Temperature measurement in the cold chain

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

2. 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

3. Holding store A simple cold chain might have the following stages Manufacture & freezing or chilling Packaging Temporary cold store Transport Transport Distribution centre Transport Retail outlet

4. 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 B C

5. 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 B C

6. 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 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

7. 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

8. 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

9. 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

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

11. 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

12. 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

13. 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 Thermocouple

14. 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

15. 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 0.385 ohms/oC • More info on resistance thermometry

16. 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

17. 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.

18. 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

19. Target temperatures Frozen Foods. The legal requirement: Quick Frozen Foodstuffsmust 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 beupdated • Normal storage and distribution temperatures are between -25 oC and –20 oC • Temperature fluctuations can be as important as absolute temperature for quality issues

20. 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

21. 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

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

23. 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

24. 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)

25. 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

26. 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”

27. 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

28. 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)

29. Undamped thermometers These are thermometers with fast response times - but this is not always what we need

30. 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

31. 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

32. 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 10 Temperature (C) 8 6 4 2 0 0 5 10 15 20 25 30 35 Time (minutes)  = Sausage;  = Leg joint at 5mm depth  = Standard undamped thermometer

33. 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 12 Temperature (C) 10 8 6 4 2 0 0 5 10 15 20 25 30 35 Time (minutes)  = Sausage;  = Leg joint at 5mm depth  = Damped FoodsaFe thermometer

34. 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

35. Damped thermometers Half-full cool box responses with precooling and 2.2 kg of gel packs • These probes were used to demonstrate the chilled food hold times of passive cool boxes for home grocery delivery- Igloo maxcold 7 6 5 4 Temperature gain (C) 3 2 1 0 0 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)

36. 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

37. Damped thermometers Damped data loggers Numerous available Most use PTFE block to slow response Hanna HI762 Digitron ThermaTag

38. 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

39. 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