Temperature measurement in the cold chain

Chris KennedyNutrifreeze Ltd

Monitoring temperature duringdistribution of chilled & frozen foods

Contents

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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

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time (minutes)

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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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= 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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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)

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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