measurement of WATER ACTIVITY in food anna divya thomas

Water activity is defined as the vapor pressure of a liquid divided by that of pure water at the same temperature

Simply, stated it is a measure of the energy status of the water in a system.

Water activity was developed to account for the intensity with which water associates with various non-aqueous constituents and solids.

Definition of aw:

aw= p/ p₀where p is the vapor pressure of

water in the substance, and p₀ is the vapor pressure of pure water at the same temperature

Higher aw substances tend to support more microorganisms. Bacteria usually require at least 0.91, and fungi at least 0.7

Measurement of Water ActivityThe measurement of water activity in

foods has been the subject of many studies

The choice of one technique over another depends on the range, accuracy, precision, and speed of measurement required

The accuracy of most methods lies in the range of 0.01–0.02 aw units.

On the basis of the underlying principles, the methods for water activity measurement have been classified into four major categories:

Measurements Based on Colligative Properties

1) Vapor Pressure MeasurementThe measurement using a vapor pressure

manometer (VPM) was first suggested by Makower and Meyers, Taylor, Sood and Heldman, Labuza , and Lewicki

aw is expressed as ratio of partial pressure of water in food to vapor pressure of pure water having same temperature

Therefore measuring the vapor pressure of water in food system is the most direct measure of aw

The method for measurement of water vapor pressure of food consists of placing 10–50 g of sample in the sample flask

The desiccant flask is filled with a desiccant material, usually CaSO4

Keeping the sample flask isolated, the system is evacuated to less than 200 μmHg; this is followed by evacuation of the sample for1–2 min.

The vacuum source is then isolated by closing the valve between the manometer legs

Upon equilibration for 30–50 min the pressure exerted by the sample is indicated by the difference in oil manometer height (ΔL1).

The sample flask is subsequently excluded from the system, and the desiccant flask is opened

Water vapor is removed by sorption onto the desiccant, and the pressures exerted by volatiles and gases are indicated by the difference in manometer legs (ΔL2)

Apiezon B manometric oil (density 0.866 g/cm3) is generally used as the manometric fluid

Temperatures of the sample (Ts) and the vapor space in the manometer (Tm) are recorded

water activity of the sample, after thermal and pressure equilibration are attained at each measurement, is calculated from formula

If Ts and Tm are different, water activity is corrected as

2)Freezing Point Depression Measurements

For aw > 0.85, freezing point depression techniques have been reported to provide accurate results

For real solutions, the relationship between the freezing point of an aqueous solution and its aw is

This method is applicable only to liquid foods and provides the aw at freezing temperature instead of at room temperature,

The error is reported to be relatively small (0.01 at 25°C).

This method, however, has the advantage of providing accurate water activity in the high range (>0.98)

And can be effectively applied to systems containing large quantities of volatile substances

Other colligative properties such as osmotic pressure and boiling point elevation have not been used for food systems.

Measurements Based on Psychrometry

Measurements of dew point and wet bulb depression of thermocouple psychrometers along with hair and electric hygrometers have been used in the measurements of aw

1) Electric HygrometerThere are 2 type of electric hygrometer

sensors capacitance or resistance Material(either salt film or hygroscopic

polymer fiber)Depending on the water vapor pressure

of surrounding air, water will absorb or desorbs within

The sensor must be calibrated to resistance or capacitance value to unit of aw

In these instrument sample is placed in sealed chamber containing sensor

The sample air and sensor must come to vapor and thermal equilibrium for accurate aw measurement

These instrument measure the entire range from 0.1 aw and accuracy between 0.01 and 0.02 aw

Hair or polymer hygrometerSeveral strands of hair are attached

between a fixed point Leaver arm to produce a deflection

when relative humidity changesSample is placed in a hygrometer

sample cup and seals lid containing build-in-polymer hygrometer

This instrument requires 3hr of equilibration at constant temperature

Thermocouple psychrometerPsychrometer measure the wet bulb

temperature which related to vapor pressure using the equation

Thermocouple is positioned above a sample in a small sealed champers

Small droplet of water is placed or condensed on thermocouple junction

As water evaporate latent heat of vaporization cools the thermocouple to wet bulb temp

2) Capacitance hygrometer

Consist of 2 charged plate separated by a polymer membrane dielectric

As membrane absorb water, its ability increases and capacitance is measured

3) Dew point hygrometerThe temperature at which dews forms

on a clean surface is directly related to the vapor pressure of air

Placing the mirror over a closed sample chamber

The mirror is cooled until dew point temperature is then used to find the relative humidity of chamber using psycho meter chart

Major drawbacks include sensor fatigue and sensor poisoning by volatiles such as glycol, ammonia, acetone, and other organic substances

Measurements Based on Isopiestic TransferThe isopiestic method relies on the

equilibration of the water activities in two materials in a closed system

Transfer of moisture may take place either through direct contact of the materials,

Thus allowing for movement of bulk, micro capillary, and gaseous water

Or by maintaining the two materials separately, thus permitting transfer to occur only through the vapor phase

Analysis of the concentration of water in some reference material such as microcrystalline cellulose or a protein at equilibrium permits determination of water activity from the calibration curve

This technique is not accurate at aw levels of less than 0.50 or over 0.90

Measurements Based on Suction (Matric) Potential

The water potential of soil the capillary suctional potential of gel and the matric potentials of food gels have been determined using the principle of a tensiometer

Accurate for high aw range, the technique is useful for materials that bind large quantities of water

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