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

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temperature measurement in the cold chain

Temperature measurement in the cold chain

Monitoring temperature during

distribution of chilled & frozen foods

Chris Kennedy

Nutrifreeze Ltd

contents
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
a simple cold chain might have the following stages

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

back to square 1
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 15
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 16
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
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
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 temperature9
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 temperature10
How to measure temperature

Bimetallic strip

  • -40 oC to 500 oC
  • Accuracy at time of manufacture about 1% of scale
how to measure temperature11
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 temperature12
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 temperature13
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

how to measure temperature14
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
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
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
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 foods18
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 temperatures19
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
cold stores
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 stores21
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
slide22

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)

bulk transport
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

bulk transport24
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)

local transport
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
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 display27
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 display28
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
Undamped thermometers

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

what temperatures to measure
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
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 thermometers32
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

damped thermometers33
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

damped thermometers34
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 thermometers35
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)

damped thermometers36
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

damped thermometers37
Damped thermometers

Damped data loggers

Numerous available

Most use PTFE block to slow response

Hanna HI762

Digitron ThermaTag

damped thermometers38
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
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