Temperature measurement in the cold chain Chris Kennedy Nutrifreeze Ltd Monitoring temperature...
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Transcript of Temperature measurement in the cold chain Chris Kennedy Nutrifreeze Ltd Monitoring temperature...
Temperature measurement in the cold chain
Chris KennedyNutrifreeze Ltd
Monitoring temperature duringdistribution of chilled & frozen foods
Contents
Throughout this presentation you’ll find links to other useful sites: ... like this one. (Note: to follow these red links you need to be in “slide show” mode. They will not work in edit mode.
Thermometry- a little revision
What temperatures to measure– Cold stores– Transport– Retail display
Damped thermometers– Home– Data logging– Along the distribution chain
A simple cold chain might have the following stages
Manufacture & freezing or chilling Packaging Temporary cold store
Transport
Transport
Holding store
Distribution centre
Transport
Retail outlet
Back to Square 1
The 0th Law of Thermodynamics:
Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other
A
C
B
Back to Square 1
The 0th Law of Thermodynamics:
Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other
A
C
B
Back to Square 1The 0th Law of Thermodynamics:
Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other
The property that determines whether two systems are in thermal equilibrium is their temperature, i.e. two systems in thermal equilibrium with each other have the same temperature
A B
C
A good thermometer
In general, when we design a thermometer we want it to quickly come into thermal equilibrium with the product whose temperature we are measuring. And we want to do this without changing that temperature by making the measurement. Hence, a good thermometer will generally have:
negligible heat capacity
a fast response (but see later)
at least one clearly identifiable, unambiguous thermal property
How to measure temperature
Thermal expansion of a liquid (mercury/alcohol)
Accuracy limited by bore uniformity…
… and calibration of scale
Calibration does not change with time
Liquid in glass thermometer
How to measure temperature
Two dissimilar metals tightly bonded (Fe/Cu)
Different coefficients of expansion cause the strip to curl (heating/cooling)
More info on bimetallic strips and other thermometers
Bimetallic strip
How to measure temperature
-40 oC to 500 oC
Accuracy at time of manufacture about 1% of scale
Bimetallic strip
How to measure temperature
Seebeck effect. Emf generated by T difference along a wire
Two different metals V=a(Tunknown-Tref)
The trick/cost is in measuring V and Tref
Thermocouple principle
How to measure temperature
Reference junction is in an isothermal block with Tref being measured by a semiconductor
Type T and type K thermocouples generate about 40V/oC so amplification required
Calibration is required for the measuring device
Thermocouple
How to measure temperature
Typically type K or type T thermocouples are used in the food industry
Type K is Chromel–Alumel
Type T is Copper-Constantan
Precision grade = +/- 1.0% or 1oC
Most probes require recalibration every 6 months to maintain accurate measurement capability
More info on thermocouples
Thermocouple
How to measure temperature
A thermistor is a semiconductor whose resistance changes with temperature
Resistance rises rapidly as temperature is reduced
Usually a bridge resistance measurement with conversion to temperature
Accuracy typically 2-3% of resistance different. May change with ageing
More info on thermistors
Thermistor
How to measure temperature - RTDs
Highly reproducible
Typical stability quoted as +/- 0.5oC per year or better
Wire-wound or thin film platinum on a ceramic substrate
PT-100 = 100 at 0oC
Requires mA current source to measure 0.385 ohms/oC
More info on resistance thermometry
Platinum resistance thermometer
Target temperatures
When storing or transporting foods, what are the temperatures we should be aiming to maintain for chilled and frozen products?
The three main issues are:– meeting the legal obligations– maintaining safety– maintaining quality
Target temperatures - chilled foodsChilled foods - legal requirements
The Food Safety (Temperature Control) Regulations 1995 …
… chilled food must be kept below 8°C. However, this is not cold enough to stop the growth of all pathogens.
Target temperatures – Chilled foods
Chilled foods - the legal requirement is 8 oC. But the following guidelines:
UK Food standards agencyEnvironmental Health officersThe Department of Health, andThe Institute of Food Science and Technology
… all advise a storage temperature of between 0 oC and 5 oC.
Manufacturers nearly all advise (and calculate shelf life) below 5oC
Target temperatures
Frozen Foods. The legal requirement:
Quick Frozen Foodstuffs must be stored and distributed below–18 oC. A reasonable time at -15 oC is allowed during local distribution.
Commission directive 92/1/EC requires monitoring equipment to be fitted in cold stores and vehicles used to distribute quick frozen foods. This regulation is about to be updated
Normal storage and distribution temperatures are between -25 oC and –20 oC
Temperature fluctuations can be as important as absolute temperature for quality issues
Cold stores The figure shows a simple small cold store
Refrigeration is often regulated on return air temperature or warmest air temperature
Local hot spots can exist due to lighting, door positioning, etc
The number of sensors will depend on size and layout of store
Info on cold store safety
Cold stores
The most important temperature is of course the food (surface?)
Air temperature is only a guide to this and will fluctuate more rapidly
Damped thermometers will give a truer record of the food temperature
The most important temperature is of course the food surface (red line)
Air temperature is only a guide to this and will fluctuate more rapidly
Damped thermometers will give a truer record of the food temperature
Simulation of temperature abuse on a case of meat productsCase of product removed to +20 oC still air
4
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0 10 20 30 40 50 60 70
time (minutes)
Te
mp
era
ture
(C
)T
em
pe
ratu
re (
C)
Time (minutes)
Bulk Transport
Most vehicles have evaporator at front of load
Thermistor or gauge read-out for driver
Control on return air temperature
Secondary sensor at warm end to monitor performance
The International Institute of refrigeration website has more info on all aspects of the cold chain
Air temperature monitoring of temperature in a controlled vehicle
Bulk TransportLoading is important for good air circulation and rapid recovery from door openings.
The transport of refrigerated foods between countries is controlled by the International Agreement of the Transport of Perishable Foodstuffs , commonly known as the ATP agreement. This agreement has been adopted into the legislation of most countries.
(a) Normal air temperature record, and (b) Poorly loaded vehicle air temperature record of chilled foods vehicle
( By permission of Cold Chain Instruments)
Local Transport
Excursions depend on size and frequency of door openings
The more variations in air temperature the less use air temperature is as a monitor
Food measurement or damped monitoring becomes more desirable
Air temperature record of a small delivery vehicle
Retail Display
According to one leading retailer:“80% of supermarket customer complaints can be traced to defects in the chain after delivery to the supermarket”
Retail Display
Air return and air-off temperatures are recorded
Issues are:
– Location– Filling– Heating/lighting– Dehydration– Packaging– Customers
Air temperature monitoring in retail display cabinets: (a) multi-deck cabinet, (b) serve-over cabinet
Retail Display Wide variation in design of
cabinets
Air off and return air temperatures should provide the extremes
Hot spots can only be detected using food temperature measurement
Temperature monitoring records of two different display cabinets.
(By permission of the University of Bristol)
Undamped thermometers
These are thermometers with fast response times - but this is not always what we need
What temperatures to measure?
Throughout the cold chain it is common to:
– Set the air-off temperature– Control using the return air temperature– And usually to monitor the hot spot temperature
All the above is necessary but we would still like to know the FOOD temperature:
– Increasingly, it is food simulant probes that help to provide this information
Damped thermometers
The use of a food simulant allows us to monitor the likely temperatures of foods
Simple food simulants (water/butter/glycerol) allow monitoring of the likely MEAN temperature of foods
Particularly useful where door openings are frequent (multidrop/retail/ home/)
But also allow monitoring of the food chain by suppliers/retailers
Damped thermometers An example FoodsaFe for use in catering and
at home Individually calibrated liquid in
glass
Time (minutes)
Tem
per
atu
re (
C)
Comparison of thermometers with actual foods after the door of a domestic fridge is left open
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= Sausage; = Leg joint at 5mm depth
= Standard undamped thermometer
Damped thermometers Seal in a food gel Now the response
corresponds to a food of similar dimensions
More info on the FoodsaFe
Comparison of thermometers with actual foods
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0 5 10 15 20 25 30 35
Time (minutes)
Te
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era
ture
(C
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= Sausage; = Leg joint at 5mm depth
= Damped FoodsaFe thermometer
Damped thermometers
Damping can also be used for thermocouple devices and data loggers
Here a type-K thermocouple is housed in the gel
The gel and dimensions can be designed to match specific food products
Damped thermometers These probes were used to demonstrate the chilled food hold
times of passive cool boxes for home grocery delivery- Igloo maxcold
Half-full cool box responses with precooling and 2.2 kg of gel packs
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Time (minutes)
Tem
pera
ture
gain
(C
)
9l wall 9l centre precool and 2.2 kg gel 9L wall
precool and 2.2kg gel 9L centre Linear (9l centre) Linear (precool and 2.2kg gel 9L centre)
Damped thermometers These probes were used to demonstrate the chilled food hold
times of passive cool boxes for home grocery delivery - and the PED Thermexx
PED Pressure formed PS cool box responses with eutectic chill plate
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Time (minutes)
Tem
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ain
(C
)
wall centre
PED pressure formed PS cool box responses with eutectic chill plate
Damped thermometersDamped data loggers
Numerous available
Most use PTFE block to slow response
Hanna HI762
Digitron ThermaTag
Damped thermometers
In bulk distribution mean temperature is not necessarily the most useful
The critical temperature is often the surface temperature
Response of surface temperature will depend on
– Product composition– Case size– Packing material
The food simulant must be designed to closely match the response of the package
Summary
Throughout the cold chain it is common to:
– set the air-off temperature– control using the return air temperature
All of this is necessary but we would still like to know the FOOD temperature:
– increasingly, food simulant probes help to provide this information