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

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Primary and Secondary Chilling

Quality Considerations

0003 In the majority of foods, fast cooling is beneficial, but

there are instances where excessively rapid chilling

rates cause quality problems in particular foods. For

example, a serious defect known as ‘woolly texture’

can be produced in rapidly cooled peaches. Biochem-

ical constraints lead to toughening if the lean tissue of

beef or lamb is reduced to 10



C or below within 10 h

of slaughter. Due to differences in the biochemistry

of pork muscle, the same temperature has to be

achieved within 3 h to cause toughening. (See Meat:

Preservation.)

0004 Different foodstuffs exhibit particular quality

advantages because of rapid chilling. In meat, the

pH starts to fall immediately after slaughter, and

protein denaturation begins. The result of this de-

naturation is a pink, proteinaceous fluid, commonly

called ‘drip’, often seen in pre-packaged joints. The

rate of denaturation is directly related to temperature,

and it follows therefore that the faster the chilling

rate, the less the drip. With both pork (Table 1)and

beef, the use of rapid rates of chilling can halve the

amount of drip loss. Fish passing through rigor mortis

above 17



C are largely unusable because the fillets

shrink and become tough. A relatively short delay of

an hour or two before chilling can demonstrably

reduce shelf life. In a different commodity area,

freshly harvested sweetcorn loses 5, 20, or 60% of

its sugar content after 24 h in air at 0, 10, and 30



respectively. Prompt cooling is clearly required if this

vegetable is to retain its desirable sweetness. Simi-

larly, the ripening of fruit can be controlled by rapid

cooling, the rate of ripening declining as temperature

is reduced and ceasing below about 4



C. (See Fish:

Processing; Ripening of Fruit.)

0005 Rapid cooling is also often desirable with cooked

products to maintain quality by eliminating the over-

cooking that occurs during slow cooling. For vitamin

retention, the time taken to reduce the center tem-

perature from 80 to 15



C is a critical factor. The

vitamin C content in nonpasteurized meals is reduced

by 1 to 12% if cooling is carried out in 0.5 h, by 2 to

17% if the time is increased to 2 h, and by 10 to 38%

when cooling takes 5 h. (See Ascorbic Acid: Proper-

ties and Determination.)

0006Vacuum-cooling is a very effective method of

cooling cooked beef joints and hams. However, the

process results in high weight losses and some reduc-

tion in eating qualities of the meat. Intermittent

spraying to replace the water loss is practiced in

vacuum-cooling of vegetables. It could have an

application with meat.

Economic Considerations

0007It is clear from restrictions already detailed that

attempts to increase chilling rates are complicated

by many factors, but there are a number of clear

advantages in production economics if faster cooling

can be achieved. Most foods are of high value, and

any increase in rate of product throughput will im-

prove cash flow and utilize expensive plants more

efficiently. For example, the cooling time of 400-g

pies can be reduced by 10 min if the air velocity is

raised from 6 to 10 m s

1

. In high-throughput baking

lines (> 1000 items per hour), the 7% increase in

throughput could justify the higher capital and run-

ning costs of larger fans. The power required by the

fans to move the air within a chill room increases with

the cube of the velocity. A fourfold increase in air

velocity from 0.5 to 2 m s

1

results in a 4.4-h (18%)

reduction in chilling time for a 140-kg beef side, but

requires a 64-fold increase in fan power. Increasing air

velocity to3 m s

1

only achieves an extra 6% reduction

in chilling time. In most practical situations, where

large items (e.g., meat carcasses, tuna, bins of vege-

tables) are being cooled, it is doubtful whether an air

velocity greater than 1 m s

1

can be justified.

0008Efficient chilling produces a reduction in weight

loss, which results in a higher yield of saleable material.

Most foods have a high water content, and the rate of

evaporation depends on the vapor pressure at the sur-

face. Vapor pressure increases with temperature, and

thus any reduction in the surface temperature will

reduce the rate of evaporation. The effect of air tem-

perature and velocity on evaporative weight loss

during chilling is dependent upon the endpoint of the

chilling process (Figure 1a and b). When chilling for a

set time, weight loss increases as the temperature

decreases and velocity increases. The opposite effect

is found when chilling to a set temperature.

0009If specified cooling schedules are to be attained,

refrigeration machinery must be designed to meet

the required heat extraction rate at all times during

the chilling cycle. Heat enters a chill room via open

doors, from personnel, through the insulation, from

lights and cooling fans, and from the cooling

tbl0001 Table 1 Drip loss after 2 days’ storage at 0



C, from leg joints

from different breeds of pig cooled at different rates

Breed Drip loss (percentby weight)

Slow Quick

Landrace 0.47 0.24

Large White 0.73 0.42

Wessex  Large 0.97 0.61

White

Pietrain 1.14 0.62

1176 CHILLED STORAGE/Quality and Economic Considerations

products. The product load is the major component

of the total heat to be extracted from a fully loaded

chill room. The rate of heat release from a food varies

with time. It is at a peak immediately after loading

and then falls rapidly.

0010The peak value is primarily a function of the envir-

onmental conditions during chilling. In commercial

systems, the peak load imposed on the refrigeration

plant is also a function of the rate at which hot food is

introduced into the chill room. Increasing the air

velocity, decreasing the air temperature, or shortening

the loading time increases the peak heat load. In beef-

chilling, for example, there is a fourfold difference in

peak load between a chill room operating at 8



0.5 m s

1

loaded over 8 h and the same room operat-

ing at 0



C, 3 m s

1

and loaded over 2 h.

0011Alternative chilling treatments can produce large

increases in throughput and reductions in weight loss

over conventional treatments (Table 2). The cash

savings that result from the reduced weight losses

can substantially increase the overall profits of many

slaughtering operations (Table 2).

Commercial Storage, Retail Display, and

Domestic Storage

Packaged Food

0012Most packaging systems substantially reduce evap-

orative weight loss from the surface of the product,

and many retard the rate of chemical degradation.

The use of modified atmospheres or opaque pack-

aging can limit color changes during the chilled

storage of meat, fish, and fruits.

0013Losses in vitamin content tend to be reduced at

lower storage and display temperatures. Vitamin C

losses from lettuce average 4.8, 5.6, and 6.6% per day

at temperatures of 1, 5, and 10



C, respectively.

4 ⬚C

7 ⬚C

13 ⬚C

4 hours

22 hours

Air velocity (m/s)

0246810

23 41

Weight loss (%)

(a) Velocity

Thin 18 h

Thin 10 ⬚C

Fat 10 ⬚C

Fat 18 h

3.0

2.5

1.5

2.0

Air temperature (⬚C)

Weight loss (%)

(b) Temperature

fig0001 Figure 1 Relationship between weight loss and (a) air velocity

for lamb carcasses and (b) air temperature for beef sides when

chilling either for a set time or to a final temperature.

tbl0002 Table 2 The weight loss, saving (£) over conventionally chilled controls, total work done at 24 h post mortem, cooling time to 10



C for

each treatment, drip loss from loin chops and annual saving for each treatment compared with the conventional chill used as a control

for an abattoir slaughtering an average of 1000 pigs per week (74 kg dead weight at £0.90 kg

1

)

Chilling treatment Weight loss at 24 h (%) Cooling time to 10



C Drip (%) Texture work (J) Saving (£)

Deep leg (h) Surfaceleg (h)

(1) Ultrarapid

Side 1.13 (0.85) >7.0 5.0

2.3 0.18 28 300

Whole carcass 1.10 (1.01) >8.0 4.0

0.9 0.21 33 600

(2) Ultrarapid two-stage 2.00 (0.29) 7.0 0.2 0.23 9 600

(3) Immersion 0.32 (2.08) 5.4 3.6 1.5 0.30 69 200

(4) High humidity 1.96 (0.21) 12.9 8.7 0.8 0.20 7 000

(5) Delayþhigh humidity 1.93 (0.24) 14.8 9.1 1.0 0.20 8 000

(6) Delayþspray 0.95 (1.22) 14.1 9.2 1.0 0.20 40 600

(7) Rapidþhigh humidity 1.53 (0.64) 11.6 1.3 0.9 0.21 21 300

(8) Rapidþconventional 1.67 (0.50) 11.5 1.6 1.0 0.24 16 600

Measured at intersection of lean and fat.

CHILLED STORAGE/Quality and Economic Considerations 1177

Larger effects of temperature are found in French

beans with equivalent losses of 1.9, 3.0, and 5.1%

per day. No losses were reported in carrots at the

three temperatures.

0014 Oranges and pineapples are examples of foods that

have an optimum temperature for vitamin C reten-

tion. In oranges, a 10% loss per month at 10



increases to 12% at 5



C and 26% at 0



C. Raising

the storage temperature above 10



C also increases

the rate of loss to 13% at 15



C and 22% at 30



A similar optimum at 10



C is found in pineapples.

0015 Vitamin B

retention is also a function of both

storage temperature and species. Loss from lettuce

and French beans increases twofold and threefold,

respectively, as the temperature is increased from 1 to



C. No change was reported in parsley or carrots.

(See Vitamin B

: Properties and Determination.)

Unwrapped Food

0016 Changes in appearance are normally the criteria that

limit storage and display of unwrapped foods, com-

mercial buyers or domestic consumers selecting fresh

or newly loaded product in preference to that dis-

played or stored for some time. Deterioration in ap-

pearance has been related to degree of dehydration in

red meat (Table 3), and similar changes are likely to

occur in other foods. Apart from any relationship to

appearance, weight loss is of considerable importance

in its own right. The direct cost of evaporative loss

from unwrapped foods in chilled display cabinets in

the UK is in excess of £5 million per annum.

0017 In the equation that governs weight loss, already

described, the mass-transfer coefficient, m, is a func-

tion of air velocity. Typical values range from ap-

proximately 2 10

8

kg m

2

1

at 0.25 m s

1

to 14 10

8

kg m

2

1

at 1.5 m s

1

. The vapor

pressure difference (P

P

) is a function of surface

temperature and dryness, and the temperature and

relative humidity of the air.

0018In storage rooms at temperatures in the range of

0–2



C, 76–90% RH, the average weight loss from

cauliflower, French beans, and green peas is in the

range of 0.1–0.4% per day. Under conditions typical

of domestic refrigeration, 4–8



C and 70–90% RH,

weight losses from the same products range from 0.3

to 3.0% per day. In a nonrefrigerated ambient,

16–24



Cand50–70% RH, losses are even higher in

the range of 1.0–4.0% per day.

0019The relative effects of air temperature, velocity, and

humidity on weight loss in retail display are demon-

strated in Figure 2. Changes in relative humidity have

a substantial effect with a reduction from 95 to 40%,

increasing weight loss over a 6-h display period by a

factor of between 14 and 18. The effect of air velocity

on weight loss is confounded by that of relative hu-

midity. Raising the air velocity from 0.1 to 0.5 m s

1

has little effect on weight loss at 95% RH but

tbl0003Table 3 Relationship between change in appearance and

evaporative weight loss (g cm

2

)

Evaporative loss Change in appearance

Up to 0.01 Red, attractive, and still wet; may lose some

brightness

0.015–0.025 Surface becoming drier; still attractive but

darker

0.025–0.035 Distinct obvious darkening; becoming dry and

leathery

0.05 Dry, blackening

0.05–0.10 Black

2 ⬚C, 0.1 m/s

6 ⬚C, 0.1 m/s

2 ⬚C, 0.3 m/s

6 ⬚C, 0.3 m/s

2 ⬚C, 0.5 m/s

6 ⬚C, 0.5 m/s

0.09

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

Weight loss (g/sq.cm)

40% RH 60% RH 95% RH

fig0002 Figure 2 Weight loss from samples of beef steak under simulated display conditions.

1178 CHILLED STORAGE/Quality and Economic Considerations

increases the loss by a factor of between 2 and 2.4 at

60% RH. A temperature change from 2 to 6



C has a

far smaller effect on weight loss than changes in either

relative humidity or velocity.

002 0 In further work, a model developed to predict the

rate of weight loss from unwrapped meat under the

range of environmental conditions found in chilled

retail displays showed that it was governed by the

mean value of the conditions. Fluctuations in tem-

perature or relative humidity had little effect on

weight loss, and any apparent effect was caused by

changes in the mean conditions.

002 1 There is a conflict between the need to make the

display attractive and convenient to increase sales

appeal and the optimum display conditions for the

product. High lighting levels increase the heat load,

and the air temperature in the cabinet rises. This

rise increases the temperature difference across the

evaporator coil, and the air entering the cabinet is

dehumidified. Consequently, the rate of evaporative

weight loss from foods on display is increased.

002 2 Studies have shown that changing a lighting com-

bination of 50 W sons and 100 W halogen lights to

100 W sons and a colour 83 fluorescent significantly

increased the weight loss (Figure 3). The increase was

similar in magnitude to that produced by 20% reduc-

tion in relative humidity. On average, the rate of

weight loss under the combination of 50 W sons and

100 W halogen (spot) lights was approximately 1.4

times less than the 100 W sons and colour 83 fluores-

cent lighting.

Seealso: Ascorbic Acid:PropertiesandDetermination;

Fish:Processing; Meat:Preservation; Ripening of Fruit;

Vitamin B

:PropertiesandDetermination

Further Reading

Anonymous (1974) ASHRAE Handbook and Product

Directory. Applications. New York: American Society

of Heating Refrigeration and Air Conditioning

Engineers.

Food Refrigeration & Process Engineering Research Centre

(FRPERC) (1997) Meat Refrigeration – Why and How?

EU Concerted Action Programme CT94 1881. Lang-

ford, UK: FRPERC.

FRPERC (2000) Publications on the refrigeration and

thermal processing of food. University of Bristol.

http://www.frperc.bris.ac.uk.

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

Barking, UK: Elsevier Applied Science.

International Institute of Refrigeration (1985) Technology

Advances in Refrigerated Storage and Transport.Or-

lando, FL: International Institute of Refrigeration –

Commissions D1, D2 & D3.

International Institute of Refrigeration (1986) Recent

Advances and Developments in the Refrigeration of

Meat by Chilling. Bristol, UK: International Institute

of Refrigeration Commission C2.

International Institute of Refrigeration (1990) Progress in

the Science and Technology of Refrigeration in Food

Engineering. Dresden: International Institute of Re-

frigeration – Commissions B2, C2, D1, D2/3.

International Institute of Refrigeration (2000) Recommen-

dations for Chilled Storage of Perishable Produce. Paris:

International Institute of Refrigeration ISBN 2-913149-

09-X

James SJ (1999) Food refrigeration and thermal processing

at Langford, UK: 32 years of research. Transactions of

the IChemE Food and Bioproducts Processing 77(C):

261–279.

Zeuthen P, Cheftel JC, Eriksson C, Gormley TR, Lonko P

and Paulus K (1990) Chilled Foods: The Revolution in

Freshness. Barking, UK: Elsevier Applied Science.

300200100 400 500

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Time on display (m)

40% RH, 100 W spots + 50 W son

65% RH, 100 W spots + 50 W son

65% RH, color 83 + 100 W son

85% RH, 100 W spots + 50 W son

fig0003 Figure 3 Weight loss from delicatessen products under different humidity and lighting conditions.

CHILLED STORAGE/Quality and Economic Considerations 1179

Microbiological Considerations

S J Walker, Campden & Chorleywood Food Research

Association, Chipping Campden, Gloucestershire, UK

Background

0001 During storage, many foods and their raw materials

will deteriorate and become unacceptable to the end

user. Food preservation techniques aim to prevent or

retard this deterioration. Chilled storage can be rarely

relied upon to prevent spoilage, but can be used to

extend the time until this becomes apparent. Such

storage has major effects on the rates of biochemical

and biological changes in foods, although this article

will focus on microbiological issues in chill stored

foods and ingredients.

0002 Foods or ingredients intended for chilled storage

may initially contain a wide variety of microorgan-

isms derived from the material itself or contaminants

arising from harvesting, storage, and processing. The

organisms that spoil chilled foods will be those that

are best adapted to these temperatures.

Effects of Chilled Storage on

Microorganisms

0003 If the temperature of chilled storage is below the

minimum growth temperature for a microorganism,

no growth will occur. However, microbial growth has

been reported for a number of microorganisms at

temperatures below 0



C. Consequently, chilled stor-

age cannot be relied upon for microbial stability. The

major effects on microbial growth are to extend the

time until an increase in numbers is apparent (the lag

time) and to reduce the rate of growth when this

occurs.

Cardinal Temperatures

0004 Microorganisms may be characterized by their abil-

ities to grow at specific temperatures, commonly re-

ferred to as the cardinal temperatures.

0005 With chilled foods, the factor of most concern is

the minimum growth temperature (MGT), which

represents the lowest temperature at which

growth of a particular microorganism can occur. If

the MGT of a microorganism is greater than 10



this microorganism will not grow during chill stor-

age. Care is needed with published MGT values. For

example, if the time period for the investigation

reporting this value were too short, the resultant

value would be overestimated. As the MGT is

affected by other factors, including the pH, salt,

preservatives, and previous heat treatments, a true

estimate can be determined only when other factors

are optimal for growth.

0006Although gradual death may occur if a microor-

ganism is stored below its MGT, this should not be

considered to be a lethal process, as, in many cases,

growth will resume if the temperature is subsequently

raised.

0007The optimum growth temperature represents the

temperature at which the biochemical processes

governing growth of a particular microorganism are

overall operating most efficiently. At this tempera-

ture, the lag phase before growth is minimized and

the growth rate maximized. As the temperature rises

above the optimum, the rate of growth decreases until

the maximum growth temperature is reached. In gen-

eral, the maximum growth temperature is only a few

degrees higher than the optimum. At temperatures

just above the maximum for growth, cell injury starts

to occur. However, if the temperature is subsequently

reduced, growth may resume, although a period of

time may be required to permit cell repair. At higher

temperatures, the inactivation of one or more critical

enzymes in the microorganism becomes irreversible,

and cell damage occurs, leading to cell death.

0008Based on the relative positions of the cardinal

temperatures, microorganisms can be divided into

four main groups, viz. psychrophiles, psychrotrophs,

mesophiles and thermophiles (Table 1). With chilled

foods, the groups of most concern are the psychro-

philes and psychrotrophs. Whilst these terms have

been used synonymously in the past, it is now

accepted that the major spoilage microorganisms of

chilled foods are psychrotrophic in nature. True psy-

chrophiles are rare in food microbiology and gener-

ally limited to some microorganisms from deepsea

fish. The minimum growth temperatures for many

of the microorganisms relevant to chilled foods are

given in Table 2.

Microbial Growth in Chilled Foods

0009Whilst microbiological spoilage of chilled foods may

take diverse forms, it generally manifests itself in a

change in the sensory characteristics. In the simplest

form, this may be due to the production of visible

tbl0001Table 1 Cardinal temperatures for microbial types

Temperature (



Minimum Optimum Maximum

Psychrophiles <0–5 12–20 20–22

Psychrotrophs <0–5 20–35 30–40

Mesophiles 10 30–40 40–45

Thermophiles 30–40 45–65 60–>80

1180 CHILLED STORAGE/Microbiological Considerations

growth, and this is common in molds that produce

large, often pigmented colonies. Bacteria and yeasts

also may produce visible (sometimes pigmented) col-

onies on foods. Other forms of spoilage include the

production of gases, slime (extracellular polysacchar-

ide material), diffusible pigments, and enzymes that

may produce softening, rotting, off-odors and off-

flavors from the breakdown of food components.

0010 When only a few spoilage microorganisms are pre-

sent, the consequences of growth may not be appar-

ent. If, however, the microorganisms multiply during

storage, the production of deleterious compounds or

deterioration in structure in the food may become

unacceptable. Some of the enzymes produced by

spoilage bacteria may remain active, even when a

thermal process has destroyed the causative microor-

ganisms in the food.

0011 The relationship between microbial numbers and

food spoilage is complex and depends on the number,

type, and activity of the microorganisms present,

the type of food, the intrinsic properties of the food

(e.g., pH, water activity, preservatives) and the

extrinsic conditions (e.g., temperature of storage,

atmosphere).

0012Spoilage is often most rapid and pronounced in

proteinaceous chilled foods such as red meats,

poultry, fish, shellfish, milk, and some dairy products.

These products allow relatively good microbial

growth as they are highly nutritious and have a high

moisture content and relatively neutral pH value.

However, care is needed to distinguish between

those microorganisms present in spoiled food and

those responsible for the spoilage defect (often called

specific spoilage organisms), which may be only a

fraction of the total microflora. Consequently, the

relationship between sensory spoilage and microbial

numbers is often only poorly correlated.

Spoilage Microorganisms

0013For this article, spoilage microorganisms have been

arbitrarily divided into six categories: Gram-negative

(oxidase positive) rod-shaped bacteria, coliform/en-

teric bacteria, Gram-positive spore-forming bacteria,

lactic acid bacteria, other bacteria, and yeasts and

molds.

0014Gram-negative (oxidase-positive) rod-shaped bac-

teria This group generally comprises the most

common spoilage microorganisms of fresh, chilled

proteinaceous products stored in air. The minimum

growth temperatures are often 3to0



C, and they

grow relatively rapidly at 5–10



C. Although they

may represent only a small proportion of the initial

microflora, they may grow and rapidly dominate the

microflora of fresh, proteinaceous, chilled, stored

foods.

0015Within this general group, the genus Pseudomonas

is most common, although other genera include Aci-

netobacter, Aeromonas, Alcaligenes, Alteromonas,

Flavobacterium, Moraxella, Shewanella, and Vibrio

species. These microorganisms are common in the

environment, particularly in water, and so may easily

contaminate foods. Often, they may proliferate on

inadequately cleaned surfaces of food processing

plant or equipment.

0016These organisms may spoil products by the

production of pigments, slime material on surfaces,

and enzymes that result in food rots, off-flavors and

off-odors. Some of the enzymes produced by Pseudo-

monas species are extremely heat-resistant and may

produce long-term defects (e.g., rancidity or age gel-

ation) in thermally processed (i.e., sterile) products.

0017Although this group is well adapted to grow at chill

temperatures, it tends to be sensitive to other factors

such as the presence of salt or preservatives, lack of

oxygen, low pH (< 5.5) and a low water activity

(< 0.98). Should these conditions prevail in a food,

these bacteria will compete less well, and so other

tbl0002 Table 2 Minimum growth temperatures of microorganisms

(Adapted from Martens, 1999)

Microorganism Minimum temperature (



Bacillus cereus

Mesophilic 15

Psychrotrophic 4

Bacillus species (spoilage) 0–15

Brochothrix thermosphacta 4

Campylobacter jejuni 32

Clostridium botulinum:

Mesophilic (proteolytic) 10

Psychrotrophic (nonproteolytic) 3

Clostridium perfringens 12

Coliforms 2–8

Escherichia coli 4

Escherichia coli O157 6.5

Lactobacillus species 0–10

Listeria monocytogenes 0

Micrococcus species 1–10

Pseudomonas species 3

Salmonella 7

Staphylococcus aureus 6 (10 for toxin)

Vibrio parahaemolyticus 5

Yeasts 10

Yersinia enterocolitica 1

Under otherwise optimal conditions. These figures are indicative only

and not necessarily representative of all strains of microorganisms in

foods, and will also depend on the composition of the food and the storage

conditions.

Depending on the strain.

From Martens T (1999) Harmonisation of Safety Criteria for Minimally

Processed Foods: Rationale and Harmonisation Report. European

Commission FAIR CT- 96 -1020.

CHILLED STORAGE/Microbiological Considerations 1181

microbial groups may cause spoilage. Overall, this

group is not heat-resistant and is readily removed by

mild thermal treatments. Consequently, their pres-

ence in heat-processed chilled foods is usually as a

consequence of postprocess contamination.

0018 Coliform/enteric bacteria This bacterial group also

consists of Gram-negative rods, but these may be

distinguished from the above group by a negative

oxidase reaction. Traditionally, microbiologists have

tended to examine for these groups separately, as

their sources, significance, and factors affecting

growth may differ. This group is frequently used as

an indicator of postprocess contamination, inad-

equate processing or potential fecal contamination.

0019 Compared with the Gram-negative (oxidase posi-

tive) rod-shaped bacteria, the coliform-enteric group

is generally less well adapted to growth at tempera-

tures of less than 5–10



C, although some may grow

at lower temperatures. However, they often dominate

the flora of foods stored at temperatures of 8–15



and are less sensitive to changes in pH compared with

the previous group. They are, however, generally sen-

sitive to low water activity, preservatives, salt and

thermal treatments. The coliform/enteric group does

not necessarily require the presence of oxygen for

growth.

0020 This group may break down carbohydrates to give

acids, which may result in souring of milk and other

foods. Other types of spoilage include the production

of pigmented growth, gases, slime, off-odors and off-

flavors; these have often been described as ‘grassy,’

medicinal, unclean, and fecal.

0021 Typical spoilage genera include Citrobacter,

Escherichia, Enterobacter, Hafnia, Klebsiella, Pro-

teus, and Serratia. These microorganisms are widely

disseminated in the environment, including in

animals. For animal products, poor slaughter and

dressing practices may contribute to their presence

in foods.

0022 Gram-positive spore-forming bacteria These are of

particular significance as they can produce

heat-resistant spores that can survive many thermal

processes that destroy vegetative cells, leaving these

organisms to grow and dominate the microflora. The

minimum growth temperatures are often quoted as 0–



C, although growth is frequently slow below 8



0023 The genera of concern in this group are Bacillus

and Clostridium. Again, these are common in the

environment, and spores may survive for tens and

possibly hundreds of years. The most common form

of spoilage is the production of large quantities of gas

that may result in pack or product blowing. The heat

resistance of psychrotrophic strains is considered to

be lower than that of mesophilic strains, but the

former group may be of concern in chilled pasteurized

foods.

0024Lactic acid bacteria At chill temperatures, lactic

acid-producing bacteria grow more slowly than

many of the bacterial groups mentioned previously.

Consequently, they are only responsible for spoilage if

growth of most other bacterial species is inhibited.

This group is more tolerant of low pH than other

spoilage bacteria and may multiply at pH values

below 4.0. The lactic acid bacteria are also more

resistant than the previously discussed spoilage bac-

teria to slight reductions in the water activity (a

and some strains are salt-tolerant. Lactic acid bacteria

usually predominate in vacuum-packed products and

in some modified-atmosphere-stored foods.

0025This bacterial group comprises both rod- and

coccus-shaped Gram-positive bacteria, and typical

genera include Carnobacterium, Lactobacillus, Leu-

conostoc, Pediococcus, and Streptococcus. Spoilage

is generally by the production of acid, which results in

souring, with or without concomitant gas produc-

tion.

0026The presence of lactic acid-producing bacteria is

not necessarily a problem, as some strains are deliber-

ately added during the manufacture of some chilled

foods (e.g., cheese, yogurts, and some salamis) and

are essential for the development of the desired prod-

uct characteristics. In addition, there is much interest

in their potential use as a novel preservation system,

as many produce natural antimicrobial compounds in

addition to acids.

0027Other bacteria Depending on the food type and

preservation system operating, other microorganisms

also may cause problems in specific chilled foods. For

example, Brochothrix thermosphacta is occasionally

present on raw meats but does not normally create a

spoilage problem. However, it can grow in atmos-

pheres with a low oxygen level and/or high carbon

dioxide concentration and so may cause problems

in vacuum-packed or modified-atmosphere-packed

meat products and produces an objectionable pun-

gent ‘cheesy’ odor.

0028Micrococcus species are Gram-positive cocci that

can grow in the presence of high salt concentrations.

They grow more slowly than many other spoilage

bacteria at chill temperatures but can cause souring

and slime production when growth of competing or-

ganisms is inhibited.

0029Yeasts and molds Yeasts and molds generally grow

more slowly than bacteria in foods, permitting

good growth and so are generally out-competed.

1182 CHILLED STORAGE/Microbiological Considerations

Therefore, this group is seldom responsible for the

spoilage of fresh proteinaceous foods. When the con-

ditions in the food are altered to restrict bacterial

growth, the role of yeasts and molds may become

more significant. They are generally more resistant

than bacteria to low pH, reduced a

values, the pres-

ence of preservatives and low temperatures. Molds

tend to require oxygen for growth, whereas many

yeasts can grow in the presence or absence of oxygen.

Most yeasts and molds are not heat-resistant and are

readily destroyed by a thermal process, although

some (e.g., Byssochlamys) may produce relatively

heat-resistant ascospores that may survive food pas-

teurization treatments.

0030 Air movements may be an important vector of

transmission, especially with mold ascospores and

so result in widespread contamination.

0031 Typical spoilage yeasts include Candida, Debaryo-

myces, Hansenula, Kluyveromyces, Rhodotorula,

Saccharomyces, Torula,andZygosaccharomyces

species. Molds that may be isolated from spoiled

chilled foods include Aspergillus, Cladosporium,

Geotrichum, Mucor, Penicillium, Rhizopus, and

Thamnidium species. Fungal spoilage may be charac-

terized by the production of highly visible, often

pigmented, growth, slime, fermentation of sugars to

form acid, gas or alcohol, and the development of

off-odors and off-flavors, often described as yeasty,

fruity, musty, rancid, and ammoniacal.

0032 As with the lactic acid bacteria, yeasts and molds

are sometimes deliberately added to food products

(e.g., mold on ripened cheeses such as Brie).

Pathogenic Microorganisms

0033 Foods may be considered to be microbiologically

unsafe owing to the presence of microorganisms

that may invade the body (e.g., Salmonella, Listeria

monocytogenes, E. coli O157:H7 and Campylo-

bacter) or those that produce a toxin that is ingested

with the food (e.g., Clostridium botulinum, Staphylo-

coccus aureus, and Bacillus cereus). The growth of

pathogenic microorganisms in foods may not neces-

sarily result in spoilage, and so the absence of dele-

terious sensory changes cannot be relied upon as an

indicator of microbial safety. Furthermore, some

toxins are resistant to heating and so may remain in

a food after viable microorganisms have been re-

moved.

0034 In general, with human pathogens, the greater the

number of cells consumed, the greater the chance of

microbial invasion, as a larger number of cells may be

able to evade/swamp the body’s defense mechanism.

Consequently, control, and preferably inhibition, of

growth in foods is essential. However, with some

invasive pathogens (e.g. Campylobacter), growth in

the food may not be necessary as the infectious dose

is low.

0035For discussion in this article, the pathogenic bac-

teria of concern for chilled foods can be arbitrarily

divided as follows.

Microorganisms capable of growth at temperatures

below 5



0036These microorganisms are potentially of greatest con-

cern as they continue to multiply, even with ‘good’

refrigeration temperatures. Although growth may

continue, temperature control is critical, and the

growth rate becomes increasingly slow as the tem-

perature is reduced. In addition, temperature control

can interact effectively with other factors (e.g., pH,

salt, preservatives) to prevent or greatly limit growth.

0037Listeria monocytogenes A wide range of foods in-

cluding meat, poultry, dairy products, seafoods and

vegetables have been reported to be contaminated

with L. monocytogenes. Although the total absence

of L. monocytogenes from raw meats, poultry, and

vegetables is difficult to ensure, the bacterium has

been isolated from products that have undergone a

thermal process designed to eliminate this bacterium.

Such isolations are of concern, as many of these

chilled foods may be consumed without further

heating. The presence of L. monocytogenes on

cooked foods suggests that postprocess contamin-

ation may have occurred. Several studies have

shown that this bacterium has been isolated from a

wide range of sites in several types of factory and may

even be spread by poorly controlled cleaning proced-

ures. Environmental control of Listeria, particularly

in key areas of production (e.g., after cooking), is

crucial to the prevention of product contamination.

0038The major concern with L. monocytogenes is its

ability to grow at low temperatures, and a minimum

growth temperature of 0.4



C has been reported.

Temperature control will, however, retard the rate

of growth. Conversely, temperature abuse during

storage of a food can exacerbate problems. Listeria

monocytogenes is more resistant than many other

vegetative bacteria to some, but not all, of the preser-

vation mechanisms used in food manufacture (e.g.,

chilling, reduced water activity). Whilst resistance

may be noted to these preservation systems when

examined individually, foods are complex, and the

combined effects of two or more preservation factors

may effectively prevent growth.

0039L. monocytogenes is not considered to be a classic-

ally heat-resistant bacterium. It is generally accepted

that conventional high-temperature, short-time

milk pasteurization (71.7



C/15 s) eliminates this

CHILLED STORAGE/Microbiological Considerations 1183

microorganism when freely suspended in milk. In

other foods, a minimum process of 70



C for 2 min

(or the thermal equivalent) ensures the effective elim-

ination of this bacterium.

0040 The symptoms of disease are protean and range

from a mild flu-like illness to meningitis, septicemia,

stillbirths, and abortions. In general, the major

symptoms of disease are restricted to the pregnant

mother, fetus, elderly, and immunocompromised,

and with these groups, the mortality level can be

high.

0041 Yersinia enterocolitica Outbreaks of disease caused

by Y. enterocolitica are relatively rare, but have im-

plicated milk, tofu, and chocolate milk. Although the

reported incidence of Y. enterocolitica in gastrointest-

inal samples is generally low, it has increased, but this

may be due not only to a true underlying increase but

also to a greater awareness of this bacterium, recog-

nition of symptoms, and improved methods. In some

European countries, disease cause by Y. enterocolitica

has surpassed that of Shigella and even rivals that of

Salmonella.

0042 A wide variety of foods have been reported to be

contaminated with Y. enterocolitica, including many

chilled products, i.e., raw and cooked meats, poultry,

seafoods, milk, dairy products, and vegetables. Care

is needed as the isolates responsible for disease gener-

ally belong to a few specific (bio-sero) types, whereas

those from foods and the environment belong to

a wide range of (bio-sero) types. Therefore, the

pathogenic significance of food isolates should be

ascertained before the food is considered as a health

risk. The types responsible for human disease

may be isolated from pigs and occasionally pork

products.

0043 The minimum reported growth temperature for

Y. enterocolitica is 1.3



C, and the bacterium

grows relatively well at chill temperatures. Storage

at refrigeration temperatures interacts with other pre-

servation factors present in foods and help prevent

growth of this bacterium. Y. enterocolitica is a heat-

sensitive bacterium and can be readily eliminated

from foods by heating. It has, however, been isolated

from cooked meats, seafoods, and pasteurized dairy

products, which is indicative of postprocess con-

tamination. Greater attention is required for the

environmental control of Y. enterocolitica in food-

manufacturing establishments.

0044 The symptoms of human yersiniosis are also vari-

able. Overall, acute gastroenteritis is the most

common symptom, particularly with children, and is

characterized by diarrhea, abdominal pain, fever,

and, less commonly, vomiting. With adolescents, ab-

dominal pain may be localized and misdiagnosed as

appendicitis. The mortality rate from human yersi-

niosis is low, and, other than cases involving append-

ectomies, the symptoms are generally self-limiting

and rarely require treatment. In some cases, second-

ary symptoms may occur several weeks after the

typical gastrointestinal symptoms disappear, e.g.,

postinfectious polyarthritis and erythema nodosum.

0045Aeromonas hydrophila The role of A. hydrophila

as an agent of foodborne disease is still a matter of

controversy. This bacterium, however, does possess

many of the characteristics of other pathogenic bac-

teria. Incidents of foodborne disease implicating

A. hydrophila have included oysters and prawns.

0046The minimum reported growth temperature for

A. hydrophila is 0.1 to 1.2



C. The bacterium is

considered to be heat-sensitive and so may be readily

eliminated from foods. Little has been published on

the presence of A. hydrophila in the processing envir-

onment, but it is likely to be isolated particularly from

wet areas.

0047Bacillus cereus The role of B. cereus as a spoilage

bacterium of chilled foods is well recognized, and

some strains may grow at temperatures as low as



C. The minimum reported growth temperatures

of pathogenic strains is usually considered to be 10–



C, although some isolates from outbreaks that

involved chilled products have been able to grow

and produce toxins at 4



C. In addition, psychro-

trophic, presumptively enterotoxigenic strains have

been isolated from pasteurized milks and some

cook–chill meats.

0048Bacillus cereus may be of particular significance in

foods that have been heated or pasteurized, as the

heat treatment may have eliminated other competitor

microorganisms. During subsequent chilled storage,

spores that survive the heat treatment may germinate

and grow. Although relatively little published infor-

mation is available, the heat resistance of psychro-

trophic B. cereus (and other related species) is

generally lower than that of the mesophilic strains.

0049Clostridium botulinum Human botulism is caused

by the ingestion of a potent neurotoxin preformed in

the food. Based on the antigenic analysis of this, seven

types can be distinguished (named A–G). The strains

responsible for disease can be divided into two main

groups. First, mesophilic types A and some strains of

B and F are proteolytic and so often cause putrefac-

tion of foods if substantial growth occurs. Second,

psychrotrophic types E and others of B and F are

nonproteolytic, and so the consequences of growth

and toxin production can occur with no discernible

change to the food’s appearance or smell. The

1184 CHILLED STORAGE/Microbiological Considerations

minimum growth temperature of the mesophilic pro-

teolytic strains is considered to be 10



C, and so these

are of limited significance with chilled foods. Type E

C. botulinum is able to grow and produce toxin after

incubation at 3.3



C for 32 days. It is now recognized

that nonproteolytic strains of types B and F are also

capable of growth and toxin production at 5



Cor

less. Therefore, these nonproteolytic strains may

grow, albeit slowly, in chilled foods.

0050 ‘Sous vide’ processing consists of packing foods

under vacuum in impermeable sealed bags that are

then heat-processed and stored chilled for extended

periods. Whilst the time and temperature of cooking

are specific to the food type, it will destroy vegetative

microbial cells but may not be sufficient to destroy

bacterial spores. The growth of nonproteolytic

C. botulinum is of particular concern, because in the

absence of air, spores may germinate and grow during

storage if temperatures are not carefully controlled

below the minimum for growth (3



C).

0051 It should be noted that the heat resistance of the

psychrotrophic nonproteolytic strains is considerably

lower than that of the mesophilic proteolytic strains.

The risk of botulism from the former group can be

minimized by the use of appropriate heating, con-

trolled chilled storage and/or alterations in the prod-

uct formulation to prevent growth.

Microorganisms capable of initiating growth at

temperatures of 5–10



0052 These are a number of other pathogenic bacteria,

which, although unable to grow at temperatures

below 5



C, may grow if temperature abuse occurs.

These include Salmonella species, Escherichia coli,

and Staphylococcus aureus, with generally accepted

minimum growth temperatures of 5–7



C. Even at

temperatures up to 10



C, the growth rate of these

bacteria is generally slow, but these bacteria do, how-

ever, cause foodborne disease, frequently implicating

chilled foods. Psychrotrophic strains of salmonellae

have very occasionally been reported.

0053 Several types of E. coli are well recognized as

agents of foodborne disease. At present, the type of

most concern is E. coli 0157:H7 and other verocyto-

toxigenic E. coli, which may produce severe hemor-

rhagic colitis. Limited growth of some strains may

occur at 5–10



0054 Whilst Staphylococcus aureus may grow at tem-

peratures as low at 6



C, disease is caused by the

ingestion of a preformed toxin. The minimum tem-

perature for toxin production is greater than for

growth and is considered to be 10–14



0055 Overall, the above bacterial species do not grow at

temperatures below 5



C, but may survive at these

temperatures. Pathogens, and spoilage bacteria, often

survive adverse conditions (e.g., low pH or high salt)

better at refrigeration temperatures compared with

higher temperatures. Therefore, if the infectious dose

of the bacterium is low and/or growth of the patho-

gen has already occurred (e.g., during slow cooling),

growth during chilled storage may not be a prerequis-

ite for disease.

Microorganisms capable of initiating growth at

temperatures above 10



0056These species include mesophilic C. botulinum, meso-

philic B. cereus and other Bacillus species, C. perfrin-

gens and Campylobacter species. In general, these do

not grow below 10



C, and growth is limited at tem-

peratures between 10 and 15



0057Of particular concern in this bacterial group are the

Campylobacter species that comprise the most com-

monly reported cause of gastrointestinal disease in

many developed countries. Although many of the

reported cases are sporadic, outbreaks have fre-

quently implicated the consumption of raw milk and

undercooked chicken. This bacterial group is un-

usual, as the minimum temperature for growth is

25–30



C, and so it will not grow on most foods.

However, as the infectious dose of the microorganism

is very low, growth may not be necessary for disease

to occur.

0058Whilst disease caused by the mesophilic spore-

forming bacteria has implicated chilled foods, this is

usually as a consequence of poor temperature control

during cooling after cooking. These bacteria may

grow extremely rapidly during a long slow cooling

regime after cooking and then persist during chilled

storage.

Conclusions

0059Chilled foods cover a wide range of commodities,

containing a large number of ingredients. The

number and types of microorganisms present are

affected by the indigenous microflora, microorgan-

isms contaminating before and after processing, the

growth rates and abilities of the microorganisms pre-

sent, the intrinsic properties of the food, the effects of

processing and packaging, and the time and tempera-

tures of storage. Consequently, the microbial safety

and spoilage of chilled foods is very complex. Con-

siderable efforts have been made by the food chain to

minimize these effects through careful raw-material

selection, product design and formulation, processing

and packaging conditions, and good temperature

control from processing through distribution, retail

display, and even consumer storage. All of these have

helped ensure the safety and quality of this important

food sector.

CHILLED STORAGE/Microbiological Considerations 1185