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

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ultimately only cereals are used, with disastrous con-

sequences. (See Macrobiotic Diets.)

0048 Embroiled in this widening saga of unconventional

diets is the use of vitamin supplements in megadoses.

It is now increasingly evident that such therapy is

metabolically hazardous, particularly for children.

Food Intolerance and Aversion

0049 There are few subjects that are so enigmatic, confus-

ing, and clouded with anecdotal clutter as that of

food adversion (intolerance) and aversion (psycho-

logical intolerance or avoidance). It has been

suggested that the term ‘allergy’ which implies an

unpleasant and immediate response should be applied

only to those with immediate allergic reactions to

food and with the presence of specific immunoglobu-

lin E (IgE) antibodies. Mast cells, basophils, eosino-

phils, macrophages, and platelets can all be activated

by IgE antibodies, causing the release of preformed or

instant mediators. (See Food Intolerance, Types; Food

Allergies; Lactose Intolerance.)

Food Intolerance

0050 Clinical manifestations of food intolerance can

be diverse and may include diarrhea, abdominal

pain, failure to thrive, urticaria, eczema, migraine,

behavioral disturbance, and food-sensitive colitis.

Food intolerance occurs in approximately 5% of the

pediatric population and can be caused by the

following:

0051 Allergic reaction(s). This mechanism is mediated

via IgE antibody, a T-cell-mediated reaction, or

the systemic formation of antigen–antibody com-

plexes.

0052 Pharmacological factors, e.g., tyramine. There is a

high concentration of tyramine in fermented

cheese and this can elevate the blood pressure

and produce symptoms. Caffeine in coffee and

tea, in the hypersensitive, can cause symptoms

such as anxiety and tachycardia.

0053 Enzyme deficiency, e.g., lactose intolerance caused

by hypolactasia. It has been suggested that a defi-

ciency of the enzyme phenolsulfotransferase may

have a role in food-induced migraine.

0054 Toxic effect. In many foods there are naturally

occurring toxins, e.g., mycotoxins from storage

of foods contaminated with molds.

0055 Food fermentation. Bacteria in the colon, particu-

larly Proteus morganii, can produce decarboxyl-

ase, an enzyme that will convert dietary histidine

to histamine. However, as histamine does not

cause an immunological reaction, its effect is

pseudoallergic.

0056Other intolerances: tartrazine, an azodye, is a

common coloring substance contained in many

foods, soft drinks, and pharmaceutical products;

benzoates or sulfur dioxide preservatives and

butylated hydroxyanisole or butylated hydroxy-

toluene antioxidants may cause urticaria and

asthma.

Acetylsalicylic acid (ASA), present in some vegetables

and a variety of fruits, can cause a range of symptoms

in sensitive patients, including asthma, urticaria, and

rhinitis; synthetic salicylates are found as flavoring

products.

Food Aversion

0057Food aversion is the result of either psychological

intolerance or food avoidance.

Pathogenesis

0058The permeability of the gastrointestinal tract mucosa

to food allergens is believed to be a major factor in the

pathogenesis of antigenic food reactions. It is known

that intestinal permeability is increased following an

acute gastroenteritis. Temporary changes in the gut

have been demonstrated after rotavirus enteritis by

studying the differential permeation of the sugars,

lactose, lactulose, and rhamnose. Studies using the

inert probe, polyethylene glycol, have shown in-

creased intestinal permeability in allergic children.

Moreover, using a radioimmunological method

for measuring serum concentration of human a-

lactalbumin, preterm infants have an increased

absorption of macromolecules compared with that

seen at term.

0059The integrity of the bowel mucosa is maintained in

part by secretory IgA and the glycocalyx (a glycopro-

tein coating the bowel enterocytes) as well as the

mucosa-associated lymph tissues (MALTs). It has

been suggested that following food challenge there

is the deposition of immune complexes in different

target organs and sites which can result in migraine,

arthalgia, or eczema.

Treatment

0060Treatment is often unsatisfactory because of difficul-

ties in identifying and then removing the offending

allergen(s).

Food Additives, Salicylates, and

Hyperactivity

0061Pediatricians and nutritionists seem to polarize them-

selves into either protagonists or vehement opponents

of this theory which postulates that hyperkinesis can

1156 CHILDREN/Nutritional Problems

be managed in 30–50% of children by eliminating

artificial food-coloring matter, preservatives, and

other additives from the diet, and by eliminating

fruits and vegetables containing natural salicylates.

The major symptoms of this psychological disorder

are overactivity, distractibility, restlessness, impulsive

behavior, and a short attention span, identifying this

syndrome in part as a conduct disorder. It is probably

not a single entity and the etiology is multifactorial.

Unfortunately, it is now accepted that as many as

50% of all children with this disorder show some of

its features in adulthood.

See also: Anorexia Nervosa; Dahi; Food Intolerance:

Types; Food Allergies; Lactose Intolerance;

Kwashiorkor; Macrobiotic Diets; Marasmus; Obesity:

Etiology and Diagnosis; Fat Distribution; Treatment;

Organically Farmed Food; Protein: Deficiency;

Vegetarian Diets

Further Reading

Bentley D and Lawson M (1988) Clinical Nutrition in

Paediatric Disorders. London: Baillie

re Tindall.

Bentley D, Lifschitz C and Lawson M (2002) Pediatric

Gastroenterology and Clinical Nutrition. London:

Remedica.

Callaghan AL, Booth IW, Moy RGD, Shaw NJ and Debelle

G (2002) Archives of Disease in Childhood 86 (suppl. 1):

A35–A36.

Ellis KJ and Nicholson M (1997) Leptin levels and body

fatness in children: effects of gender, ethnicity and sexual

development. Pediatric Research 42(4): 484–488.

Garrow JS, James WPT and Ralph A (2000) Human

Nutrition and Dietetics. Edinburgh: Churchill Living-

stone.

Mann J and Truswell S (2000) Essentials of Human

Nutrition. Oxford: Oxford University Press.

Stockman JA (2002) Yearbook of Pediatrics. St Louis:

Mosby.

Walker AF (ed.) 1990 Applied Human Nutrition for Food

Sciences and Home Economists. Chichester: Horwood

(Ellis).

Whitaker RC, Wright JA, Pepe MS, Seidel KD and Dietz

WH (1997) Predicting obesity in young adulthood from

childhood and parental obesity. New England Journal of

Medicine 25 337(13): 869–873.

World Health Organization (1998) Obesity: Preventing and

Managing the Global Epidemic. Geneva: World Health

Organization.

CHILL FOODS

Effect of Modified-atmosphere

Packaging on Food Quality

A G Smith, The University of Huddersfield,

Huddersfield, UK

B P F Day, Campden & Chorleywood Food Research

Association, Gloucestershire, UK

Introduction

0001 One form of food preservation technology that has

grown in importance in recent years, often in combin-

ation with chilled foods, is modified-atmosphere

packaging (MAP). The market for products using

MAP has grown rapidly because of the benefits that

this technique offers, particularly in respect to main-

taining the quality of food throughout the extended

shelf-life that the application of the method achieves.

Estimates of sales indicate that around 1.6 billion

modified-atmosphere (MA) packs of food were sold

in 1990; this had risen to 2.6 billion by 1997 and is

expected to exceed 3.3 billion in 2002.

0002MAP can be described as a food preservation tech-

nique in which the composition of the atmosphere

surrounding the food, inside its hermetically sealed

packaging, is different from the normal composition

of air. Within the food packages air is replaced by a

blanket of gas, a mixture of gases, or even no gas at all

in the case of vacuum-packing, which is a form of

MAP. Under MAP the growth of bacteria, molds, and

yeasts that cause food to deteriorate is inhibited and

other forms of physiological decay are slowed down.

When applied effectively, MAP extends the shelf-life

of food – the period of time for which it might be

considered to be of saleable quality.

0003Consumers increasingly demand the natural qual-

ities of fresh foods in raw, part-prepared, and even

ready-to-eat products, whilst also preferring them to

be free from additives. Retailers wish to provide the

widest possible range of products for the consumer to

choose from but need to be assured that items will

have a long enough life on the display shelf to make

them viable to stock. MAP contributes towards the

fulfillment of these diverse demands by extending

the shelf-life of food items whilst maintaining the

product’s original quality.

CHILL FOODS/Effect of Modified-atmosphere Packaging on Food Quality 1157

0004 Unlike controlled-atmosphere bulk storage, where

precise control of the gaseous components in a food

warehouse atmosphere is possible, in MAP there is no

way of varying the concentration of these compon-

ents once the packaging has been sealed. Highly per-

ishable foods will only achieve their full potential in

terms of extended life when packed in MA if they are

handled hygienically throughout any processing and,

in most cases, also held chilled.

0005 Chilled products already sold in MAP include

cheeses, cooked and cured meats, cooked and dressed

vegetable products, crustaceans, dairy and bakery

goods, fish, kebabs, pasta, pizza, red (uncooked)

meats, ready meals, whole and prepared fresh fruit

and vegetables (Figure 1), and this range continues to

expand rapidly.

0006 MAP cannot in itself enhance the quality of food,

be it raw or prepared in some way. The technology

can only be used to extend the reasonable life of a

food product because it can preserve food at or very

near to its original quality due to its ability to inhibit

the various forms of deterioration that naturally occur

when products are stored in air. Any positive impact

that MAP has on maintaining food quality will be

limited by the need for foods:

0007 to be of prime quality in all respects in the first

instance

0008 to be handled in an appropriate manner that min-

imizes risk from any form of contamination or

deterioration prior to and throughout the packing

process

0009to be held at appropriate temperatures after

packing and prior to sale (ideally between 0 and



C for chilled foods).

With these issues in mind, this chapter aims to outline

the effects of MAP on the quality of food purveyed to

consumers. The typical gases used in MAP and their

applications are briefly considered and the impact of

this technique on the quality of the major categories

of chilled foods is discussed.

The Gases Used in MAP

0010It is necessary for manufacturers to utilize food-grade

gases for the safe and hygienic MAP of food products.

These high-purity gases are produced in hygienic con-

ditions applicable to food production systems and are

stored in dedicated containers made and maintained

to the standards required in any food-handling envir-

onment.

0011The gases most commonly used for food MAP are

those found naturally in air: carbon dioxide (CO

nitrogen (N

), and oxygen (O

). The effectiveness and

safety of other gases that might be used for MAP are

being examined and gases such as argon, helium,

nitrous oxide, and sulfur dioxide are already permit-

ted for food use in the European Union (EU). Other

gases, such as carbon monoxide, hydrogen, and chlor-

ine, are also being investigated, although their com-

mercial application may be limited due to negative

effects on food quality and regulations relating to use.

0012Air consists of N

(78.09%), O

(20.95%), argon

(0.93%), and CO

(0.03%), and in the majority of

MAP applications, N

, and CO

are currently the

only gases used. The ratio of each gas employed in

MAP mixtures varies significantly from air according

to the food commodity being packed. It is the careful

maintenance and monitoring of the proportions of

each gas and the utilization of correct mixtures of

gases for specific products that are important to

insure that the benefits of MAP are fully exploited.

Carbon Dioxide

0013Whilst CO

provides particular benefits in concen-

trations that can enable the shelf-life of food to be

extended considerably, it can also have some disad-

vantageous effects on food quality. CO

effectively

inhibits most bacteria (aerobic strains) and molds

that need O

to survive. Generally, the greater the

concentration of CO

around packaged foods, the

longer its shelf-life can be extended.

0014CO

is easily absorbed into fats or water which are

key constituents of many perishable foods. The mild

carbonic acid that forms from the absorption of CO

Carcass

meat

31%

Cooked meat

and meat

products

15%

Snack foods

11%

Fish and shellfish

Fresh fruit

and vegetables

Ready meals

Poultry

Dried foods

Fresh pasta

Bakery products

Dairy products

Other

fig0001 Figure 1 Relative importance of major food sectors to the

modified-atmosphere packaging retail market (UK) 1997. Data

from MSI Data Report (1998).

1158 CHILL FOODS/Effect of Modified-atmosphere Packaging on Food Quality

into the food’s natural moisture has further pre-

servative effects. However, this factor can lead to

‘pack collapse,’ as the package’s gas volume reduces

due to such CO

absorption, and, although foods

might still be edible, it can also result in other un-

desirable changes in the sensory properties of some

foods.

0015 The determination of the proportion of CO

used in

MAP gas mixes will relate to achieving an acceptable

balance between extending shelf-life and avoiding the

potentially undesirable effects. However, the concen-

tration of CO

should not normally be below 20% if

it is required to inhibit mold growth effectively

(Table 1).

Nitrogen

0016 This inert gas is useful for excluding or reducing the

relative proportion of O

in the atmosphere around

food. It is also used to counter the effects of pack

collapse caused by the absorption of CO

by fatty or

moist food items, which is particularly likely to occur

under chilled storage.

0017 Whilst N

does not support the growth of aerobic

microorganisms, it is neither absorbed by foods nor

does it have the mildly acidifying effect of CO

. This

means that on its own it is not as effective as CO

for

inhibiting microorganisms. However, N

has little

impact on the sensory qualities of food which makes

it particularly useful for gas-packing foods that might

be adversely affected in this way by CO

, such as

creams and other dairy products (Table 1). N

also used in the head space of beverage containers,

to flush out O

, and in packaging dry food which does

not require the bacteria and mold-inhibiting proper-

ties of CO

yet needs to be protected from oxidative

deterioration.

Oxygen

0018It is the O

in the air that causes most of the spoilage

problems that MAP seeks to reduce. Not only is

oxygen needed for survival by many microorganisms,

it can also cause oxidative deterioration in foods.

Hence, MAP largely originated from the objective of

excluding O

from around foods to obviate potential

quality problems. However, there are applications

when a controlled amount of oxygen in MA gas

mixes is necessary when packing certain food com-

modities.

0019For example, O

can maintain acceptable color

quality in raw meats and is needed to avoid anaerobic

respiration in fruits and vegetables. O

is also used in

packing white fish and seafood products because it

inhibits Clostridium botulinum, a highly toxic anaer-

obic pathogen. These benefits mean that carefully

tbl0001 Table 1 Recommended gas mixtures for retail modified-atmosphere packaging of a selection of food commodities

Food item CO

(%)

Most raw red meats (beef, lamb, pork) 30 70

Venison/wild boar 20 80

Raw offal 20 80

Raw poultry/game

30 70

Raw fish and seafood crustaceans/molluscs 40 30 30

Raw oily fish 40 60

Cooked, cured, and processed meats 30 70

Cooked, cured, and processed fish/seafood 30 70

Cooked, cured, and processed poultry and game products 30 70

Ready meals and cook-chill products 30 70

Fresh pasta products 50 50

Bakery products 50 50

Dairy products – hard cheeses (not mold-ripened varieties)

100

Dairy products – grated and soft cheeses (not mold-ripened varieties)

30 70

Dairy products – other products, including butter, cream, custard, margarine, yogurts

100

Dried foods 100

Combination products (pies, pastries, pizzas, battered products) 30 70

Cooked/sauced/stuffed vegetable products 30 70

Most liquid food and beverages (beers, cider, cordials, yogurt drinks, oils, wines) 100

Carbonated drinks 100

Fresh whole and prepared fruit and vegetables 5 5 90

Gas mixtures relate to retail packaging only. Bulk and primal storage require different optimum gas mixes.

Dark poultry meat such as skinned and diced turkey leg meat requires a gas mixture similar to red meats to retain acceptable color quality.

and N

inhibit mold growth, therefore mold-ripened cheeses are not normally modified-atmosphere packaged.

Aerosol creams excepted, which are packaged under nitrous oxide (N

O).

Adapted from Air Products (1995) The Freshline Guide to Modified Atmosphere Packaging (MAP). Basingstoke, Hants: Air Products, with permission.

CHILL FOODS/Effect of Modified-atmosphere Packaging on Food Quality 1159

monitored amounts of O

may be used in MAP for

certain major food commodities.

0020 Recent research has indicated that a novel form of

MAP utilizing high levels of O

(over 70%) has the

potential to maintain both quality and food safety,

particularly for respiring fruit and vegetable items

and where products involve a combination of respir-

ing and nonrespiring chilled foods, such as sand-

wiches, pizzas, and ready meals.

0021 Recommended gas mixtures to be used for maxi-

mizing shelf-life whilst maintaining sensory quality of

a range of major food commodities being packed for

retail sale are summarized in Table 1.

Other Gases

0022 As noted, numerous other gases are being examined

for use in MAP. Some are already permitted for speci-

fied food use in the EU such as argon (E938), helium

(E939), nitrous oxide (E942), and sulfur dioxide

(E220). Of these, argon (Ar) appears to inhibit

enzyme activity and tissue fermentation more effect-

ively than N

. Ar may also be better at displacing O

from the cells of food than N

, so might further

reduce the occurrence of oxidative deterioration.

Nitrous oxide (N

O) and Ar are also claimed to en-

hance the effect of antimicrobial agents used in food

products. Research is continuing into the develop-

ment of novel forms of MAP using such alternative

gases.

Major Chilled Food Categories and the

Effect of MAP

Red Meat

0023 Today’s consumers demand bright red meat. A gener-

ation or more ago, butchers’ meat was expected to

have ‘hung’ well, and, if anything, exhibit a deep red

color, which was seen as an indicator of tenderness

and flavor, but most meat purchasers today would

probably infer that such meat was ‘past its best.’

Retailers are largely responsible for promoting and

displaying meat of a fresh red hue and this is now

perceived as a key quality indicator. MAP has a major

contribution to make in respect to extending the life

of meat in a desired red color. This is because gas

mixtures containing high levels of O

react with the

natural postmortem blood chemistry to maintain a

‘consumer-attractive’ appearance for longer. Typic-

ally, raw meat will have a shelf-life of up to 4 days,

but packaged in a modified gas mix, this can be

doubled.

0024 MAP producers acknowledge that gas mixes that

are high in O

contribute to the maintenance of color

quality preferred by today’s consumers. In fact, if

meat is packed in a CO

-rich atmosphere, excluding

which many microorganisms need to thrive, shelf-

life could be extended for considerably longer. How-

ever, the consequent innocuous darkening of the meat

color that would occur in the presence of CO

regarded as an undesirable quality characteristic and

hence this is not done.

0025Raw meats require cooking before they are edible.

If the cooking process is undertaken effectively, there

should be minimal risk of food poisoning. It is there-

fore considered that, whilst O

-rich atmospheres

might support aerobic microflora, the risks are ac-

ceptably small if meat is hygienically handled prior

to packing and held chilled prior to sale. Thus, MAP

protects the meat from contamination and enables

the maintenance of visually appealing fresh color

during the period of extended shelf-life, a quality

characteristic which is highly valued by both the

retailer and the consumer.

Cooked and Cured Meats

0026Cooked meats are considered a ‘high-risk’ food since

they do not undergo further processing prior to con-

sumption. It is therefore imperative that producers of

such products employ hazard minimization tech-

niques during handling, preparation, cooking, and

packaging of such foods. If this is done then MAP

has an important role to play in the safe extension of

product shelf-life whilst simultaneously insuring that

the product exhibits all the expected qualities of a

freshly cooked item.

0027As can be seen in Table 1, the gas mixture advised

for MAP of cooked meats and poultry excludes O

The cooking process will kill any vegetative patho-

gens and also fix color, unlike raw meat. Without the

need for O

for the maintenance of visual quality, a

mixture of N

and CO

is used for MAP, affording

the advantage of increased inhibition of microbial

deterioration. The maximal extension of shelf-life is

dependent on appropriate chilled storage, ideally be-

tween 0 and 3



0028Some lightly cured meats, such as bacon, may re-

quire cooking before consumption but because they

are usually treated with salts such as nitrites, their

pink coloring is assured, hence the same CO

gas mixture as for cooked meats is recommended.

Keeping quality of these products may be adversely

affected if lower salt formulations are developed

however. It is also imperative that appropriate storage

temperatures are rigorously maintained below 8



and preferably between 0 and 3



0029Other cured products, such as salami, do not need

further cooking and the curing process is a traditional

preserving method that may even be effective enough

1160 CHILL FOODS/Effect of Modified-atmosphere Packaging on Food Quality

to insure considerable shelf-life, even if stored in am-

bient conditions. However, fattier meat products

may be subject to oxidative rancidity, so packing in

an MA without O

can be useful to inhibit this

process.

0030 Although vacuum-packing has been successfully

employed in the production of retail cooked and

cured meat products with enhanced shelf-life, gas-

filled MAP provides a better-looking product that is

easier to handle after removal of the packaging. How-

ever, vacuum skin packaging (VSP), where a lightly

applied, highly ductile plastic barrier laminate forms

itself around the food during the vacuum process,

without squashing it, has been seen to be a particu-

larly useful innovation by retailers for some sliced

cooked meat products. VSP not only achieves the

benefit of easy separation of meat slices but also

enables packs to be stacked vertically for more at-

tractive and efficient merchandising.

Fish

0031 MAP is particularly useful for extending the shelf-

life of raw fish which normally deteriorates rapidly

after exposure to air. The controlled proportions of

gases used in MAP can contribute towards reducing

the formation of peroxides and other chemicals,

which create characteristic off-odors and taints in

fish.

0032 Fish are particularly susceptible to oxidative ran-

cidity; however, MAP gas mixtures cannot contain

too high a concentration of CO

alone as absorption

into the moist tissues will cause packs to collapse.

absorption also exacerbates drip and affects

color and flavor adversely, especially in seafood. O

must also be incorporated into the gas mixture used in

MAP of white fish, shellfish, and crustaceans to

insure sensory quality and also to inhibit the growth

of toxin-forming anaerobes, such as Clostridium

botulinum. Oily fish, however, such as mackerel, her-

ring, tuna, and salmon, need an MAP gas mixture

consisting of CO

(40%) and N

(60%) alone. This

is because oxidative deterioration is considered the

major potential quality problem.

0033 From catch through to MAP, fish must be handled

hygienically and rigorously stored in temperature-

controlled conditions. The fish species and season,

affecting such factors as fat content, will also influ-

ence the keeping quality of the product. The exten-

sion of shelf-life and the maintenance of acceptable

food quality using MAP can only be fully exploited if

the product being packaged is initially of prime

quality and has been handled appropriately at all

times prior to packaging. Overall shelf-life and ac-

ceptable quality can be prolonged by MAP for up to 6

days – around twice as long as fish would keep held

chilled in air.

Fruit and Vegetables

0034Fresh fruit and vegetables continue to respire and

ripen after harvesting. MAP has an important role

in extending the life of these commodities in that

it can be used to reduce the rate of respiration and

delay ripening. By slowing textural softening, chlor-

ophyl degradation, and other ripening processes and

by reducing the impact of chilling injury and other

physiological problems, product quality can be main-

tained for up to five times longer than if stored in air.

This means that shelf-life can be increased from 2 to 5

days for highly perishable items, and increased from

up to 7 to around 35 days for hardier fruit and vege-

tables. The potential increase in shelf-life that can be

attained through the use of MAP is inversely propor-

tional to the rate of respiration naturally occurring.

0035There are certain technical issues that must be con-

sidered when packaging fruit and vegetables in MAs.

The natural respiration process tends to deplete O

and cause CO

levels to increase. A minimum level of

is required in the gas mixture to avoid anaerobic

respiration occurring, as this would result in the de-

velopment of undesirable sensory qualities in the

food. Without any O

present there would also be an

increased risk of food poisoning by pathogenic anaer-

obes such as Clostridium species, the hardy spores of

which, should they be present in any soil remnants,

might germinate if storage temperatures are not well

controlled at any time.

0036The packaging itself must be carefully selected to

insure that the correct level of gas permeability can

occur. In order to achieve optimal conditions, the MA

within the packaging must be maintained such that

the gas mix contains between 2 and 10% of both O

and CO

. With commodities such as beansprouts,

broccoli, leeks, mushrooms, and peas, which exhibit

a high rate of respiration, traditional MAP films are

not permeable enough to maintain the gas mixture.

With these foodstuffs, a microperforated film is more

appropriate for their level of respiration, but has the

consequent disadvantage of potentially allowing in-

gress of microorganisms and other forms of organo-

leptic deterioration to occur.

0037Recent research has shown that freshly prepared

vegetables packed in high O

atmospheres (> 70%)

may offer a solution to the disadvantages of gas mix-

tures currently in use. High O

MAP has been shown

to prevent the formation of anaerobic fermentation

byproducts and inhibit both enzymic deterioration

and the growth of microorganisms. Research is cur-

rently continuing in this area.

CHILL FOODS/Effect of Modified-atmosphere Packaging on Food Quality 1161

Dairy Products

0038 Ideal gas mixtures for the range of dairy products

vary according to the likely types of deterioration to

which specific items are most susceptible (Table 1).

0039 As hard-cheese quality is most likely to be affected

by mold growth, an atmosphere consisting solely of

is most appropriate for MAP. Acceptable eating

quality can be maintained and shelf-life trebled from

around 4 weeks to up to 3 months by using MAP.

0040 Softer cheeses are more likely to deteriorate by

separation and can be affected by yeasts, bacteria,

and oxidative rancidity. A mixture of CO

and N

is recommended to counteract the likely pack-

deforming absorption of CO

by these moister prod-

ucts. For the most perishable products, MAP can help

to maintain quality whilst doubling normal shelf-life

to around 2 weeks and for many items this could be

considerably longer.

0041 It is because of the mold-inhibiting properties of

and N

that cheeses that incorporate natural

molds are not normally packed using MA technology.

0042 Creams and similar dairy products are packed

solely in N

because flavor quality would be adversely

affected by CO

absorption. N

would also be recom-

mended for packaging butter and yogurts should

MAP be applied to such products.

Ready Meals

0043The range of chilled products available from the

major retailers has increased greatly in recent

years. Major retailers have developed their own

brands, often in collaboration with large manufac-

turers who have the technical capability to produce

cook-chill products safely and hygienically.

0044Within the UK, current regulations stipulate that

the maximum shelf-life for cook-chill products util-

ized in commercial operations is limited to a

maximum of 5 days. This does not apply to retail

foods for the domestic market, which means that,

by using MAP, or by cooking under vacuum, as

in the sous-vide process, shelf-lives of ready meals

and the like can be safely extended considerably

further.

0045Whilst proper storage is essential, ideally below



C, food safety and eating quality can be main-

tained for up to 10 days without any appreciable

risk when using MAP technology or sous-vide

methods to produce such foods.

0046Temperature control is imperative throughout the

production process, be it to insure proper cooking, or

to insure the minimization of risk from bacterial

growth after chilling. This is particularly important

for preventing the germination of hardy spores of

tbl0002 Table 2 Examples of active packaging systems and their applications

Active packaging system Active element Actual/potential foodapplications

scavengers 1. Iron-based

2. Metal/acid

3. Metal catalyst (e.g., platinum)

4. Ascorbate/metallic salts

5. Enzyme-based

Bread, cakes, cooked rice, biscuits, pizza, pasta, cheese, cured

meats and fish, coffee, snack foods, dried foods and beverages

scavengers/emitters 1. Iron oxide/calcium hydroxide

2. Ferrous carbonate/metal halide

3. Calcium oxide/activated charcoal

4. Ascorbate/sodium bicarbonate

Coffee, fresh meats, and fish, nuts and other snack foods

products and sponge cakes

Ethylene scavengers 1. Potassium permanganate

2. Activated carbon

3. Activated clays/zeolites

Fruit, vegetables and other horticultural products

Preservative releasers 1. Organic salts

2. Silver zeolite

3. Spice and herb extracts

4. BHA/BHT antioxidants

5. Vitamin E antioxidants

Cereal, meats, fish, bread, cheese, snack foods, fruit and

vegetables

Ethanol emitters 1. Encapsulated ethanol Pizza crusts, cakes, bread, biscuits, fish and bakery products

Moisture absorbers 1. PVA blanket

2. Activated clays and minerals

3. Silica gel

Fish, meats, poultry, snack foods, cereals, dried foods,

sandwiches, fruit and vegetables

Flavour/odor absorbers 1. Cellulose triacetate

2. Acetylated paper

3. Citric acid

4. Ferrous salt/ascorbate

5. Activated carbon/clays/zeolites

Fruit juices, fried snack foods, fish, cereals, poultry, dairy

products and fruit

BHA, butylated hydroxyanisole; BHT, butylated hydroxytoluene.

1162 CHILL FOODS/Effect of Modified-atmosphere Packaging on Food Quality

Bacillus or Clostridium species which might remain

after cooking.

0047 The quality of cook-chill meals has improved dra-

matically in recent years, but MAP can only maintain

the quality of the product as originally produced. This

means that retailers must insure that product formu-

lation incorporates efficient organoleptic assessments

as well as risk control methods such as Hazard An-

alysis Critical Control Points (HACCP) if they wish to

guarantee that consumers receive products of accept-

able quality in all respects.

Conclusion – The Future of MAP

0048 Innovations in MAP and related packaging technolo-

gies are continuously being made. In recent years

these innovations have improved the utility and func-

tion of such packaging. This has resulted in MAP

providing additional benefits that have a positive

impact on the quality of food as it reaches the end

consumer, particularly through retailers.

0049 Innovations in film materials have allowed the de-

velopment of vacuum MAP methods such that VSP

(vacuum skin packaging) can be formed around food

without squashing it, unlike earlier vacuum pack

technology.

0050 Active packaging refers to systems and devices

which incorporate certain additives, such as oxygen

scavengers, carbon dioxide emitters, ethylene scaven-

gers, moisture absorbers, and other ‘active’ elements

(Table 2).

0051 The overall aim of these developments is to act

as ‘freshness enhancers’ and hence to achieve even

longer extensions in the shelf-life of foods, whilst

maintaining sensory and nutritional quality and mi-

crobial safety. Advances in biotechnology, the science

of materials and packaging methods are continuously

being made and hence will influence the future devel-

opment of MAP.

0052 Future innovations in packaging in general may be

driven by environmental concerns such as the impli-

cations arising from the increasing problem of dis-

posing of packaging waste. Consumers’ desire for

easier-to-use packs, along with other added-value fea-

tures, may also influence future developments in food

packaging. However, the potential to prolong the

shelf-life of food whilst retaining desirable fresh qual-

ities is likely to remain the key benefit and purpose of

employing MAP.

See also: Chilled Storage: Principles; Attainment of

Chilled Conditions; Quality and Economic Considerations;

Microbiological Considerations; Use of Modified-

atmosphere Packaging; Packaging Under Vacuum;

Controlled-atmosphere Storage: Applications for Bulk

Storage of Foodstuffs; Effects on Fruit and Vegetables;

Sensory Evaluation: Sensory Characteristics of Human

Foods; Food Acceptability and Sensory Evaluation;

Spoilage: Bacterial Spoilage; Fungi in Food – An

Overview; Molds in Spoilage; Yeasts in Spoilage

Further Reading

Air Products (1995) The Freshline Guide to Modified

Atmosphere Packaging (MAP). Basingstoke, Hants: Air

Products.

Day BPF (1997) Modified-atmosphere packaging markets,

Europe. In: Brody AL and Marsh KS (eds) The Wiley

Encyclopedia of Packaging Technology, 2nd edn, pp.

656–659. John Wiley.

Day BPF (1999) Principles – chilled storage of foods. In:

Robinson R, Batt C and Patel P (eds) Encyclopedia of

Food Microbiology, pp. 403–410. London: Academic

Press.

Day BPF (1999) Applying MAP to chilled ready meals. In:

Savoury Foods Industry Guide – Europe, pp. 32–34.

Uxbridge, Middlesex: Tusset RAI.

Day BPF (2000) Chilled food packaging. In: Stringer M and

Dennis C (eds) Chilled Foods – A Comprehensive Guide,

2nd edn, pp. 135–150. Cambridge, UK: Woodhead.

Farber JM and Dodds KL (eds) (1995) Principles of Modi-

fied-Atmosphere Packaging and Sous Vide Product

Packaging. Lancaster, Pennsylvania: Technomic.

O’Connor-Shaw RE and Reyes VG (1999) Use of modified-

atmosphere packaging – chilled storage of foods. In:

Robinson R, Batt C and Patel P (eds) Encyclopedia of

Food Microbiology, p. 410. London, UK: Academic

Press.

CHILL FOODS/Effect of Modified-atmosphere Packaging on Food Quality 1163

CHILLED STORAGE

Contents

Principles

Attainment of Chilled Conditions

Quality and Economic Considerations

Microbiological Considerations

Use of Modified-atmosphere Packaging

Packaging Under Vacuum

Principles

S James, University of Bristol, Langford,

North Somerset, UK

Basic Theory

0001 The complete chill chain for a food will contain many

of the following unit operations: prechilling prepar-

ation, primary or secondary chilling, storage, trans-

portation, and display. Storage, transport, and display

are temperature maintenance operations. Chilling

results in a substantial decrease in the mean tempera-

ture of the product, whereas during the preparatory

treatment, there can be a range of temperature

responses from a large gain to a small decrease in

the temperature of the foodstuff.

0002 The initial and final temperatures, and the max-

imum and minimum desirable rates of temperature

reduction during chilling, depend on the food being

processed. In all cases, the final product temperature

must be above the initial freezing point of the food,

but in a number of products, biological factors dictate

higher storage temperatures.

0003 The principle factors that control the rate of heat

and mass transfer during cooling are independent of

the foodstuff. Heat can be lost from the surface of a

body by four basic mechanisms: radiation, conduc-

tion, convection or evaporative cooling. (See Heat

Transfer Methods.)

0004 To determine the rate of heat transfer (Q

) by radi-

ation from food, the following approximation may

be applied:

¼ e A sðT

 T

Þ, ð1Þ

where e is the emissivity, A is the surface area of the

food, s the Stefan–Boltzman constant, and T

and T

the temperature of the surface and the enclosure,

respectively. To achieve substantial rates of heat loss

by radiation, large temperature differences between

the product and the enclosure are required. Such

differences are not normally present during food

cooling operations, except in the initial chilling of

bakery products. However, researchers in Russia

have used banks of refrigerated plates to enhance

radiant heat transfer during the chilling of beef

carcasses.

0005The rate of one-dimensional heat transfer (Q

)by

conduction is given by:

¼ kA

, ð 2Þ

where k is the thermal conductivity of the medium

through which the heat is passing, and qT/qx is the

temperature gradient.

0006Physical contact between the product and a cooler

surface is required to extract heat by conduction. The

irregular shape of most foodstuffs precludes this

mechanism in many applications. Plate coolers are

routinely used to freeze fish and offal, which have

soft pliable surfaces. Belt freezers are used for steaks,

fish fillets, and other thin products. However, the rate

at which heat can be conducted away from the

surface is not the sole criterion that governs the time

taken to cool a product. Heat must also be conducted

from within the product to its surface before it can be

removed. Most foodstuffs are poor conductors of

heat, and this imposes a severe limitation on attain-

able chilling times for either large individual items or

small items cooled in bulk.

0007The rate of mass transfer (M) from the surface of an

unwrapped food is described by the equation::

M ¼ mAðP

 P

Þ, ð3Þ

where m is the mass transfer coefficient, A the area, P

the saturated vapor pressure at the surface, a

the

water activity, and P

the vapor pressure above the

food surface. Q

, the heat loss due to evaporation,

can be obtained by multiplying M by the appropriate

latent heat. The heat lost in the evaporation of water

from the surface of the product is a minor com-

ponent of the total heat loss for most foodstuffs. In

1164 CHILLED STORAGE/Principles

spray-cooling systems, the rate of heat removal is

enhanced by evaporation from the surface. However,

the main advantage of spray cooling is a reduction in

weight loss. In a number of specific cases, i.e., vacuum

cooling, evaporation is the primary cooling agent.

0008 Most food-chilling systems rely on convection

as the principal means of heat removal. The rate of

one-dimensional heat transfer (Q

) by convection is

given by::

¼ h

AðT

 T

Þ, ð4Þ

where h

is the convection or film heat transfer coef-

ficient, and T

is the temperature of the cooling

medium. The most common media are air or water,

although sugar solutions, salt brines, and other

refrigerants have been used. Each combination of

product and cooling system can be characterized by

a specific surface heat-transfer coefficient. The value

of the coefficient depends on the shape and surface

roughness of the foodstuff and, to a much greater

degree, on the thermophysical properties and velocity

of the medium. Values range from 5 W m

2

1

for

still air to over 500 W m

2

1

for agitated water.

Chilling Methods for Solid Foodstuffs

Moving Air

0009 This is the most widely used method as it is econom-

ical, hygienic, and relatively noncorrosive to equip-

ment. Systems range from the most basic, in which a

fan blows the cooled air around an insulated room, to

products on conveyors passing through blast chilling

tunnels or spirals. Low rates of heat transfer are

attained in air-cooling systems, but this is not import-

ant if conduction within the product is the rate-

controlling factor. A major disadvantage is excessive

dehydration from the surface of any unwrapped

product, whilst the need to avoid surface freezing

limits the lowest air temperature that can be used. In

practice, air distribution is a major, often overlooked,

problem.

0010 A batch system where warm food is placed in

refrigerated rooms is the most common method of

chilling. Individual items such as carcasses, tuna,

or bunches of bananas are hung from rails, smaller

products are placed on racks or pallets, and bulk

fruits and vegetables are placed in large bins.

0011 Conveying products overcomes problems of

uneven air distribution since each item is subjected

to the same velocity/time profile. In the simplest

continuous systems, the food is suspended from an

overhead conveyer and moved through a refrigerated

room. This process is often used in air chilling of

poultry or in prechilling of pork carcasses. Some

small cooked products are continuously chilled on

racks of trays, which are pulled or pushed through a

chilling tunnel using a simple mechanical system. In

more sophisticated plants, the racks are conveyed

through a chilling tunnel in which the refrigeration

capacity and air conditions can be varied throughout

the length of the tunnel.

0012In larger operations, it is more satisfactory to

convey the cooked products through a linear tunnel

or spiral chiller. Linear tunnels are far simpler con-

structions than spirals but require more space.

Hydrocooling/Immersion Cooling

0013Hydrocooling is probably the least expensive method

of achieving rapid cooling in small products. The

product is immersed in, or sprayed with, cool water,

either at ambient or near 0



C. Practical systems vary

from simple unstirred tanks to plants where the prod-

uct is conveyed through agitated tanks or under banks

of sprays. Such systems are typically used for celery,

asparagus, peas, sweet corn, carrots, peaches, etc.

0014Immersion chilling also has application with larger

products. Most poultry to be frozen is initially cooled

by immersion in chilled water or an ice/water mix-

ture. The procedure is very fast, and the birds gain

weight during the process. The maximum weight gain

is controlled by legislation.

0015An alternative system to air or immersion is

spray chilling. Practical spray-chilling systems use a

combination of air and sprays for the initial part of

the chilling period and then air only for the rest of the

chilling cycle. The sprays of cold water at 2–3



C are

applied not continuously but in short bursts. The

main advantage claimed for these systems is reduced

weight loss.

Plate Cooling

0016With thinner materials, a plate-cooling system has the

potential to halve the cooling time required in an air-

blast system. Continuous horizontal and rotary plate

freezing systems have been produced commercially

and could be modified to operate at higher tempera-

tures. They tend to be expensive, especially if auto-

matic loading and unloading are required, but have

low running costs. Designs have been produced for

continuous belt coolers similar to those used for belt

freezing.

Ice or Ice/Water Chilling

0017Chilling with crushed ice or an ice/water mixture is

simple, effective, and commonly used for fish cooling.

The individual fish are packed in boxes between

layers of crushed ice, which extract heat from the

fish and consequently melt. The temperature of the

CHILLED STORAGE/Principles 1165