Temperature measurement in the cold chain

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Temperature measurement in the cold chain
Monitoring temperature during distribution of chilled & frozen foods Chris Kennedy Nutrifreeze Ltd

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 Holding store Transport Distribution centre Transport Retail outlet

Back to Square 1 A B C The 0th Law of Thermodynamics:
Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other A B C

Back to Square 1 A B C The 0th Law of Thermodynamics:
Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other A B C

Back to Square 1 A B C The 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
Liquid in glass thermometer Thermal expansion of a liquid (mercury/alcohol) Accuracy limited by bore uniformity… … and calibration of scale Calibration does not change with time

How to measure temperature
Bimetallic strip 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

How to measure temperature
Bimetallic strip -40 oC to 500 oC Accuracy at time of manufacture about 1% of scale

How to measure temperature
Thermocouple principle 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

How to measure temperature
Thermocouple Reference junction is in an isothermal block with Tref being measured by a semiconductor Type T and type K thermocouples generate about 40mV/oC so amplification required Calibration is required for the measuring device

How to measure temperature
Thermocouple 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

How to measure temperature
Thermistor 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

How to measure temperature - RTDs
Platinum resistance thermometer Highly reproducible Typical stability quoted as +/- 0.5oC per year or better Wire-wound or thin film platinum on a ceramic substrate PT-100 = 100W at 0oC Requires mA current source to measure ohms/oC More info on resistance thermometry

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 foods
Chilled 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 agency Environmental Health officers The Department of Health, and The 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 oC and –20 oC Temperature fluctuations can be as important as absolute temperature for quality issues

Cold stores Info on cold store safety
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

Simulation of temperature abuse on a case of meat products
Case of product removed to +20 oC still air 4 Temperature (C) Time (minutes) 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

Air temperature monitoring of temperature in a controlled vehicle
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 Air temperature monitoring of temperature in a controlled vehicle The International Institute of refrigeration website has more info on all aspects of the cold chain

Bulk Transport Loading 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)

Air temperature record of a small delivery vehicle
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 Comparison of thermometers with actual foods after the door of a domestic fridge is left open 18 16 14 12 Temperature (C) 10 8 6 4 2 5 10 15 20 25 30 35 Time (minutes)  = 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 16 14 Temperature (C) 12 10 8 6 4 2 5 10 15 20 25 30 35 Time (minutes)  = 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

Half-full cool box responses with precooling and 2.2 kg of gel packs
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 7 6 5 4 Temperature gain (C) 3 2 1 50 100 150 200 250 300 350 400 -1 Time (minutes) 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)

PED pressure formed PS cool box responses with eutectic chill plate
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

Damped thermometers Damped data loggers Numerous available
Damped thermometers Damped 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

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