Temperature Measurement in The Cold Chain.

1. Temperature Measurement in The Cold Chain. Monitoring of temperatures during the distribution of chilled and frozen foods. Chris Kennedy- NutriFreeze Ltd.

2. Contents • Throughout this presentation you’ll find links to other useful sites: ...like this one. • 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/ 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 • So in general when we design a thermometer we will want it to come quickly into thermal equilibrium with the product whose temperature we are measuring. We want to do this without changing that temperature by making the measurement. Hence a good thermometer will generally have: • Negligible heat capacity • Fast response (but see later) • At least one clearly identifiable, unambiguous thermal property.

8. How to measure temperature Liquid in glass • 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 Bi-metallic strip • Two dissimilar metals tightly bonded (Fe/Cu) • Different coefficients of expansion cause strip to curl. (heating/ cooling) • More Info on bimetallic strips and other thermometers

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

11. How to measure temperature Thermocouples • 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 Thermocouples • Ref junction is in an isothermal block with Tref measured by a semiconductor. • T and K type thermocouples generate around 40mV/oC so amplification required. • Calibration is required for the measuring device

13. Typically K or t type used in the food industry K-type Chromel–Alumel T-type Cu-Constantan Precision grade = +/- 1.0% or 1oC. Most probes require recalibration every 6 months to ensure this More info on thermocouples How to measure temperature Thermocouples

14. How to measure temperature Thermistors • A thermisitor is a semi conductor 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 aging. • More info on thermistors

15. How to measure temperature-RTD’s Platinum resistance thermometry • 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 Chilled Foods- Legal requirements The Food Safety (Temperature Control) Regulations 1995 “Chilled food must be kept below 8°C” But this is not cold enough to stop the growth of all pathogens

17. Target temperatures Chilled Foods- Legal requirements 8oC But… 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 and 5oC Manufacturers Nearly all advise (and calculate shelf life) below 5oC

18. Target temperatures Frozen Foods Legal requirements Quick Frozen Foodstuffs must be stored and distributed below –18oC. A reasonable time at -15oC 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 and –20oC • Temperature fluctuations can be as important as absolute temperature for quality issues.

19. 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

20. 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

21. 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. 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's website has more info on all aspects of the cold chain

23. Bulk Transport • Loading is important for good air circulation and rapid recovery from door openings.

24. 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

25. 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”

26. Retail Display • Again air return and air off temperatures are recorded • Issues are: • Location • Filling • Heating/lighting • Dehydration • Packaging • Customers

27. 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.

28. Undamped thermometersThese are thermometers with fast response times but this is not always what we need.

29. What temperatures to measure? • Throughout the cold chain it is common to • Set the air off temperature • Control using the return air temperature • Monitor the hot spot 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

30. 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

31. Damped thermometers • An example. • FoodsaFe for use in catering and at home • Individually calibrated liquid in glass

32. Damped thermometers • Seal in a food gel • Response corresponds to a food of similar dimensions. More info on the FoodsaFe

33. Damped thermometers • A similar philosophy can be used thermocouple devices and data logging. • Here a K-Type thermocouple is housed in the gel. • The gel and dimensions can be designed to match specific food products

34. Damped thermometers • These probes were used to demonstrate the chilled food hold times of passive cool boxes for home-grocery delivery- Igloo maxcold

35. 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

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

37. Damped thermometers • In bulk distribution however mean temperature is not necessarily the most useful • The critical temperature is often the surface temperature • Response of this temperature will depend on • Product composition • Case size • Packing material • The food simulant must be designed to closely match the response of the package in this case

38. 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