Temperature Measurement in The Cold Chain.

Chris Kennedy- NutriFreeze Ltd.

Monitoring of temperatures during the distribution of chilled and frozen foods.

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

A simple cold chain might have the following stages

Manufacture & Freezing/ 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

C

B

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.

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

How to measure temperature

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

Bi-metallic strip

How to measure temperature

-40 to 500oC Accuracy around 1% of scale at time of

manufacture.

Bi-metallic 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.

Thermocouples

How to measure temperature

Ref junction is in an isothermal block with Tref measured by a semiconductor.

T and K type thermocouples generate around 40V/oC so amplification required.

Calibration is required for the measuring device

Thermocouples

How to measure temperature

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

Thermocouples

How to measure temperature

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

Thermistors

How to measure temperature-RTD’s

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 therm

ometry

Platinum resistance thermometry

Target temperaturesChilled 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

Target temperatures

Chilled Foods- Legal requirements 8oC

But…GuidelinesUK 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

ManufacturersNearly all advise (and calculate shelf life) below 5oC

Target temperatures

Frozen FoodsLegal requirementsQuick Frozen Foodstuffs must be stored and distributed below –18oC. A

reasonable time at -15oC is allowed during local distributionCommission 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.

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?) 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 +20C still air.

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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's 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.

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

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 Again air return and air

off temperatures are recorded

Issues are:

– Location– Filling– Heating/lighting– Dehydration– Packaging– Customers

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.

Undamped thermometersThese 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– 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

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 glassComparison of thermometers with actual foods

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Sausage Leg Joint (5mm) Standard Thermometer

Damped thermometers Seal in a food gel 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 (5mm) Foodsafe Thermometer

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

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 2.2kg 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

Damped thermometersDamped data loggersNumerous available.Most use PTFE block to slow response

Hanna HI762

Digitron ThermaTag

                                                              

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

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