Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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‘coolant’ remains at a constant 0



C until all the ice

has turned into water. The process is labor-intensive,

although automatic filling systems have been de-

veloped. Cabbage and root crops are also cooled

with crushed ice.

Vacuum Cooling

0018 Solid products with a large surface area-to-volume

ratio and an ability to release internal water are

amenable to vacuum cooling. The products are

placed in a vacuum chamber, and the resultant evap-

orative cooling removes heat from the food. In gen-

eral terms, a 5



C reduction in product temperature is

achieved for every 1% of water that is evaporated.

Prewetting is commonly applied to facilitate cooling

without any loss of weight.

0019 Vacuum cooling is rapid and economical to operate

because of low labor costs, but the capital cost of

the large vacuum vessels is high. Large amounts of

lettuce, celery, cauliflower, green peas, and sweetcorn

are vacuum-cooled.

Cryogenic Cooling

0020 Avoiding surface freezing of the product is the main

problem in using liquid nitrogen or solid carbon di-

oxide for chilling. Continuous chilling systems using

liquid nitrogen either immerse the product in the

liquid, spray the nitrogen on to the surface, or vapor-

ize the nitrogen in a forced draught and pass it over

the surface of the foodstuff.

0021 Direct spraying of liquid nitrogen on to a food

product, whilst it is conveyed through an insulated

tunnel, is the most common method. Surface freezing

is still a problem, but an extra refrigeration effect is

obtained by precooling the food with the cold gas

produced by the vaporization.

Chilling Methods for Liquid Foodstuffs

0022 The majority of liquid foodstuffs require cooling after

a heat-processing operation such as cooking, pasteur-

ization, or sterilization. Milk is cooled at the point of

collection to maintain its quality, unpasteurized fruit

juices are cooled immediately after production, whilst

fermented beverages are often cooled during primary

and secondary fermentation, and before storage. (See

Fermented Milks: Products from Northern Europe;

Milk: Processing of Liquid Milk; Pasteurization:

Principles.)

0023 In simple or small-scale processes, containers of hot

liquids are allowed to cool in ambient air or placed

inside chill rooms. Other cooling systems for liquid

foods rely on direct expansion refrigeration, the use

of a secondary refrigerant that is passed through or

around the foodstuff, vacuum cooling, or a combin-

ation of liquid and vacuum.

0024Batch coolers for liquid foods range in capacity

from 100 to 10 000 l with the foodstuff contained in

a stainless steel vessel. The cooling medium may cir-

culate through the jacket of the vessel, through a coil

immersed in the liquid, or both. Most vessels are

provided with agitators to improve convective heat

transfer and stop temperature stratification. One

common method used to decrease cooling times in a

closed vessel is to apply a vacuum to produce

evaporative cooling.

0025Continuous cooling used to be achieved in falling

film or surface coolers in which the hot liquid was

pumped over the top of a horizontal bank of refriger-

ated coils and flowed down over the cooled surfaces.

These systems have now been replaced by totally

enclosed coolers. Double-pipe coolers have also

been employed in specialized applications but have a

limited heat-transfer surface. Multiplate coolers are

extensively used for liquid foods. They have the

highest available heat-transfer surface, lowest mater-

ial requirements, and maximum efficiency, and are

very flexible in operation and easy to clean. In certain

applications, such as beer and wine cooling, multi-

tube coolers that have a much higher resistance to

pressure and can use primary refrigerants have

advantages over multiplate coolers.

0026Scraped surface heat exchangers can have advan-

tages in the cooling of very viscous liquid foods and

where surface fouling is a potential problem. Cur-

rently, there is interest in the use of pumpable ice in

the form of an ice ‘pig’ to clean heat exchangers and

pipeline systems for liquid foods.

Chilled Storage

0027Theoretically, there are clear differences between the

environmental conditions required for chilling, which

is a heat removal/temperature reduction process, and

storage where the aim is to maintain temperature.

However, in many air-based systems, chilling and

storage take place in the same chamber, and even

where two separate facilities are used, in many cases,

not all the required heat is removed in the chilling

phase. With some fruits and vegetables, the rate of

respiration during storage is sufficient to require heat

removal if product quality is to be maintained. (See

Storage Stability: Mechanisms of Degradation;

Parameters Affecting Storage Stability.)

Bulk Storage Rooms

0028Most unwrapped meat and poultry and all types of

wrapped foods are stored in large air-circulated

1166 CHILLED STORAGE/Principles

rooms. To minimize weight loss, air movement

around the unwrapped product should be the min-

imum required to maintain a constant temperature.

With wrapped products, low air velocities are also

desirable to minimize energy consumption. However,

many storage rooms are designed and constructed

with little regard to air distribution and the mainten-

ance of low localized velocities over products. Hori-

zontal throw refrigeration coils are often mounted in

the free space above the racks or rails of product, and

no attempt is made to distribute the air around the

products. Using air socks, it is claimed that an even

air distribution can be maintained with localized

velocities not exceeding 0.2 m s

1

Controlled-atmosphere Storage Rooms

0029 Controlled-atmosphere storage rooms were de-

veloped for specialized fruit stores, especially those

for apples. Interest is growing in the application of this

technique to other commodities including meat and

fish. In addition to the normal temperature control

plant, these stores also include special gas-tight seals

to maintain an atmosphere, which is normally lower

in oxygen and higher in nitrogen and carbon dioxide

than air. An additional plant is required to control the

concentration, generate nitrogen and consume

oxygen. (See Controlled-atmosphere Storage: Appli-

cations for Bulk Storage of Foodstuffs.)

0030 There is growing interest in the use of controlled

atmosphere retail packs to extend the chilled storage

and display life of meat and meat products. Since the

packs insulate the products, efficient precooling before

packaging is important.

Transportation

0031 Developments in temperature-controlled transporta-

tion systems for products have been one of the main

factors leading to the rapid expansion of the chilled

food market. The sea transportation of chilled meat

from Australasia to European and other distant

markets, and road transportation of chilled products

throughout Europe and the Middle East, are now

common practice. Air freighting was initially used

for high-value perishable products such as straw-

berries, asparagus, and live lobsters. It is now increas-

ingly used to provide consumers with a year-round

supply of locally out-of-season products such as green

beans, mangetout, and green on carrots.

Overland Transport

0032 Overland transportation systems range from 12-m

refrigerated containers for long-distance road or

rail movement of bulk chilled products to small

uninsulated vans supplying food to local retail

outlets. The majority of current road transport

vehicles for chilled foods are refrigerated using either

mechanical, eutectic plates or liquid nitrogen cooling

systems.

0033Many types of independent engine and/or electric

motor-driven mechanical refrigeration units are avail-

able for lorries or trailers. One of the most common is

a self-contained ‘plug’ unit mounted in an opening

provided in the front wall of the vehicle. The condens-

ing section is on the outside and the evaporator on the

inside of the unit, separated by an insulated section,

which fits into the gap in the wall. Units have one or

two compressors, depending upon their capacity,

which can be belt-driven from the vehicle but are

usually driven direct from an auxiliary engine. This

engine may use petrol from the vehicle’s supply or an

independent tank, or liquid petroleum gas. Many are

equipped with an additional electric motor for standby

use or for quiet running, e.g., when parked or on a

ferry. Irrespective of the type of refrigeration equip-

ment used, the product will not be maintained at its

desired temperature during transportation unless it is

surrounded by air or surfaces at or below that tem-

perature. This is usually achieved by a system that

circulates moving air, either forced or by gravity,

around the load. Inadequate air distribution is prob-

ably the principal cause of product deterioration and

loss of shelf-life during transport. Conventional forced

air units usually discharge air over the stacked or

suspended products either directly from the evapor-

ator or through ducts towards the rear cargo doors.

Because air takes the path of least resistance, it circu-

lates through the channels, which have the largest

cross-sectional area. These tend to be around, rather

than through, the product. If products have been

cooled to the correct temperature before loading and

do not generate heat, they only have to be isolated

from external heat ingress. Many trucks are now

being constructed with an inner skin that forms a

return air duct along the side walls and floor, with

the refrigerated air being supplied via a ceiling duct.

Sea Transport

0034Recent developments in temperature control, pack-

aging, and controlled atmospheres have increased

substantially the range of foods that can be trans-

ported around the world in a chilled condition.

Control of the oxygen and carbon dioxide levels in

shipboard containers has allowed fruits and vege-

tables such as apples, pears, avocado pears, melons,

mangoes, nectarines, blueberries, and asparagus to be

shipped from Australia and New Zealand to markets

in the USA, Europe, Middle East, and Japan. With

conventional vacuum packing, it is difficult to achieve

a shelf-life in excess of 12 weeks with beef and 8

CHILLED STORAGE/Principles 1167

weeks for lamb. However, a study has found that a

shelf-life of up to 23 weeks at 2



C can be achieved

in cuts of lamb that are individually packed in evacu-

ated bags of linear polyethylene, and then placed in a

foil laminate bag that is gas-flushed and filled with

a volume of CO

approximately equal to that of

the meat. Similar storage lives are currently being

achieved with beef primals transported from Austra-

lia and South Africa to the EU. Assuming good stand-

ards of preparation and prompt cooling, the times

given in Table 1 could be used as approximate guide-

lines for long-distance meat shipment.

0035 These times rely on the meat being at or below the

storage temperature before loading. The 2– 4-week

advantage of transporting meat at  1.5



C rather

than 0



C will be lost if the meat is loaded at a tem-

perature above 0



C. Cooling in the center of a load of

meat is very slow, and the meat will be well into its

journey before the desired temperature is achieved.

0036 International Standard Organization (ISO) con-

tainers for food transport are 6 or 12 m long, hold

up to 26 tonnes of product and can be ‘insulated’ or

‘refrigerated.’ The refrigerated containers incorporate

insulation and have refrigeration units built into their

structure. Insulated containers either utilize plug-type

refrigeration units or may be connected directly to an

air-handling system in a ship’s hold or at the docks.

Refrigerated containers are easier to transport over

land than the insulated types, but have to be carried

on deck when shipped because of problems in operat-

ing the refrigeration units within closed holds. On

board ship, they are therefore subjected to much

higher ambient temperatures and consequently larger

heat gains, which make it far more difficult to control

product temperatures. Close temperature control is

most easily achieved in containers that are placed in

insulated holds and connected to the ship’s refriger-

ation system.

Air Transport

0037 Although air freighting of foods offers a rapid method

of serving distant markets, there are many problems

because the product is unprotected by refrigeration

for much of its journey. Up to 80% of the total journey

time is spent on the tarmac or transport to and from

the airport. Perishable cargo is usually carried in

standard containers, sometimes with an insulating

lining and/or dry ice, but is often unprotected on

aircraft pallets.

0038Studies in Australia have led to the following rec-

ommendations for air transport of chilled foods:

0039Insulated containers should always be used to

reduce heat gain.

0040Product should always be precooled and held at

the required temperature until loading.

0041With products that deteriorate after any surface

freezing, dry ice should not be used.

0042Containers should be filled to capacity.

0043A thermograph should accompany each consign-

ment.

Retail Display

0044The retail display of chilled food is probably still the

weakest link in the cold chain. In the UK, the Food

Hygiene (Amendment) Regulations 1990 required

food retailers to maintain the temperature of certain

chilled foods below 5



C and others 8



C during stor-

age, transport, and display. This legislation, which

contained severe financial penalties for temperature

infringements, produced a marked improvement in

the design, construction, and operation of retail dis-

play cabinets. However, the legislation has since been

relaxed, and there is currently less emphasis on the

maintenance of food temperatures.

0045The required retail display life and consequent en-

vironmental conditions for wrapped chilled products

differ from those for unwrapped products. The de-

sired display life for wrapped meat, fish, vegetables,

and processed foods ranges from a few days to many

weeks and is primarily limited by microbiological

considerations. Retailers of unwrapped fish, meat,

and delicatessen products normally require a display

life of one working day.

0046Display cabinets for delicatessen products are

available with gravity or forced convection coils,

and the glass fronts may be nearly vertical or angled

up to 20



. In the gravity cabinet, cooled air from the

raised rear-mounted evaporator coil descends into the

display well by natural convection, and the warm air

rises back to the evaporator. In the forced-circulation

cabinets, air is drawn through an evaporator coil by a

fan and then ducted into the rear of the display. It then

returns to the coil after passing directly over the prod-

ucts, or forming an air curtain, via a slot in the front

of the cabinet and a duct under the display shelf.

0047Although the same cabinets can be used for

wrapped foods, most are sold from multideck

cabinets with single or twin air curtain systems. It is

important that the front edges of the cabinet shelves

do not project through the air curtain since the

tbl0001 Table 1 Recommended transportation life of meat

Vacuumpack 0



C Vacuumpack1. 5



CCO

1. 5



Pork 6 weeks 8 weeks

Lamb 7 weeks 10 weeks >12 weeks

Beef 10 weeks 14 weeks

1168 CHILLED STORAGE/Principles

refrigerated air will then be diverted out of the cab-

inet. However, if narrow shelves are used, the curtain

may collapse, and ambient air can be drawn into the

display well. External factors such as the store ambi-

ent temperature, the siting of the cabinet, and poor

pretreatment and placement of products substantially

affect cabinet performance. Computational fluid dy-

namics is increasingly being used as a valuable mod-

eling tool to improve air distribution and temperature

control in retail display cabinets and retail stores

themselves.

See also: Controlled-atmosphere Storage: Applications

for Bulk Storage of Foodstuffs; Fermented Milks:

Products from Northern Europe; Fish: Fish as Food; Heat

Transfer Methods; Milk: Processing of Liquid Milk;

Pasteurization: Principles; Storage Stability:

Mechanisms of Degradation; Parameters Affecting

Storage Stability

Further Reading

Anonymous (1994) ASHRAE Handbook. Refrigeration.

New York: American Society of Heating, Refrigeration

and Air Conditioning Engineers.

Anonymous (2000) Recommendations for Chilled Storage

of Perishable Produce. Paris: International Institute of

Refrigeration.

Gormley TR (1990) Chilled Foods – The State of the Art.

Barking, UK: Elsevier Applied Science.

James SJ (1999) Food Refrigeration 1990 to 2020 A.D.

Institute of Refrigeration Centenary Publication.

James C and James SJ (1996) Past, Present and Future

Methods of Cooling Foods. Publication of MAFF Fel-

lowship, University of Bristol.

James SJ and James C (2002) Meat Refrigeration. Cam-

bridge, UK: Woodhead Publishing.

Jowitt R, Escher F, Hallstrom B et al. (1983) Physical Prop-

erties of Foods. Barking, UK: Elsevier Applied Science.

Perry RH and Chilton CH (1973) Chemical Engineers’

Handbook, 5th edn. Tokyo: McGraw-Hill.

Stringer M and Dennis C (2000) Chilled Foods: a Compre-

hensive Guide, 2nd edn. Cambridge, UK: Woodhead

Publishing.

Attainment of Chilled

Conditions

S James, University of Bristol, Langford,

North Somerset, UK

Background

0001 The purpose of the chill chain is first to reduce the

temperature of a food to below a set temperature and

henceforth maintain it at or below that temperature.

Since the growth of both pathogenic and food spoil-

age organisms is very temperature-dependent, the

rapid attainment and maintenance of low tempera-

tures are important to food safety and shelf-life. In

this article, the cooling rates and temperatures that

can be attained and maintained are discussed together

with the limiting physical factors.

0002The speed at which a food can be cooled can be

limited by either the rate of heat removal from its

surface or internal conduction. Heat removal from

the surface is a direct function of the surface heat

transfer coefficient (h). Typical values of h range

from 5 W m

2



1

for slow-moving air to 500 W

2



1

for agitated water. Table 1 shows that,

at low values of h, a 10-fold increase, 5 to 50 W m

2



1

, makes a substantial, 3.2- to 4.2-fold reduction

in cooling time. A further 10-fold increase from 50 to

500 W m

2



1

decreases the cooling time three-

fold for a 2-cm-thick slab but only results in a 60%

reduction at a thickness of 8 cm. This indicates that

in the thicker material internal heat conduction is

becoming rate-controlling.

Primary Chilling

0003Primary chilling after harvest or slaughter is usually

applied to large individual items or small items cooled

in bulk. Air cooling is often used in primary chilling

because internal conduction controls the rate the

products can be cooled. Data from the cooling of

beef sides (Figure 1) illustrate the relative influence

of air temperature and air velocity. The data have

been presented as a plot of the logarithm of tempera-

ture against time and, provided air temperatures are

chosen to avoid substantial surface freezing, it is quite

feasible to determine the cooling time for any other

air temperature. The fractional unaccomplished tem-

perature on the y-axis can be replaced by the meat

temperature calculated from y ¼ (tt

)/(t

t

), where

t is the meat temperature, t

the initial meat tempera-

ture, and t

the air temperature.

0004Cooling in air at a constant 4



C, compared with



C, at 3 m s

1

will increase the time to reach 7



Cin

tbl0001Table 1 Predicted cooling time (h) from 40



Cto2



C at the

center

Coolingmethod h (Wm

2



1

) Meat thickness (cm)

2.0 4.0 8.0

Air (still) 5 5.0 11.0 24.0

Air (5 m s

1

) 50 1.2 2.8 7.4

Plate 360 0.7 1.8 5.5

Immersion 500 0.4 1.2 4.4

CHILLED STORAGE/Attainment of Chilled Conditions 1169

the deep leg of a 100-kg side from 20.3 to 27.7 h

(36% increase). At 0.5 m s

1

the same rise in air

temperature for a 220-kg side will increase the

chilling time from 45.9 to 68.3 h (49% increase).

0005 The shelf-life of vegetables is largely governed by

the amount of water that is lost from the vegetables.

Leafy vegetables have a relatively short shelf-life

because of the potential for high weight of evapor-

ation. Their shelf-life can be greatly improved if the

field heat is removed immediately after harvesting

and the temperature of vegetables subsequently main-

tained at their optimum. For most winter vegetables,

which have their origins in temperate climates, this

temperature is close to 0



C. Some tropical fruits and

vegetables are damaged if the temperature is reduced

too low and they should, therefore, be chilled and

maintained at warmer temperatures.

0006 Vegetables are cooled in many ways. One conveni-

ent bulk system is to pack the vegetables into large

crates which are then stacked in front of a chiller and

air is forced to flow through the crates back to the

chiller in order to effect a fast rate of cooling. Weight

loss will be rapid during the first parts of the process

but when the temperature of the surface of the vege-

tables approaches that of the air then the relative

humidity of the air will become important. Artificial

ways of increasing the humidity of the air are

common for vegetable cooling. The most normal

system utilizes an ice bank from which chilled water

at 0



C is drawn. This is passed over packing to

extend the surface area of water through which air

from the chill room is drawn. The resulting air is at

very high humidity and with a temperature of close to



C. By controlling the temperature of the water and

bypassing the air, the bulk humidity and temperature

of the air can be controlled to lower relative humid-

ities and warmer temperatures than these.

0007Vegetables can be readily cooled with running

water. This reduces the evaporation of water to zero

and even small weight gains can be achieved. The

water can also be used to wash the vegetables. Subse-

quent storage should be in very high humidities to

prevent further deterioration.

0008Leafy vegetables can be cooled in a vacuum cooling

process. In this process, the vegetables are placed in

a vacuum chamber and a vacuum is drawn. The

Chiller

air

temperature

⬚c

38.5

Side weights kg

100

140

180

220

Approximate average for British abattoirs

Air speed (m s

−1

)

0.5

0 8 16 24 32 40 48

Time (h) postmortem

56 64 72 80 88 96 104

0.02

0.01

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Deep-leg temperature (⬚C)

fig0001 Figure 1 Relationship between deep-leg temperature and cooling time for beef sides. y is the fractional unaccomplished tempera-

ture which can be used to calculate cooling under different chiller temperatures.

1170 CHILLED STORAGE/Attainment of Chilled Conditions

evaporation of water from the plant is used to pro-

mote rapid cooling. The small loss, 2% in the case of

lettuces, is considered acceptable, given that the sub-

sequently chilled lettuces have a longer shelf-life than

those chilled by conventional means.

Secondary Chilling

0009 Cooking rarely eliminates all food-poisoning organ-

isms and a number survive as spores which will

germinate and grow if cooling rates are slow. Rapid

reduction in surface temperature retards microbial

growth and consequently extends shelf-life. This is

especially important when chilling cooked products

that will eventually be consumed cold or in a warm

reheated state. Rapid rates of cooling are required

without surface freezing. The problem is complicated

in two-compartment ready-meal consumer packs.

These typically contain rice or pasta in one compart-

ment, and a meat- or fish-based product in the

second. The thermal properties of the two items are

often very different and they may be filled to different

depths. Air at 10



Cand5ms

1

produces a cooling

time of 34 min, but substantial quantities of the prod-

uct are frozen. At an air velocity of 0.5 m s

1

the

cooling time is doubled but only a small area of the

rice is frozen. With higher air temperatures, the extent

of freezing is reduced. A cooling time of approxi-

mately 0.75 h is achieved at 5



C, 5 m s

1

with ac-

ceptable freezing.

0010 The importance of achieving a minimum required

air velocity around small products is clearly demon-

strated by data obtained from cooling pork pies

(Figure 2). To guarantee that all the crust remained

above 2



C on the unwrapped 400-g (70-mm high,

95-mm diameter) pies, an air temperature of 1.5

+ 0.5



C was used. At this temperature, a small

increase in air velocity from 0.5 to 1.0 m s

1

reduced

the cooling time by 85 min (almost 30%). Even at

very high velocities (> 6.0 m s

1

) appreciable reduc-

tions in cooling time were still being achieved.

0011Rapid rates of temperature reduction can be

achieved in trays of cooked product such as mince,

baby foods, and poultry portions when cooled under

vacuum.

0012Slow cooling of large hams and cooked meat has

been implicated in a number of food-poisoning out-

breaks. It is now recommended that large uncured

joints are cooled in < 8 h and cured joints in < 10 h.

In laboratory studies three different cooling methods

– forced air at 0



C, 1.2 m s

1

, immersion in water at



C, and cooling under vacuum – for large (6.8–

7.3 kg) hams, and other meats were compared

(Table 2). Although cooling times are much faster

under vacuum, weight losses (typically 7.5–9.5%)

are much higher than those in air (typically 2–3%).

Storage, Transport, and Display

0013In general, after initial chilling, as a chilled product

moves along the chill chain it becomes harder to

control and maintain its temperature. Temperatures

of bulk packs of chilled product in large storerooms

are far less sensitive to small heat inputs than single

consumer packs in transport or open display cases.

If primary and secondary cooling operations are

400-g pork pies

10.0 m s

−

6.0 m s

−1

1.0 m s

−1

0.5 m s

−1

30 60 90 120 150 180 210 240 270 300

Time

(

min

)

Center temperature (⬚C)

fig0002 Figure 2 Temperature at slowest cooling point in 400-g pork pies at 1.5



C air temperature and velocities of 10, 6, 1, and 0.5 m s

1

CHILLED STORAGE/Attainment of Chilled Conditions 1171

efficiently carried out, then the food will be reduced

below its required temperature before it is placed in

store. In this situation the store’s refrigeration system

is only required to extract extraneous heat that enters

through the walls, door openings, etc.

0014 Efficient cooling is also important before transpor-

tation. A study of different transportation methods

found that the deep temperature in beef sides and

quarters at the time of their loading into transport

vehicles in three USA plants ranged from 6 to 18



(Figure 3). In rail wagons the surface temperature

declined during the first 24 h and was subsequently

maintained at a temperature of 0 + 1



C. In the road

vehicles the surface temperature fell slowly during the

whole journey and had not attained a steady min-

imum value when unloaded. On average the deep

temperature of sides in rail wagons reached 1



after 72 h. Temperatures in quarters in road vehicles

were still above 2



C after 120 h.

0015 Even when temperature-controlled dispatch bays

are used, there is a slight heat pick-up during loading.

In bulk transportation, the resulting temperature rise

is small and the refrigeration system of the vehicle

rapidly returns the product to the required tempera-

ture. Larger problems exist in local multidrop distri-

bution to individual stores. There are substantial

difficulties in maintaining the temperature of chilled

foods transported in small refrigerated vehicles that

conduct multidrop deliveries to retail stores and

caterers. The vehicles have to carry a wide range of

products and operate under diverse ambient condi-

tions. During any one delivery run, the chilled prod-

uct can be subjected to as many as 50 door openings,

where there is heat ingress directly from outside and

from personnel entering to select and remove prod-

uct. The design of the refrigeration system has to

allow for extensive differences in load distribution,

dependent on different delivery rounds, days of the

week, and the removal of product during a delivery

run. A refrigeration system’s ability to respond to

sudden demands for increased refrigeration is often

restricted by the power available from the vehicle. All

these problems combine to produce a complex inter-

active system. Computer programs such as CoolVan

are now available to aid the design and operation of

local delivery systems.

0016Surveys carried out in the UK, Denmark, and

Sweden (Figure 4) have revealed a very wide range

6.5

7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5 16.5 17.5 18.5

Deep-leg temperature (⬚C)

No. of occurrences

fig0003 Figure 3 Number of occurrences of deep-leg temperatures in different ranges when loaded into railway wagons or road trailers.

Open columns, railway wagons; cross-hatched columns, road trailers; filled columns, road and rail.

tbl0002 Table 2 Cooling time to 10



C for different cooked meats

Typ e o f m e a t Weigh t

(kg)

Initial temperature

(



Cooling time (min) to10



Cin

Air Immersion Vacuum

Beef

M. s e mitendin o s u s 0.9 75 150 100 52

Forequarter 2.7 75 390 282 43

Silverside 2.7 75 338 240 61

Boneless turkey 6.4 85 526 411 36

Boned out ham 7.1 80 761 456 57

1172 CHILLED STORAGE/Attainment of Chilled Conditions

of temperatures within foods on retail display. Legis-

lation on maximum temperatures for the display of

chilled foods is in place in many European countries.

Domestic

0017 Although legislation such as that introduced in

the UK should insure that food producers and

retailers maintain acceptable product temperatures

during the distribution chain, they lose control when

the product leaves the retail store. After a chilled

product is removed from a display cabinet it spends

a period outside a refrigerated environment whilst

it is carried around the store and then transported

home.

0018Investigations have compared unprotected trans-

portation in a car boot with that in an insulated box.

Initial product temperatures measured when the food

reached the car ranged from 4 to over 20



C. Some

product temperatures on samples placed in the boot

rose to approaching 40



C(Table 3) during the 1-h

car journey whilst most of the samples placed in the

insulated box cooled during the car journey, except

for a few at the top of the box which remained at

their initial temperature. Small products, i.e., prawns,

showed the highest temperature changes during

transport. Thicker products like cooked chicken

were less influenced. After being placed in the domes-

tic refrigerator it required approximately 5 h before

the temperature was reduced below 7



tbl0003 Table 3 Maximum temperature (



C) measured and increase in bacterial numbers (generations) during 1 h in car followed by 5 h in a

domestic refrigerator

Product Conditions Maximum temperature Pseudomonas Clostridium

Ambient car 25 1.5 0.4

Cool box in car 13 <0.4 0

Raw chicken Ambient car 24 1.6 0.2

Cool box in car 4 0 0

Cooked chicken Ambient car 28 1.8 0.7

Cool box in car 12 0 0

Prawns Ambient car 37 1.3 1.6

Cool box in car 14 0 0

Brie cheese Ambient car 28 2.2 0.8

Cool box in car 12 0 <0.1

−8 −6 −4 −2

1 3 5 7 9 11 13 15

Temperature of food (⬚C)

fig0004 Figure 4 Temperature of chilled foods in retail display cabinets in the UK (open columns), Denmark (cross-hatched columns), and

Sweden (filled columns). Data from

Bøgh-Sørensen L (1980) Product temperatures in chilled cabinets. In: 26th European Meeting of

Meat Research Workers, Colorado, paper 22,

Olsson P (1990) Chill cabinet surveys. In: Processing and Quality of Foods. Volume 3.

Chilled Foods: The Revolution in Freshness, pp. 3.279–3.288. London: Elsevier Applied Science and

Rose SA (1986) Microbiological

and temperature observations on pre-packaged ready-to-eat meats retailed from chilled display cabinets. In: Recent advances and

developments in the refrigeration of meat by chilling, pp. 463–470. Paris: International Institute of Refrigeration.

CHILLED STORAGE/Attainment of Chilled Conditions 1173

−5 to −4.1 ⬚C

−4 to −3.1 ⬚C

−3 to −2.1 ⬚C

−2 to −1.1 ⬚C

−1 to −0.1 ⬚C

0−0.9 ⬚C

1−1.9 ⬚C

2−2.9 ⬚C

3−3.9 ⬚C

4−4.9 ⬚C

5−5.9 ⬚C

6−6.9 ⬚C

7−7.9 ⬚C

8− 8.9 ⬚C

9−9.9 ⬚C

10−10.9 ⬚C

11−11.9 ⬚C

12−12.9 ⬚C

13−13.9 ⬚C

14−14.9 ⬚C

0 5 10 15 20 25

(a)

0 5 10 15 20 25 30 35 40

(b)

−5 to −2.9 ⬚C

−3 to −0.9 ⬚C

−1 to −0.9 ⬚C

1−2.9 ⬚C

3−4.9 ⬚C

5−6.9 ⬚C

7−8.9 ⬚C

9−10.9 ⬚C

11−12.9 ⬚C

13−14.9 ⬚C

15−16.9 ⬚C

fig0005 Figure 5 (a) Food temperatures and (b) air temperatures measured in domestic refrigerators in 252 households in the UK.

(a) percentage of food with temperature in stated range; (b) percentage of refrigerators with overall average temperatures within

stated range.

1174 CHILLED STORAGE/Attainment of Chilled Conditions

0019 Predictions made using a mathematical model that

calculated bacterial growth from temperature/time

relationships indicated that increases of up to 1.8

generations in bacterial numbers (Table 3) can occur

during this transport and domestic cooling phase. The

model assumes that bacteria require a time to accli-

matize to a change in temperature (the lag phase) and

that no acclimatization had occurred during display.

If this rather optimistic assumption was not made,

then up to 4.2 generations of Pseudomonas and

growth of both Salmonella and Listeria were pre-

dicted. Very small increases in bacterial numbers

(< 0.4 generations: Table 3) were predicted when the

insulated box was used, due to the lower product

temperatures.

0020 Chilled foods spend a period between a few min-

utes and many days stored in domestic refrigerators.

The average food temperature measured in a survey

of refrigerators in 252 UK households was approxi-

mately 6



C(Figure 5a). However, in over 15% of the

refrigerators the average food temperature was above



C. In a typical UK refrigerator, a chilled food would

spend over 35% of its time in an air temperature

above 7



C(Figure 5b).

See also: Chilled Storage: Principles; Quality and

Economic Considerations; Vegetables of Temperate

Climates: Leaf Vegetables