Temperature measurement
in the cold chain
Monitoring temperature during
distribution of chilled & frozen foods
Chris Kennedy
Nutrifreeze Ltd
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
Holding store
Transport
Transport
Retail outlet
Distribution centre
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
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
B
A
C
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
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 0.385 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
-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
Simulation of temperature abuse on a case of meat products
Case of product removed to +20 oC still air
14
4
Temperature (C) (C)
Temperature
12
10
8
6
4
2
0
0
10
20
30
40
50
60
70
time (minutes)
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
Bulk Transport

Most vehicles have
evaporator at front of load

Thermistor or gauge readout 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.
(a) Normal air temperature record, and
(b) Poorly loaded vehicle air temperature record of chilled foods vehicle
( By permission of Cold Chain Instruments)
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.
Local Transport
Air temperature record of a small delivery vehicle

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

Air return and air-off
temperatures are recorded

Issues are:
–
–
–
–
–
–
Air temperature monitoring in retail display cabinets:
(a) multi-deck cabinet, (b) serve-over cabinet
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
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
Temperature (C)
16
14
12
10
8
6
4
2
0
0
5
10
15
20
25
30
Time (minutes)
 = Sausage;  = Leg joint at 5mm depth

= Standard undamped thermometer
35
Damped thermometers
Seal in a food gel
Now the response
corresponds to a food of
similar dimensions
More info on the FoodsaFe



Temperature (C)
Comparison of thermometers with actual foods
16
14
12
10
8
6
4
2
0
0
5
10
15
20
25
30
Time (minutes)
 = Sausage;  = Leg joint at 5mm depth

= Damped FoodsaFe thermometer
35
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
7
6
Temperature gain (C)

5
4
3
2
1
0
0
50
100
150
200
250
300
350
-1
Time (minutes)
9l wall
precool and 2.2kg gel 9L centre
9l centre
Linear (9l centre)
precool and 2.2 kg gel 9L wall
Linear (precool and 2.2kg gel 9L centre)
400
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
Pressure
formedPS
PS cool
cool box
withwith
eutectic
chill plate
PED PED
pressure
formed
boxresponses
responses
eutectic
chill plate
7
6
Temperature Gain (C)

5
4
3
2
1
0
0
50
100
150
200
250
-1
Time (minutes)
wall
centre
300
350
400
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