest. When the AquaLab senses that the mirror per-formance has dropped to unacceptable levels, it will dis-play the triangular warning sign after the sample has beenmeasured. To see what the mirror performance value is,press the upper right button when the triangle appears,and it will show you the value. At this point, you shouldstop sampling and clean the chamber. If the triangle is stillon the screen after cleaning, the mirror is most likely stilldirty or a volatile in your sample is contaminating the mir-ror. Contact Decagon for more assistance.

7. PROBLEM:The following screen comes up after turning on themachine:

SOLUTIONS:

• The block is not plugged in to the motherboard. Open the case and check to make sure that the small ribbon cable that connects the block to the mother-board is snapped and locked in place.

• One or more components has failed on the

block failure


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block’s circuit board. If the block is properly plugged in to the motherboard and this message appears, it is likely that one or more of the compo-nents have failed on the block’s circuit board. If you press -exit- at this prompt, the instrument will not have values for the component that has failed, which will lead to incorrect readings. If this message appears and you continue to sample, Decagon cannot be liable for errors in reading that may occur. Contact Decagon for a solution to this problem.

Component Performance ScreenIf, after cleaning your instrument and reading the othertroubleshooting hints, you have reason to believe that oneof the components of your AquaLab may be causing mea-surement error, you can access a screen that will displayvalues for component performance. This is done either byholding down the lower right button while turning on theinstrument, or by pressing the upper left button while inthe linear offset adjustment main menu (Change linearoffset? Y/N).After it initializes, the following screen will appear:

This screen gives you four values. The top left value is thevalue the thermocouple is reading. It is basically the differ-

-Exit-

sensors 3.21 0.030

23.5 1100


73

ence in temperature between the block and the mirror. Itshould typically have a value of 3, ±0.3. If this is zero,there is something wrong with the thermocouple. The topright value is the value read by the thermopile, which is thetemperature difference between the block and what it“sees” below it (the sample, when reading). This valueshould be around zero, but will change when you changethe drawer position. The bottom left value is the blocktemperature. This value should be around ambient tem-perature. The bottom right value is the mirror reflectancevoltage, in units of millivolts. This value should normallybe between 400 and 2400mV, and should be steady.

You can’t change anything in this screen, but it is here togive you an indication of the component performance. Ifyou notice that any of these values are not what theyshould be, contact Decagon for further instruction. Pressthe button next to -Exit- to get back to the mainmenu.

AquaLabFurther Reading

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13. Further Reading

Water Activity Theory and MeasurementBousquet-Ricard, M., G. Qualyle, T. Pharm, and J.C. Chef-

tel. (1980). Comparative study of three methods ofdetermining water activity in intermediate mois-ture foods. Lebensmittel-Wissenschaft und-Tech-nologie. 13:169-173.

Chirife, J., G. Favetto, C. Ferro-Fontan, and S. Resnik.(1983). The water activity of standard saturatedsalt solutions in the range of intermediate mois-ture foods. Lebensmittel-Wissenschaft und-Tech-nologie. 16:36-38.

Duckworth, R. (1975). Water Relations of Foods. Aca-demic Press, New York.

Gomez-Diaz, R. (1992). Water activity in foods: Determi-nation methods. Alimentaria. 29:77-82.

Gómez, R. and Fernandez-Salguero J. (1992). Water activ-ity and chemical composition of some food emul-sions. Food Chemistry. 45:91-93.

Greenspan, L. (1977). Humidity fixed points of binary sat-urated aqueous solutions. Journal of Research ofthe National Bureau of Standards - A.Physics andChemistry. 81A:89-96.


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Karmas, E. (1981). Measurement of moisture content.Cereal Foods World. 26(7):332-334.

Kitic, D., D.C. Pereira-Jardim, G. Favetto, S.L. Resnik, andJ. Chirife. (1986). Theoretical prediction of thewater activity of standard saturated salt solutionsat various temperatures. Journal of Food Science.51:1037-1042.

Labuza, T.P. and R. Contreras-Medellin. (1981). Predictionof moisture protection requirements for foods.Cereal Foods World. 26(7):335-343.

Labuza, T.P., K. Acott, S.R. Tatini, and R.Y. Lee. (1976).Water activity determination: A collaborative studyof different methods. Journal of Food Science.41:910-917.

Prior, B.A. (1979). Measurement of water activity in foods:A review. Journal of Food Protection. 42(8):668-674.

Reid, D.S. (1976). Water activity concepts in intermediatemoisture foods. In: Intermediate Moisture Foods.Davies, R., G.G. Birch, and K.J. Parker (ed.)Applied Science Publishers, London. pp. 54-65.

Richard, J. and T.P. Labuza. (1990). Rapid determinationof the water activity of some reference solutions,culture media and cheese using a new dew pointapparatus. Sciences des Aliments. 10:57-64.

Roa, V. and M.S. Tapia de Daza. (1991). Evaluation ofwater activity measurements with a dew point elec-tronic humidity meter. Lebensmittel-Wissenschaftund-Technologie. 24(3):208-213.


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Roos, K.D. (1975). Estimation of water activity in interme-diate moisture foods. Food Technology. 29:26-30.

Scott, V.N. and D.T. Bernard. (1983). Influence of temper-ature on the measurement of water activity offood and salt systems. Journal of Food Science.48:552-554.

Snavely, M.J., J.C. Price, and H.W. Jun. (1990). A compari-son of three equilibrium relative humidity measur-ing devices. Drug Development and IndustrialPharmacy. 16(8):1399-1409.

Stamp, J.A., S. Linscott, C. Lomauro, and T.P. Labuza.(1984). Measurement of water activity of salt solu-tions and foods by several electronic methods ascompared to direct vapor pressure measurement.Journal of Food Science. 49:1139-1142.

Stoloff, L. (1978). Calibration of water activity measuringinstruments and devices: Collaborative study. Jour-nal of Association of Official Analytical Chemists.61:1166-1178.

Troller, J.A. (1983). Methods to measure water activity.Journal of Food Protection. 46:129-134.

Troller, J.A. and J.H.B. Christian. (1978). Water Activityand Food. Academic Press, New York.

Troller, J.A. and V.N. Scott. (1992). Measurement of wateractivity (aw) and acidity. In: Compendium ofMethods for the Microbiological Examination ofFoods. Vanderzant, C. and D.F. Splittstoesser (ed.)American Public Health Association, Washington,D.C. pp. 135-151.


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van den Berg, C. (1985). Water activity. In: Concentrationand Drying of Foods. MacCarthy, D. (ed.) Elsevier,London. pp. 11-35.

van den Berg, C. (1991). Food-water relations: Progressand integration, comments and thoughts. In:Water Relationships in Foods. Levine, H. and L.Slade (ed.) Plenum Press, New York.

van den Berg, C. and Bruin. (1981). Water activity and itsestimation in food systems: Theoretical aspects.In: Water Activity: Influences on Food Quality.Rockland, L.B. and G.F. Stewart (ed.) AcademicPress, New York. pp. 1-61.

Vega, M.H. and G.V. Barbosa-Canovas. (1994). Predictionof water activity in food systems: A review on the-oretical models. Revista Espanola De Ciencia YTecnologia De Alimentos. 34:368-388.

Vega, M.H., B. Romanach, and G.V. Barbosa-Canovas.(1994). Prediction of water activity in food sys-tems: A computer program for predicting wateractivity in multicomponent foods. Revista Espan-ola De Ciencia Y Tecnologia De Alimentos.34:427-440.

Vos, P.T. and T.P. Labuza. (1974). Technique for measure-ments of water activity in the high aw range. Jour-nal of Agricultural and Food Chemistry. 22:326-327.

Voysey, P. (1993). An evaluation of the AquaLab CX-2 sys-tem for measuring water activity. Digest, Microbi-ology Section. 124


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Food Quality and SafetyBrandt, L. (1996). Bound for success. Controlling water

activity gives technologists the edge in developingsafe, shelf-stable foods. Food Formulating. Sep-tember:41-48.

Chirife, J. and B.M. Del-Pilar. (1994). Water activity, glasstransition and microbial stability in concentrated/semimoist food systems. Journal of Food Science.59:921-927.

Chirife, J. and M.P. Buera. (1995). A critical review ofsome non-equilibrium situations and glass transi-tions on water activity values of foods in themicrobiological growth range. Journal of FoodEngineering. 25:531-552.

Chirife, J. and M.P. Buera. (1996). Water activity, waterglass dynamics, and the control of microbiologicalgrowth in foods. Critical Reviews in Food Scienceand Nutrition. 36(5):465-513.

Franks, F. (1982). Water activity as a measure of biologicalviability and quality control. Cereal Foods World.27(9):403-407.

Franks, F. (1991). Water activity: a credible measure offood safety and quality? Trends in Food Scienceand Technology. March:68-72.

Hardman, T.M. (1988). Water and Food Quality. ElseiverPress, London.

Kress-Rogers, E. (1993). Food quality measurement. FoodIndustry News. September:23-26.

Levine, H. and L. Slade. (1991). Water Relationships in


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Foods. Plenum Press, New York.Mannheim, C.H., J.X. Liu, and S.G. Gilbert. (1994). Con-

trol of water in foods during storage. Journal ofFood Engineering. 22:509-532.

McMeekin, T.A. and T. Ross. (1996). Shelf life prediction:Status and future possibilities. International Jour-nal of Food Microbiology. 33:65-83.

Nelson, K.A. and T.P. Labuza. (1994). Water activity andfood polymer science: Implications of state onarrhenius and WLF models in predicting shelf life.Journal of Food Engineering. 22:271-289.

Rockland, L.B. and G.F. Stewart. (1981). Water Activity:Influences on Food Quality. Academic Press, NewYork.

Rockland, L.B. and S.K. Nishi. (1980). Influence of wateractivity on food product quality and stability. FoodTechnology. 34:42-59.

Seow, C.C., T.T. Teng, and C.H. Quah. (1988). Food Pres-ervation by Moisture Control. Elsevier, New York.

Taoukis, P., W. Breene, and T.P. Labuza. (1988). Intermedi-ate moisture foods. Advances in Cereal Scienceand Technology. 9:91-128.

Water Activity and MicrobiologyBeuchat, L.R. (1981). Microbial stability as affected by

water activity. Cereal Foods World. 26(7):345-349.Chen, H.C. (1995). Seafood microorganisms and seafood

safety. Journal of Food and Drug Analysis. 3:133-144.


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Farber, J.M., F. Coates, and E. Daley. (1992). Minimumwater activity requirements for the growth of List-eria monocytogenes. Letters In Applied Microbi-ology. 15:103-105.

Garcia de Fernando, G.D., O. Diaz, M. Fernandez, andJ.A. Ordonez. (1992). Changes in water activity ofselected solid culture media throughout incuba-tion. Food Microbiology. 9:77-82.

Gibson, A.M., J. Baranyi, J.I. Pitt, M.J. Eyles, and T.A.Roberts. (1994). Predicting fungal growth: Theeffect of water activity on Aspergillus flavus andrelated species. International Journal of FoodMicrobiology. 23:419-431.

Goaleni, N., J.E. Smith, J. Lacey, and G. Gettinby. (1997).Effects of temperature, water activity, and incuba-tion time on production of aflatoxins and cyclopi-azonic acid by an isolate of Aspergillus flavus insurface agar culture. Applied and EnvironmentalMicrobiology. 63:1048-1053.

Hocking, A.D. and B.F. Miscamble. (1995). Water relationsof some Zygomycetes isolated from food. Myco-logical Research. 99:1113-1118.

Hocking, A.D., B.F. Miscamble, and J.I. Pitt. (1994). Waterrelations of Alternaria alternata, Cladosporium cla-dosporioides, Cladosporium sphaerospermum,Curvularia lunata and Curvularia pallescens.Mycological Research. 98:91-94.

Houtsma, P.C., A. Heuvelink, J. Dufrenne, and S. Noter-mans. (1994). Effect of sodium lactate on toxin


81

production, spore germination and heat resistanceof proteolytic Clostridium botulinum strains. Jour-nal of Food Protection. 57:327-330.

Kuntz, L.A. (1992). Keeping microorganisms in control.Food Product Design. August:44-51.

Li, K.Y. and J.A. Torres. (1993). Water activity relation-ships for selected mesophiles and psychrotrophsat refrigeration temperature. Journal of Food Pro-tection. 56:612-615.

Marauska, M., A. Vigants, A. Klincare, D. Upite, E.Kaminska, and M. Bekers. (1996). Influence ofwater activity and medium osmolality on thegrowth and acid production of Lactobacillus caseivar. alactosus. Proceedings of the Latvian Acad-emy of Sciences Section B Natural Exact andApplied Sciences. 50:144-146.

Miller, A.J. (1992). Combined water activity and soluteeffects on growth and survival of Listeria monocy-togenes Scott A. Journal of Food Protection.55:414-418.

Nakajo, M. and Y. Moriyama. (1993). Effect of pH andwater activity on heat resistance of spores of Bacil-lus coagulans. Journal of the Japanese Society forFood Science and Technology. 40:268-271.

Nolan, D.A., D.C. Chamblin, and J.A. Troller. (1992). Min-imal water activity levels for growth and survivalof Listeria monocytogenes and Listeria innocua.International Journal of Food Microbiology.16:323-335.


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Petersson, S. and J. Schnuerer. (1995). Biocontrol of moldgrowth in high-moisture wheat stored under air-tight conditions by Pichia anomala, Pichia guillier-mondii, and Saccharomyces cerevisiae. Appliedand Environmental Microbiology. 61:1027-1032.

Pitt, J.I. and B.F. Miscamble. (1995). Water relations ofAspergillus flavus and closely related species. Jour-nal of Food Protection. 58:86-90.

Quintavalla, S. and G. Parolari. (1993). Effects of tempera-ture, aw and pH on the growth of Bacillus cells andspore: A response surface methodology study.International Journal of Food Microbiology.19:207-216.

Saad, R.R. (1992). Effect of water activity on growth andlipids of xerophilic fungi, Aspergillus repens andAspergillus amstelodami. Zentralblatt Fuer Mikro-biologie. 147:61-64.

Santos, J., T.M. Lopez-Diaz, M.L. Garcia-Lopez, M.C.Garcia-Fernandez, and A. Otero. (1994). Mini-mum water activity for the growth of Aeromonashydrophila as affected by strain, temperature andhumectant. Letters In Applied Microbiology.19:76-78.

Tapia de Daza, M.S., Y. Villegas, and A. Martinez. (1991).Minimal water activity for growth of Listeriamonocytogenes as affected by solute and tempera-ture. International Journal of Food Microbiology.14:333-338.

Tokuoka, K. and T. Ishitani. (1991). Minimum water activ-


83

ities for the growth of yeasts isolated from high-sugar foods. Journal of General and AppliedMicrobiology. 37:111-119.

Ucar, F. and I. Guneri. (1996). The effect of water activity(aw), pH and temperature on the growth of osmo-philic yeasts. Turkish Journal of Biology. 20:37-46.

Wijtzes, T., P.J. Mcclure, M.H. Zwietering, and T.A. Rob-erts. (1993). Modelling bacterial growth of Listeriamonocytogenes as a function of water activity, pHand temperature. International Journal of FoodMicrobiology. 18:139-149.

Zwietering, M.H., T. Wijtzes, J.C. De-Wit, and R.K. Van'T.(1992). A decision support system for predictionof the microbial spoilage in foods. Journal of FoodProtection. 55:973-979.

Water Activity in FoodsMeat and SeafoodChen, N. and L.A. Shelef. (1992). Relationship between

water activity, salts of lactic acid, and growth ofListeria monocytogenes in a meat model system.Journal of Food Protection. 55:574-578.

Clavero, M.R.S. and L.R. Beuchat. (1996). Survival ofEscherichia coli O157:H7 in broth and processedsalami as influenced by pH, water activity, andtemperature and suitability of media for its recov-ery. Applied and Environmental Microbiology.62:2735-2740.

Duffy, L.L., P.B. Vanderlinde, and F.H. Grau. (1994).


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Growth of Listeria monocytogenes on vacuum-packed cooked meats: Effects of pH, a w, nitriteand ascorbate. International Journal of FoodMicrobiology. 23:377-390.

Fernandez-Salguero J., R. Gómez, and M.A. Carmona.(1994). Water activity of Spanish intermediate-moisture meat products. Meat Science. 38:341-346.

Gómez, R. and Fernandez-Salguero J. (1993). Note: Wateractivity of Spanish intermediate moisture fishproducts. Revista Espanola De Ciencia Y Tecnolo-gia De Alimentos. 33:651-656.

Hand, L. (1994). Controlling water activity and pH insnack sticks. Meat Marketing and Technology.May:55-56.

Lee, M.B. and S. Styliadis. (1996). A survey of pH andwater activity levels in processed salamis and sau-sages in Metro Toronto. Journal of Food Protec-tion. 59:1007-1010.

Luecke, F.K. (1994). Fermented meat products. FoodResearch International. 27:299-307.

Minegishi, Y., Y. Tsukamasa, K. Miake, T. Shimasaki, C.Imai, M. Sugiyama, and H. Shinano. (1995). Wateractivity and microflora in commercial vacuum-packed smoked salmons. Journal of the FoodHygienic Society of Japan. 36:442-446.

Rocha-Garza, A.E. and J.F. Zayas. (1996). Quality ofbroiled beef patties supplemented with wheatgerm protein flour. Journal of Food Science.


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61:418-421.Shimasaki, T., K. Miake, Y. Tsukamasa, M.A. Sugiyama, Y.

Minegishi, and H. Shinano. (1994). Effect of WaterActivity and Storage Temperature on the Qualityand Microflora of Smoked Salmon. NipponSuisan Gakkaishi. 60:569-576.

Untermann, F. and C. Muller. (1992). Influence of aw valueand storage temperature on the multiplication andenterotoxin formation of staphylococci in dry-cured raw hams. International Journal of FoodMicrobiology. 16:109-115.

Williams, S.K., G.E. Rodrick, and R.L. West. (1995).Sodium lactate affects shelf life and consumeracceptance of fresh (Ictalurus nebulosus, marm-oratus) fillets under simulated retail conditions.Journal of Food Science. 60:636-639.

Dairy ProductsFresno, J.M., M.E. Tornadijo, J. Carballo, P.J. Gonzalez,

and A. Bernardo. (1996). Characterization and bio-chemical changes during the ripening of a Spanishcraft goat's milk cheese (Armada variety). FoodChemistry. 55:225-230.

Hong, Y.H. (1991). Physical and chemical properties ofthe process cheese on the domestic market.Korean Journal of Animal Science. 33:387-391.

Kombila, M.E. and C. Lacroix. (1991). The effect of com-binations of salt, lactose and glycerol on the wateractivity (AW) of cheese spreads. Canadian Institute


86

of Food Science and Technology Journal. 24:233-238.

Pisecky, J. (1992). Water activity of milk powders. Milch-wissenschaft. 47:3-7.

Tornadijo, E., J.M. Fresno, J. Carballo, and S.R. Martin.(1993). Study of Enterobacteriaceae throughoutthe manufacturing and ripening of hard goats'cheese. Journal of Applied Bacteriology. 75:240-246.

Valik, L. and F. Gorner. (1995). Effect of water activityadjusted with different solutes on growth and Lac-tic acid production by Lactobacillus helveticus.Folia Microbiologica. 40:472-474.

Vivier, D., M. Rivemale, J.P. Reverbel, R. Ratomahenina,and P. Galzy. (1994). Study of the growth of yeastsfrom feta cheese. International Journal of FoodMicrobiology. 22:207-215.

Vivier, D., R. Ratomahenina, and P. Galzy. (1994). Charac-teristics of micrococci from the surface of Roque-fort cheese. Journal of Applied Bacteriology.76:546-552.

Fruits and VegetablesAyub, M., R. Khan, S. Wahab, A. Zeb, and J. Muhammad.

(1995). Effect of crystalline sweeteners on thewater activity and shelf stability of osmoticallydehydrated guava. Sarhad Journal of Agriculture.11:755-761.

Beveridge, T. and S.E. Weintraub. (1995). Effect of


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blanching pretreatment on color and texture ofapple slices at various water activities. FoodResearch International. 28:83-86.

Hubinger, M., F.C. Menegalli, R.J. Aguerre, and C. Suarez.(1992). Water vapor adsorption isotherms ofguava, mango and pineapple. Journal of Food Sci-ence. 57:1405-1407.

Jimenez, M., M. Manez, and E. Hernandez. (1996). Influ-ence of water activity and temperature on the pro-duction of zearalenone in corn by three Fusariumspecies. International Journal of Food Microbiol-ogy. 29:417-421.

Kiranoudis, C.T., Z.B. Maroulis, E. Tsami, and K.D. Mari-nos. (1993). Equilibrium moisture content andheat of desorption of some vegetables. Journal ofFood Engineering. 20:55-74.

Makower, B. and G.L. Dehority. (1943). Equilibrium mois-ture content of dehydrated vegetables. Industrialand Engineering Chemistry. 35(2):193-197.

Maltini, E., D. Torreggiani, B.R. Brovetto, and G. Bertolo.(1993). Functional properties of reduced moisturefruits as ingredients in food systems. FoodResearch International. 26:413-419.

Marin, S., V. Sanchis, and N. Magan. (1995). Water activity,temperature, and pH effects on growth of Fusar-ium moniliforme and Fusarium proliferatum iso-lates from maize. Canadian Journal ofMicrobiology. 41:1063-1070.

Monsalve-Gonzalez, A., G.V. Barbosa-Canovas, and R.P.


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Cavalieri. (1993). Mass transfer and texturalchanges during processing of apples by combinedmethods. Journal of Food Science. 58:1118-1124.

Tapia de Daza, M.S., C.E. Aguilar, V. Roa, and R.V. Diazde Tablante. (1995). Combined stress effects ongrowth of Zygosaccharomyces rouxii from anintermediate moisture papaya product. Journal ofFood Science. 60:356-359.

Zeb, A., R. Khan, A. Khan, M. Saeed, and S.A. Manan.(1994). Influence of crystalline sucrose and chemi-cal preservatives on the water activity and shelfstability of intermediate banana chips. SarhadJournal of Agriculture. 10:721-726.

Zhang, X.W., X. Liu, D.X. Gu, W. Zhou, R.L. Wang, and P.Liu. (1996). Desorption isotherms of some vegeta-bles. Journal of the Science of Food and Agricul-ture. 70:303-306.

Baked Goods and CerealsAramouni, F.M., K.K. Kone, J.A. Craig, and D.-Y.C. Fung.

(1994). Growth of Clostridium sporogenes PA3679 in home-style canned quick breads. Journalof Food Protection. 57:882-886.

Cahagnier, B., L. Lesage, and M.D. Richard. (1993). Mouldgrowth and conidiation in cereal grains as affectedby water activity and temperature. Letters InApplied Microbiology. 17:7-13.

Clawson, A.R. and A.J. Taylor. (1993). Chemical changesduring cooking of wheat. Food Chemistry. 47:337-


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343.Gómez, R., Fernandez-Salguero J., M.A. Carmona, and D.

Sanchez. (1993). Water activity in foods with inter-mediate moisture levels: Bakery and confectioneryproducts: Miscellany. Alimentaria. 30:55-57.

Harris, M. and M. Peleg. (1996). Patterns of texturalchanges in brittle cellular cereal foods caused bymoisture sorption. Cereal Chemistry. 73:225-231.

Michniewicz, J., C.G. Biliaderis, and W. Bushuk. (1992).Effect of added pentosans on some properties ofwheat bread. Food Chemistry. 43:251-257.

Ramanathan, S. and S. Cenkowski. (1995). Sorption iso-therms of flour and flow behaviour of dough asinfluenced by flour compaction. Canadian Agri-cultural Engineering. 37:119-124.

Roessler, P.F. and M.C. Ballenger. (1996). Contaminationof an unpreserved semisoft baked cookie with axerophilic Aspergillus species. Journal of FoodProtection. 59:1055-1060.

Seiler, D.A.L. (1979). The mould-free shelf life of bakeryproducts. FMBRA Bulletin. April(2):71-74.

Sumner, S.S., J.A. Albrecht, and D.L. Peters. (1993).Occurrence of enterotoxigenic strains of Staphylo-coccus aureus and enterotoxin production in bak-ery products. Journal of Food Protection. 56:722-724.

Tesch, R., M.D. Normand, and M. Peleg. (1996). Compari-son of the acoustic and mechanical signatures oftwo cellular crunchy cereal foods at various water


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activity levels. Journal of the Science of Food andAgriculture. 70:347-354.

Weegels, P.L., J.A. Verhoek, A.M.G. de Groot, and R.J.Hamer. (1994). Effects of gluten of heating at dif-ferent moisture contents: I. Changes in functionalproperties. Journal of Cereal Science. 19:31-38.

Beverages/Soups/Sauces/PreservesCarson, K.J., J.L. Collins, and M.P. Penfield. (1994). Unre-

fined, dried apple pomace as a potential foodingredient. Journal of Food Science. 59:1213-1215.

Durrani, M.J., R. Khan, M. Saeed, and A. Khan. (1992).Development of concentrated beverages fromAnna apples with or without added preservativesby controlling activity of water for shelf stability.Sarhad Journal of Agriculture. 8:23-28.

Ferragut, V., J.A. Salazar, and A. Chiralt. (1993). Stability inthe conservation of emulsified sauces low in oilcontent. Alimentaria. 30:67-69.

Ibarz, A., J. Pagan, and R. Miguelsanz. (1992). Rheology ofclarified fruit juices: II. Blackcurrant juices. Journalof Food Engineering. 15:63-74.

Kusumegi, K., T. Takahashi, and M. Miyagi. (1996).Effects of addition of sodium citrate on the pas-teurizing conditions in "Tuyu", Japanese noodlesoup. Journal of the Japanese Society for Food Sci-ence and Technology. 43:740-747.

Sa, M.M. and A.M. Sereno. (1993). Effect of temperatureon sorption isotherms and heats of sorption of


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quince jam. International Journal of Food Scienceand Technology. 28:241-248.

Pharmaceuticals/CosmeticsAhlneck, C. and G. Zografi. (1990). The molecular basis

of moisture effects on the physical and chemicalstability of drugs in the solid state. InternationalJournal of Pharmaceutics. 62:87-95.

Cochet, N. and A.L. Demain. (1996). Effect of wateractivity on production of beta-lactam antibioticsby Streptomyces clavuligerus in submerged cul-ture. Journal of Applied Bacteriology. 80:333-337.

Costantino, H.R., R. Langer, and A.M. Klibanov. (1994).Solid-phase aggregation of proteins under phar-maceutically relevant conditions. Journal of Phar-maceutical Sciences. 83:1662-1669.

Enigl, D.C. and K.M. Sorrels. (1997). Water Activity andSelf-Preserving Formulas. In: Preservative-Freeand Self-Preserving Cosmetics and Drugs: Princi-ples and Practice. Kabara, J.J. and D.S. Orth (ed.)Marcel Dekker, pp. 45-73.

Hageman, M.J. (1988). The role of moisture in protein sta-bility. Drug Development and Industrial Phar-macy. 14(14):2047-2070.

Heidemann, D.R. and P.J. Jarosz. (1991). Performulationstudies involving moisture uptake in solid dosageforms. Pharmaceutical Research. 8(3):292-297.

Friedel, R.R. and A.M. Cundell. (1998). The application ofwater activity measurement to the microbiological


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attributes testing of nonsterile over-the-counterdrug products. Pharmacopeial Forum. 24(2):6087-6090.

Kontny, M.J. (1988). Distribution of water in solid phar-maceutical systems. Drug Development andIndustrial Pharmacy. 14(14):1991-2027.

Zografi, G. (1988). States of water associated with solids.Drug Development and Industrial Pharmacy.14(14):1905-1926.

Zografi, G. and M.J. Kontny. (1986). The interactions ofwater with cellulose- and starch-derived pharma-ceutical excipients. Pharmaceutical Research.3(4):187-193.

MiscellaneousBell, L.N. and T.P. Labuza. (1992). Compositional influ-

ence on the pH of reduced-moisture solutions.Journal of Food Science. 57:732-734.

Bell, L.N. and T.P. Labuza. (1994). Influence of the low-moisture state on pH and its implication for reac-tion kinetics. Journal of Food Engineering. 22:291-312.

Bell, L.N. (1995). Kinetics of non-enzymatic browning inamorphous solid systems: Distinguishing theeffects of water activity and the glass transition.Food Research International. 28:591-597.

Brake, N.C. and O.R. Fennema. (1993). Edible coatings toinhibit lipid migration in a confectionery product.Journal of Food Science. 58:1422-1425.


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Dole, M. and L. Faller. (1950). Water sorption by synthetichigh polymers. Journal of the American ChemicalSociety. 12:414-419.

Fernandez-Salguero J., R. Gómez, and M.A. Carmona.(1993). Water activity in selected high-moisturefoods. Journal of Food Composition and Analysis.6:364-369.

Lomauro, C.J., A.S. Bakshi, and T.P. Labuza. (1985). Eval-uation of food moisture sorption isotherm equa-tions. Part I: Fruit, vegetable and meat products.Lebensmittel-Wissenschaft und-Technologie.18:111-117.

Lomauro, C.J., A.S. Bakshi, and T.P. Labuza. (1985). Eval-uation of food moisture sorption isotherm equa-tions. Part II: Milk, coffee, tea, nuts, oilseeds,spices and starchy foods. Lebensmittel-Wissen-schaft und-Technologie. 18:118-124.

Yasuda, H., H.G. Olf, B. Crist, C.E. Lamaze, and A. Peter-lin. (1972). Movement of water in homogeneouswater-swollen polymers. In: Water Structure at theWater Polymer Interface. Jellinek, H.H.G. (ed.)Plenum Press, New York/London.

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Appendix A

Preparing Salt SolutionIf you choose to mix a saturated salt solution for use as averification standard, we recommend that you use theapproved AOAC method. This method is as follows:

1. Select a reagent-grade salt and place it in a test con-tainer to a depth of about 4cm for more soluble salts (lower aw), to a depth of about 1.5 cm for less soluble salts (high aw), and to an intermediate depth for inter-mediate salts.

2. Add distilled water in increments of about 2mL, stir-ring constantly.

3. Add water until the salt can absorb no more water, evidenced by the presence of free liquid. Keep the amount of free liquid to the minimum needed to keep the solution saturated with water. If you plan on using this solution over a long term period, seal the solution well to prevent losses from evaporation. Below is a table of saturated salt solutions and their respective water activities at various temperatures. Please note that these values are based on averaged published data, and the standard errors shown reflect Greenspan’s standard error for each salt solution, not the AquaLab’s accuracy in measuring the salt. AquaLab

AquaLabAppendix A

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measures all samples with an accuracy of ±0.003 aw.

4. Saturated salt solutions are very temperature-sensitive and their values are not as accurate as the verification standards offered by Decagon.

Adapted from Greenspan (1977). Rounded to nearest thousandth.

Table 2: Water Activity of Selected Salt Solutions

Saturated Solution

aw at 20° C aw at 25° C

Lithium Chloride

0.113 ± 0.003 0.113 ± 0.003

MagnesiumChloride

0.331 ± 0.002 0.328 ± 0.002

Potassium Carbonate

0.432 ± 0.003 0.432 ± 0.004

Magnesium Nitrate

0.544 ± 0.002 0.529 ± 0.002

Sodium Chloride

0.755 ± 0.001 0.753 ± 0.001

Potassium Chloride

0.851 ± 0.003 0.843 ± 0.003

Potassium Sulfate

0.976 ± 0.005 0.973 ± 0.005

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Appendix B

Aqualab will measure these standards to ±0.003aw

Table 3: Temperature Correction of Decagon’s Verification Standards

Temp.(°C)

H2O 0.5mKCl

6.0mNaCl

8.57mLiCl

13.41mLiCl

15.0 1.000 0.984 0.761 0.492 0.238

20.0 1.000 0.984 0.760 0.496 0.245

25.0 1.000 0.984 0.760 0.500 0.250

30.0 1.000 0.984 0.760 0.504 0.255

35.0 1.000 0.984 0.760 0.508 0.261

40.0 1.000 0.984 0.760 0.512 0.266

AquaLabDeclaration of Conformity

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Declaration of Conformity

Application of Council 89/336/EECDirective:

Standards to which EN81-1conformity is declared: EN500082-1

Manufacturer’s Name: Decagon Devices, Inc.2365 NE Hopkins CourtPullman, WA 99163USA

Type of Equipment: AquaLab wateractivity meter.

Model Number: Series 3, Series 3TE

Year of First Manufacture: 1999

This is to certify that the AquaLab water activity meter, manufactured by Decagon Devices, Inc., a corporation based in Pullman, Washington, USA meets or exceeds the standards for CE compliance as per the Council Direc-tives noted above. All instruments are built at the factory at Decagon and pertinent testing documentation is freely available for verication. This certification applies to all AquaLab Series 3 models, including, but not limited to, the Series 3 and 3TE.

AquaLabCertificate of Traceability

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Certificate of Traceability

Decagon Devices, Inc.2365 NE Hopkins CourtPullman WA 99163tel: (509) 332-2756fax: (509) [email protected]

This is to certify that AquaLab water activity meters are manufactured utilizing temperature standards with calibra-tion traceable to the National Institute of Standards and Technology (NIST).

AquaLabIndex

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IndexAaccessories 12accuracy 5AquaLab

and chilled mirror dewpoint technique 6and temperature 7limitations of 9series 3 5series 3TE 5, 9theory 47

Bbeeper 19

changingblock

sensors 56block failure 66, 71buttons

for linear offset settings 28for menu selection 15

Cc for continuous mode 17cautions 41CE compliance 97charcoal 36, 59checking calibration 59chilled-mirror technique 6cleaning 55

inside case 58

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optical sensor 58sensor block 56thermopile 58tools needed for 55

coated samplesshortening read time for 34

communication settings 46component performance indicator 72components 12computer interface 44continuous sampling mode 17cotton swabs

not used for cleaning 55customer service 1Czech 16

DDanish 16data

download format 46Declaration of Conformity 97dehydrated samples 35distilled water 26dried samples

shortening read time for 34dry samples 42, 70

Ee-mail address 2error messages 64

"sample too dry" 70"sample too hot" 69triangle on screen 70

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exit 20

Ffan

inside sample chamber 7fax number 2features 11flashing. See LEDFrench 16further reading 74fuse

changing 66

GGerman 16

HHyperterminal

using for downloading 45

IItalian 16

Llanguages

changing 16LED 19linear offset

causes for 24definition 24how to adjust for 26, 26–31menu 20when to verify for 26

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loaners 63location

for sampling 13low water activity 37, 42

Mmain menu 14, 15maintenance 55manual 1menus

main menu 15system configuration 18

molalityof verification standards 25

NNIST

traceability 98Norwegian 16

Oosmotic effects 51

Ppeltier cooler 6pharmaceuticals 91Portuguese 16preparing salt solutions 94propylene glycol 36, 67

cleaning out of chamber 36

Rread time

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affected by sample temp. 38long read time 7, 34, 65, 67

readingscautions 41how AquaLab takes 39, 40taking readings 39

references 74baked goods and cereals 88beverages, soups, sauces, preserves 90dairy products 85food quality and safety 78meat and seafood 83microbiology 79pharmaceuticals 91water activity theory 74

relative humidity 5, 49repair

costs 63instructions for 61shipping 61

Ssalt standards. See verification standardssample

slow water-emitting 67sample cups

cleaning 33filling level 33

sample equilibration screen 7, 23sample preparation 32sample too dry 42, 70sample too hot 38, 41, 65samples

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coated 34dehydrated 35dried 34low water activity 37multi-component 32needing special preparation 34not at room temperature 38slow water-emitting 35surface area of 35viscous 35

sampling modescontinuous 17normal 15

saturated salts 94seller’s liability 3series 3 5series 3TE 5, 9

important tips 22shipping for repair 61sorption isotherms

relating water activity to water content 53Spanish 16spilling the sample 41Swedish 16

Ttechnical support 1telephone number 2temperature

effects on water activity 49hot samples 41of instrument 41samples not at room temp. 38

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temperature control 7reasons for 8

theorywater activity 74

timeformat in data download 46long read times 67

toll-free number 2triangle

mirror performance indicator 65, 70troubleshooting 64

Vverification standards 24

aw readings too high/low for 68compared to saturated salts 95for linear offset 28long read times for 68types 25

volatiles 9, 36special sampling block for 36

Wwarm-up 14warranty 2warranty card 2water activity

adjusting for offset 29Aqualab and 5definition 5, 47displayed 14effect on food 6, 47low 42

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of verification standards 25stability diagram 48theory 47

water contentdefinition 47methods for determining 47vs. water activity 47, 53

water potentialfactors in determining 51matric effects 52osmotic effects 51relation to water activity 50

AquaLab Series 3 v5

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