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

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See also: Chilled Storage: Principles; Attainment of

Chilled Conditions; Quality and Economic Considerations;

Microbiological Considerations; Use of Modified-

atmosphere Packaging; Packaging Under Vacuum; Chill

Foods: Effect of Modified-atmosphere Packaging on Food

Quality; Cleaning Procedures in the Factory: Types of

Detergent; Types of Disinfectant; Overall Approach;

Modern Systems; Food Poisoning: Classification;

Tracing Origins and Testing; Statistics; Economic

Implications; Spoilage: Bacterial Spoilage

Further Reading

Alcock SJ (1987) Growth characteristics of food-poisoning

organisms at suboptimal temperatures. Campden Food

Preservation Research Association Technical Memoran-

dum No. 440.

Bell C and Kyriakides A (1998) Listeria: A Practical Ap-

proach to the Organism and its Control in Foods.

London: Blackie Academic & Professional.

Betts GD and Everis L (2000) Shelf-life determination and

challenge testing. In: Stringer MF and Dennis C (eds)

Chilled Foods: A Comprehensive Guide,2

edn, pp.

259–286. Cambridge, UK: Woodhead.

Borch E, Kant-Muermans M-L and Blixt Y (1996) Bacterial

spoilage of meat and cured meat products. International

Journal of Food Microbiology 33: 103–120.

Cousin MA (1982) Presence and activity of psychrotrophic

microorganisms in milk and dairy products: a review.

Journal of Food Protection 45: 172–207.

Dainty RH (1996) Chemical/biochemical detection of spoil-

age. International Journal of Food Microbiology 33:

19–34.

Filtenborg O, Frisvad JC and Thrane U (1996) Moulds in

food spoilage. International Journal of Food Microbiol-

ogy 33: 85–102.

Gould GW and Russell NJ (1991) Major food-poisoning

and food-spoilage microorganisms. In: Russell NJ and

Gould GW (eds) Food Preservatives. London: Blackie

Academic & Professional.

Huis in’t Veld JHT (1996) Microbial and biochemical spoil-

age of foods: an overview. International Journal of Food

Microbiology 33: 1–18.

Martens T (1999) Harmonisation of Safety Criteria for

Minimally Processed Foods: Rationale and Harmon-

isation Report. European Commission FAIR CT96–

1020.

Mitscherlich E and Marth EH (1984) Microbial Survival in

the Environment. Berlin: Springer-Verlag.

Pitt JI and Hocking AD (1985) Spoilage of fresh and perish-

able foods. In: Fungi and Food Spoilage. Sydney: Aca-

demic Press, pp. 365–382.

Walker SJ and Betts GD (2000) Chilled foods microbiology.

In: Stringer MF and Dennis C (eds) Chilled Foods: A

Comprehensive Guide, 2nd edn., pp. 153–186. Cam-

bridge, UK: Woodhead.

Whitfield FB (1998) Microbiology of food taints. Inter-

national Journal of Food Science and Technology 33:

31–51.

Use of Modified-atmosphere

Packaging

T R Robertson, Massey University, Palmerston North,

New Zealand

Definition of Terms

0001‘Modified atmosphere packaging’ (MAP) is the term

applied to different packaging technologies that use

an internal atmosphere composition that has a differ-

ent composition to air (20.6% oxygen/78% nitrogen)

to increase the shelf-life of food products. The in-

ternal atmosphere is usually made up of mixtures of

oxygen (O

), carbon dioxide (CO

) and nitrogen (N

with the possible addition of very small amounts of

other gases such as carbon monoxide (CO), ethanol,

sulfur dioxide (SO

) and noble gases (e.g., argon).

An alternative often used to describe MAP is ‘gas

packaging.’

0002‘Controlled atmosphere packaging’ is sometimes

used incorrectly to describe modified atmosphere

packages, as it is not possible to control the atmos-

phere inside a primary package (the package that is

directly in contact with the product), but it could be

used to describe the controlled storage conditions

inside specialist shipping containers.

0003‘Vacuum packaging (VP)’ involves the ‘complete’

removal of air from the package, usually resulting in

levels of below 1%. The product is placed in an

oxygen barrier film, the air evacuated, and the prod-

uct sealed. The evaluated pack collapses around the

product. In the case of ‘skin packaging’ systems, the

packaging material forms a second skin around

the product. Skin packaging involves placing a prod-

uct on to a substrate material such as a thermoformed

plastic tray. A specialized polymer film is heated to

the required softening temperature then draped over

the product and tray, and a vacuum draws the film

down and around the product and the tray to make a

secure and attractive product. This packaging system

has been used very successfully to retail shelled shell-

fish and marinated steak products.

0004‘Active packaging’ is the inclusion of specific addi-

tives into the packaging film or within the packaging

to modify the headspace atmosphere and extend the

product shelf-life. Active packaging systems include

oxygen absorbents, carbon dioxide adsorbents or

emitters, ethanol emitters, and ethylene absorbents.

This is a developing technology and will gain more

acceptance as system costs are reduced, although the

use of oxygen absorbents is well established in Japan.

1186 CHILLED STORAGE/Use of Modified-atmosphere Packaging

0005 Although the benefits of MAP can be numerous, it

is important that the microbiological safety consider-

ations of using anaerobic conditions are understood.

Anaerobic conditions can promote the growth of

pathogens, and hurdles must be in place to insure

the safety of the products. This is especially true of

high-moisture foods, meat, fish, and ready-to-eat

meals, where temperature control is critical to insure

safe MAP foods. The use of good manufacturing

practices, to insure that the packaged product has a

low initial microbial count, and the use of MAP can

provide a useful extension to the product shelf-life.

The control of the temperature during the storage and

distribution cool chain is equally important to MAP

fruit and vegetables where temperature abuse can

cause increased biochemical activity, thereby also

increasing the rate of product-quality deterioration.

Principal Factors in MAP

0006 The principal factors in a successful MAP operation

are the selection of:

0007 a gas or gas mixture to minimize the selected

deteriorative reactions in the food to insure the

required shelf-life;

0008 a suitable packaging material to maintain the

desired gas composition; and

0009 an appropriate packaging machine.

For MAP to provide the stated shelf-life, the critical

factor is control of the chill chain (refrigeration

systems in the processing, storage, distribution, and

retailing chain) to insure that no temperature abuse

(i.e., temperatures outside the specified range for a

particular product) occurs.

Choice of Gas

0010 The choice of gas, or gas mixture, used to replace the

air in the package requires knowledge of the nature of

the food and its principal mode(s) of deterioration.

Oxidative reactions and the growth of microorgan-

isms are the two common deteriorative modes, and

both are affected by the oxygen present in the pack-

age atmosphere. Reduction or removal of oxygen

will reduce the oxidation reactions and prevent the

growth of some microorganisms, whereas addition of

will inhibit the growth of other microorganisms.

However some microbes, including some pathogens

are not affected by high CO

and low O

levels and

must be controlled by good manufacturing practices

to insure low microbial numbers and temperature

control.

0011 Oxygen is a key component of oxidative degrad-

ation reactions occurring in food, combining readily

with fats and oils causing rancidity, and other food

components causing color, flavor, and odor changes.

Oxygen is, however, a necessary requirement for

normal respiration metabolism of whole and minim-

ally processed fresh fruit and vegetables. Oxygen

levels below the minimum required (as low as 1–

2%) by the produce can result in anaerobic respir-

ation and the development of off-flavors and aromas.

0012Carbon dioxide is present in the atmosphere in very

low levels (0.03%). It is highly soluble in water, and

will be absorbed by the food until equilibrium is

attained. For example, at 0



C and a partial pressure

of 101 kPa of CO

, the solubility of CO

is 3.4 g per

kilogram of water. The absorption of the CO

by the

food in a sealed package creates a partial pressure

difference between the inside and outside of the pack-

aging, resulting in partial or complete collapse of the

package around the food. This CO

absorption can

also result in off-flavors, excess purge by muscle

foods, and discoloration of fresh produce.

0013Nitrogen is used to purge air from a package to

achieve sufficiently low levels of oxygen to prevent

aerobic microbial spoilage. It is also used as a filler

gas in MAP to reduce the concentration of other gases

in the package and to keep the pack from collapsing

as the CO

is absorbed by the product. The percent-

age of N

used in gas mixtures in combination with

often has to be determined experimentally to

obtain the necessary positive effects of the CO

while maintaining the appearance of the pack with

the level of N

Packaging-material Selection

0014Most MAP relies on the correct choice of packaging

material. A low water-vapor transmission rate and a

high gas barrier are usually required to maintain the

modified atmosphere, except for fresh produce where

specific barrier properties are required to maintain

the specified gas mixture, often using quite permeable

materials. Most MAP are based on monolayer or

multilayer thermoplastic polymers, and/or polymer

trays to maintain the product and atmospheric integ-

rity. High barrier multilayer films will often incorpor-

ate either metalized film or an aluminum foil layer,

and the cost of the film usually increases with the

increasing barrier properties. These films will still

allow some gas transmission, even at chill tempera-

tures. Thus, over the shelf-life of the MAP food, there

will be movement of gases across the package. The

movement of these gases can be mathematically mod-

eled to allow accurate prediction of this gas move-

ment, although there is a comparative lack of gas

permeability data for thermoplastic polymers over

the range of temperatures 0–40



C and at the high

humidity levels common in chilled food storage

conditions.

CHILLED STORAGE/Use of Modified-atmosphere Packaging 1187

0015 The packaging material needs to have the necessary

tensile strength to withstand machine handling, and

subsequent storage, distribution, and retailing. The

package must also be designed so that the end user

can open, heat (if required), and consume the food

easily.

Applications of MAP

Red Meats

0016 Fresh red meat undergoes both oxidation, resulting in

color and flavor changes, and microbial deterior-

ation. The color of the meat is determined by the

oxidation state of the pigment myoglobin. The oxy-

genated state, oxymyoglobin, is a bright red color

associated by the consumer with fresh, high-quality

red meat, whereas the reduced form, deoxymyoglo-

bin, is a purple color. The surface of the meat is

contaminated during the slaughtering process with

microorganisms, and the presence of O

encourages

the spoilage bacteria, especially the Pseudomonas

species, which will produce putrid, unpleasant smell-

ing compounds. The systems used for MAP of fresh

meat can be classified into the following groups:

0017 VP involves the exclusion of air from the pack

atmosphere, so the lack of oxygen prevents the

oxidation deteriorative reactions and suppresses

the growth of spoilage bacteria, especially the Pseu-

domonas species. The delay of these forms of de-

terioration through packaging significantly extends

the shelf-life of chilled fresh meat. VP has been used

for 2–9-kg primal cuts and subprimal cuts of meat

for international and local distribution to retail

outlets. These packs use high-gas-barrier films

that often include shrink characteristics. The meat

cut is placed into the bag, with any necessary

protection to prevent the bone piercing the bag,

vacuum-sealed, and then passed under a hot-

water spray to shrink the bag tightly over the cut.

Shrinking the bag produces a second skin effect on

the product, eliminates folds and capillaries, im-

proves seal strength and oxygen barrier, and re-

duces purge or drip from the meat. However, VP

without the shrink will still provide a useful exten-

sion to fresh chilled meat shelf-life, for beef (10–12

weeks) and lamb (6–8 weeks).

0018 High-oxygen MAP systems have atmosphere gas

mixtures of about 30% CO

and up to 70% O

to extend the color stability and delay the microbial

spoilage of display packaged meat. This system is

very suitable for centralized production of retail

display meat packages where the oxygen insures

that the meat has the required red color of fresh

meat. These systems are not suitable for long

storage times, where the high levels of oxygen can

lead to off-odors and rancidity.

0019Low-oxygen MAP includes mixtures of CO

and

, with less than 1% oxygen, and achieves a shelf-

life similar to that of VP. One-hundred percent

nitrogen flush can be used and has been shown to

be as effective as VP with reduced purge.

0020High-CO

MAP has very high levels of CO

, and in

some commercial operations includes 100% CO

atmosphere. The absence of O

retards the forma-

tion of brown metmyoglobin in the meat, and the

suppresses the growth of spoilage bacteria.

However, because CO

is very soluble in both

water and fat, especially at chiller temperatures,

excess CO

must be added to the package to

allow for solubility. Thus, the package must be

overflushed with CO

, then allowed to remain in

the chiller for 24 h to allow for the CO

adsorption

and pack-size reduction, before the final cartoning

operation can be completed. These 100% CO

packages have resulted in a chilled shelf-life for

lamb and beef of up to 16 weeks.

Poultry

0021Poultry are usually contaminated with large bacterial

populations that consist primarily of spoilage bac-

teria but can include significant numbers of patho-

gens. Poultry has a higher pH compared with red

meat and so provides an ideal environment for the

growth of these bacteria. Vacuum packages are diffi-

cult to form around whole dressed birds because of

their irregular shape. Thus, the shelf-life of vacuum-

packed poultry is usually short, limited to about 2

weeks. There are now an increasing range of cut-up

portions (e.g., breasts, wings, fillets) and other further

processed products, both raw and cooked, available.

High-CO

atmospheres will increase the product

shelf-life. For chicken meat, 20% CO

/70% N

can be as effective as 80% CO

/20% N

mix over

1 week’s storage at 2



C and thus is very useful for

retail packs. However, if a longer shelf-life is required,

as in bulk packs, higher levels of CO

are needed. A

storage life of up to 35 days has been achieved under

certain closely controlled conditions, although

commercially, 10–21 days at 1toþ2



C would

be expected using an 80–100% CO

/0–20% N

mixture.

Fish and Shellfish

0022Fish encompasses a great diversity of species and

habitats. Fish differ from terrestrial foods in that the

1188 CHILLED STORAGE/Use of Modified-atmosphere Packaging

interior of their muscles is not sterile, they undergo

rapid enigmatic breakdown of their proteins, even in

low-oxygen environments, and they often grow in

uncontrolled environments. Thus, fish are only as

clean as the water from which they are taken. Thus,

the commercial use of MAP to extend the shelf-life of

fish has been limited by the potential of Clostridium

botulinum nonproteolytic types B and E, Vibrio para-

haemolyticus, and Listeria monocytogenes growth

and toxin production in refrigerated MAP fish, with-

out any evidence of spoilage.

0023 The gas mixtures used vary according to the fish

species, with low oxygen concentrations being used

with fatty fish to suppress oxidative rancidity. Vacuum

and MAP using high CO

have been used to extend

the short shelf-life of various kinds of fish, e.g., gas

mixtures of 30% O

, 40% CO

and 30% N

for

nonfatty fish and 40% CO

and 60% N

for smoked

and fatty fish.

0024 As inhibition of normal spoilage bacteria of fish is

limited, extensions of shelf-life are not usually as

dramatic as that which can be achieved with meat,

and the need to maintain low temperatures through-

out processing, storage, distribution, and retailing is

not diminished. If the effort that is widely applied to

controlling temperatures and turnover during pro-

duction, distribution, and storage of MAP had been

applied to earlier packaging systems, these may have

provided similar shelf-life extensions as the MAP

system.

Fruits and Vegetables

0025 Fresh, whole, and minimally processed fruits and

vegetables are living, respiring tissue, and therefore,

under MAP conditions, care must the taken to insure

that the gas mixture in the atmosphere does not

become depleted of oxygen or contain too much

. The increased shelf-life or storage time for pro-

duce comes with a combination of lower tempera-

tures and lower O

levels, thereby slowing down the

rate of respiration. Reduced respiration leads to a

reduced depletion of carbohydrate reserves, hence

less weight loss, slower ripening, and longer storage

times. This suppression of the rate of respiration con-

tinues down to 2–4% O

, depending on the produce

species and storage temperature. At lower levels, the

respiration becomes anaerobic, resulting in fermenta-

tion metabolism producing off-flavors, off-odors, and

undesirable volatiles. The increasing CO

level in the

gas mixture also helps suppress the rate of respiration

in some produce. Reduced O

levels and increased

levels together can reduce respiration more

than either alone. The lower limits for O

and the

upper limits of CO

vary between different fruit and

vegetable species and can vary within one produce

species as a result of different growing and handling

conditions. Temperature control of fruit and vege-

table MAP is very important because the rate of

change in the gas permeability of the polymeric

films and the rate of respiration of the produce are

different with the same temperature changes. The

produce will use up the available oxygen very rapidly

and start anaerobic respiration. To increase the com-

plexity of fresh produce MAP, the selection of the

packaging film is difficult as it has to release CO

a rate similar to the production and let oxygen into

the pack at the rate at which it is used by the produce.

Thus, different produce will require a film with dif-

ferent gas transmission rates to insure the correct

maintenance of the desired gas mixture. Despite

almost 60 years of research and hundreds of publica-

tions reporting many experiments that have been

conducted in this area, there is surprisingly little com-

mercialization of MAP for horticultural produce.

Current areas of research and commercialization in-

clude the development of models to aid the design of

perforated film packaging to minimize weight loss

(moisture loss) while controlling the development of

a modified atmosphere. The use of high-oxygen 30–

80-kPa atmospheres and noble gases, specifically

argon modified atmosphere to extend the produce

shelf-life. Current research indicates that exposure

to superatmospheric O

concentration may stimulate,

have no effect on, or reduce the rates of respiration

and ethylene production, depending on the commod-

ity, maturity and ripeness stage, O

concentration,

storage time and temperature, and concentrations of

and C

present in the atmosphere. The rapid

growth of fresh-cut or minimally processed produce

including retail salads, baby peeled carrots, apple

slices, and many fruit and vegetables prepared

for the service industry has been possible due to

the improved quality and shelf-life of cut produce

in MAP.

0026There are two methods of creating modified atmos-

phere (MA) conditions within packages of fruit and

vegetables: passive MA and active MA. Passive MA

involves sealing the produce in the bag and letting the

product respiration and the film gas transmission rate

develop the required gas mixture. Alternatively,

the desired gas mixture can be added during bag

sealing, and the respiration and film gas transmission

rates will maintain this gas mixture. However, it is

the produce respiration rate, package oxygen and

carbon dioxide gas transmission rates, and storage

temperature that determine the composition of the

pack atmosphere, not the initial gas mixture used in

the bag.

CHILLED STORAGE/Use of Modified-atmosphere Packaging 1189

Ready-to-eat Meals, ‘Meal Solutions,’ Delicatessen

Foods

0027 Ready-to-eat meals, or prepared food products, are a

rapidly increasing supermarket category involving

chilled meals that only require the minimum of prep-

aration, if any, before consumption. These products

can be referred to as ‘delicatessen’ and include multi-

component food products. For a product to be eaten

hot, the only preparation is heating, whereas for

products like salads and sandwiches, no preparation

is required. These ready-to-eat meals can range in

quality from cheap meals, or snacks to expensive

gourmet-style food dishes, and include pasta, pizza,

precooked meats, sandwiches, precooked French

fries, complete dishes, meat based dishes, pasta

based dishes, and chilled bakery products, and

includes ‘sous vide’ or cook/chill products. MAP

is being used to increase the shelf-life of these

products, along with control of the storage, distribu-

tion, and retail chill chain to prevent temperature

abuse.

0028 The shelf-life of these products is usually limited by

two factors: microbial growth and oxygen sensitivity

of the product. Thus, the packaging system used for

ready-to-eat products should exclude O

and provide

some microbial suppressing action, and usually in-

volves a mixture of CO

and N

. These atmospheres

provide several advantages, including reduction of

oxidative rancidity, lack of growth of aerobic spoilage

organisms, suppression of mold growth by CO

minimal moisture loss through the package, and re-

duced oxidative breakdown of flavor and aroma

volatiles.

0029 Prepackaged sandwiches have grown in popularity

over the last decade, and their sale has extended from

large retail outlets to snack bars, lunch bars, and

petrol service stations. An increase in shelf-life would

be of great advantage in allowing more flexibility in

the distribution of these products. The hygiene and

safety of these MAP products during manufacture

and the use of good manufacturing practices, com-

bined with the use of good refrigeration practice,

are critical. Typical gas compositions include 30%

/70% N

,5%CO

/95% N

or a minimum of

70% CO

. The higher rate of of CO

can cause

pack collapse, as the gas is absorbed by the product,

hence the use of higher levels of N

. The specific

combination used will be a compromise, delaying

microbial growth without causing collapse of the

packs.

0030 ‘Sous Vide’ involves vacuum packaging of a pre-

pared food in a multilayer composite polymer film

pouch, cooking the packaged product, followed

by rapid cooling, and then chilled storage. These

products can have a shelf-life of 2–3 weeks under

optimum chiller conditions. An alternative style of

high-quality ready-to-eat meals involves precooking

the individual food components and then assembling

these components into the package before gas pack-

aging with the desired atmosphere of CO

and

chilling.

0031Modified atmosphere packaging for fresh pasta

products is common. Typical gas compositions in-

clude 100% N

or 70–80% CO

and 20–30% N

mixtures resulting in a shelf-life, if stored under 4



of up to 4 weeks.

See also: Clostridium: Occurrence of Clostridium

botulinum; Contamination of Food; Convenience

Foods; Fish: Spoilage of Seafood; Meat: Preservation;

Oxidation of Food Components; Packaging: Packaging

of Liquids; Packaging of Solids; Aseptic Filling; Spoilage:

Bacterial Spoilage

Further Reading

Barry-Ryan C and O’Briene (1998) Novel high oxygen and

nobel gas modified atmosphere packaging for extending

the quality and shelf-life of fresh prepared produce. IIR

Proceedings Series ‘Refrigeration Science and Technol-

ogy’ 6: 417–424.

Blakistone BA (1998) Principles and Applications of Modi-

fied Atmosphere Packaging of Foods, 2nd edn. London:

Blackie Academic & Professional.

Brody AL (ed.) (1989) Controlled/Modified Atmosphere/

Vacuum Packaging of Foods. Trumball, CT: Food and

Nutrition Press.

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

fied-Atmosphere and Sous Vide Product Packaging.

Lancaster, PA: Technomic.

Gill CO (1990) Controlled atmosphere packaging of chilled

meat. Food Control 2: 74–78.

Inns R (1987) Modified atmosphere packaging. In: Paine

FA (ed.) Modern Processing, Packaging and Distribu-

tion Systems for Food. Glasgow: Blackie & Son.

Kader AA and Ben-Yehoshua S (2000) Effects of super-

atmospheric oxygen levels on postharvest physiology

and quality of fresh fruits and vegetables. Postharvest

Biology and Technology 2: 1–13.

Kader AA, Zagory D and Kerbel (1988) Modified atmos-

phere packaging of fruits and vegetables. CRC Critical

Review of Food Science and Nutrition 28(1): 1–30.

Matz SA (1989) Bakery Technology: Packaging, Nutrition,

Product Development, Quality Assurance. Barking, UK:

Elsevier Science.

Robertson GL (1993) Food Packaging: Principles and

Practice. New York: Marcel Dekker.

Rooney ML (1995) Active Food Packaging. London:

Blackie Academic & Professional.

Zagory D (1997) Modified atmosphere packaging. In:

Brody AL and Marsh KS (eds) The Wiley Encyclopedia

of Packaging Technology. New York: Wiley.

1190 CHILLED STORAGE/Use of Modified-atmosphere Packaging

Packaging Under Vacuum

M Rossi, Packaging Technical Center, Sealed Air s.r.l.,

Passirana di Rho, Milano, Italy

The Role of Oxygen in Food Spoilage

0001 The presence of oxygen is one of the major factors of

spoilage of foods and causes:

0002 oxidation reactions, damaging vitamins, fatty

substances, pigments, flavoring substances that

are often catalyzed by enzymes;

0003 growth and activity of aerobic microorganisms

(aerobic bacteria, yeasts, molds). (See Oxidation

of Food Components.)

0004 It is therefore essential, in order to prolong the

freshness of foodstuffs, to eliminate the presence of

oxygen in contact with the foodstuff itself and to

prevent further access of oxygen during storage.

Vacuum packaging is one of the simplest and most

widely used systems to achieve this objective. This

type of packaging has enjoyed increasing success

since the late 1950s, in parallel with the development

of the technology of plastics owing also to the

changes in the distribution chain of perishable foods

requiring increased hygiene and storage life.

0005 This article outlines the principles of vacuum-

packaging technology and the features of the

packages utilized, and describes the main fields of

application of vacuum packaging to chilled foods.

Vacuum Packaging: A Definition

0006 ‘Vacuum packaging’ is a term improperly but com-

monly used to define a packaging system that implies

the reduction of the partial pressure of atmospheric

gases (oxygen being 20% of them) inside a package.

A vacuum inside a package can be obtained mainly

through two systems:

0007 steam flushing of the headspace;

0008 sucking of air from the package headspace by

means of equipment (vacuum chamber, nozzle)

based on a vacuum pump.

The former system is mainly utilized in hot packaging

of shelf-stable products, which are generally pack-

aged in rigid containers. The elimination of air is

achieved by means of a steam flush that replaces

atmospheric gases inside the package; steam conden-

sation in the package headspace after chilling reduces

the inner gaseous pressure. The latter system is com-

monly utilized for packaging of perishable foodstuffs

that have to be stored in chilled conditions. In this

case, packaging equipment based on a vacuum pump

is generally utilized in combination with flexible

packages, which, after evacuation, are closed hermet-

ically (by means of a seal or sometimes a tight clip)

and collapse on the packaged product once the pack-

age is exposed to atmospheric pressure. Depending on

the type of equipment employed, a residual pressure

of atmospheric gases as low as 500 Pa can be obtained

in the package.

0009Figure 1 illustrates the main systems utilized for

evacuating flexible packages:

0010nozzle (Figure 1a);

0011single vacuum chamber (Figure 1b);

0012divided vacuum chamber (Figure 1c).

Packaging Materials Utilized for Vacuum

Packaging

0013Plastics are the main raw material utilized for manu-

facturing the flexible materials employed in vacuum

packaging of chilled foods. These materials must have

the following properties:

0014flexibility;

0015mechanical resistance to various forms of abuse

(abrasion, puncture, flex cracking);

0016gas-barrier properties adequate to the application

requirements (generally expressed as gas perme-

ation rates);

0017thermosealability or clippability;

0018good optics (haze and gloss);

0019printability;

0020suitable dimensional behavior (dimensional stabil-

ity to heat, formability after heating, shrinkability

after heating).

To combine and balance to the required level the

above properties, it is often necessary to mix different

types of individual materials. Different resins can be

mixed together (resin blends), resin additives (such as

plasticizers, pigments, stabilizers, slip agents, etc.)

can be used, and, more commonly, a multilayer

material is produced, each layer being composed of

a distinct individual material that imparts its proper-

ties to the overall structure.

0021Individual components most commonly used in

manufacturing flexible materials utilized for vacuum

packaging are listed below, together with their abbre-

viations and main properties:

0022polyethylene (PE): sealability, formability, moisture

barrier, low cost;

0023polypropylene (PP): moisture barrier, thermal

resistance, dimensional stability;

CHILLED STORAGE/Packaging Under Vacuum 1191

Atmospheric pressure

Atmospheric pressure Atmospheric pressure

Product

Vacuum pump

(a)

Ballooning Ballooning

Closing device open

Product

Step a

Step b

Step c

Step b

Step a

Package is closed

(b) (c)

fig0001 Figure 1 Main systems utilized for obtaining vacuum packages. (a) Nozzle system. Air is extracted from the package (a bag or a

pouch) through a nozzle, and then the package is closed. This is the simplest way of extracting the air, but it does not allow high levels

of vacuum in the package. (b) Most common system utilized for evacuating bags, pouches, and thermoformed packages. (c) Divided

vacuum chamber. This allows a better evacuation of the package headspace by using two separate chambers where a vacuum is

applied sequentially. Reproduced from Chilled Storage: Packaging Under Vacuum, Encyclopaedia of Food Science, Food Technology and

Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

1192 CHILLED STORAGE/Packaging Under Vacuum

.0024 ethylene–vinylacetate copolymer (EVA): easy seal-

ability, thermal shrink properties;

0025 ionomers: mechanical strength, easy sealability,

grease resistance, formability;

0026 polyamides (PA): mechanical strength, gas barrier,

formability;

0027 polyesters (PET): mechanical resistance, heat

resistance, gas barrier;

0028 ethylene–vinylalcohol copolymer (EVOH): gas

barrier, easy processability in coextrusion;

0029 polyvinylidenechloride (PVDC): gas barrier, grease

barrier.

PE, PP, EVA, and ionomers are classified in the wide

family of resins called polyolefins. Aluminum foils

and vacuum-metallized plastic films are also utilized

because of the excellent gas-barrier properties of

aluminum.

0030 Multilayer materials are manufactured using

various techniques:

0031 coextrusion: the molten resins are combined in the

final structure by extruding them through a round

or flat extrusion die, which keeps them separate in

discrete layers;

0032 lamination: previously extruded plastic films and,

sometimes, aluminum foil, are joined together by

means of glues (glue lamination) or with resins

that have adhesive properties (extrusion lamin-

ation);

0033 coating: pre-extruded films are coated with a layer

of molten or dissolved resin (a latex).

Package Configurations Used for Vacuum

Packaging of Chilled Foods

0034 These can be classified into four main categories:

shrink bags such as Cryovac*, pouches, thermo-

formed packages, and skin packages such as Dar-

fresh*.

Shrink Bags

0035 Shrink bags are available in the form of premade bags

that can be prepared in different packages (e.g., taped

bags) to allow their utilization on automatic equip-

ment. The main feature of these bags is their ability to

shrink when exposed for a short time to heat (for

instance, a few seconds at 90



C). This behavior is

the consequence of treatment imparted to the pack-

aging material during its production (oriented poly-

meric chains that retain a built-in tension, making

them able to shrink when relaxed by heating).

0036Shrinking increases the packaging material thick-

ness (which varies between 40 and 120 mm), improv-

ing mechanical resistance and gas-barrier properties,

determines a tighter package around the product,

limiting dripping out of juices in moist products

such as meat, and improves the appearance by elim-

inating excess of packaging material around the prod-

uct. Shrink bags, at the onset of their introduction on

the market in the 1950s, were monolayer PVDC ma-

terials, but subsequent technological evolution gave

rise to complex coextruded multilayer structures

having polyolefins as the main components and gas-

barrier layers made of PVDC or EVOH.

0037Some materials are electronically cross-linked to

improve mechanical properties.

0038Shrink bags are used for vacuum packaging of

industrial units of fresh meat, processed meat and

cheese, and for consumer units of processed meat

and cheese.

Pouches

0039Pouches can be utilized in two forms:

0040premade plastic envelopes of different sizes that are

loaded with the product, then evacuated and sealed

in vacuum chambers;

0041in-line prepared pouches from machines using roll-

stock material (horizontal form fill seal and vertical

form fill seal machines).

The above machines, starting from a flat web, pro-

duce a film tubing that can be either horizontal or

vertical (hence the different definitions), which is

filled with the product, sealed transversely, and cut

into final packages.

0042The machines can be equipped with a vacuum

chamber to evacuate the pouch before final sealing.

Form fill seal machines are widely employed in food

packaging; however, their utilization for vacuum

packaging of perishable foods is rather limited as ther-

moforming is preferred whenever a high packaging

output and automation of packaging operation are

required.

0043Premade pouches are commonly utilized for

vacuum packaging of industrial units of fresh red

meat such as whole primal cuts, processed meat

(ham, bacon, salami, bologna) and cheese. Typical

structures of materials for pouches, obtained by

means of glue or extrusion lamination, are based on

bioriented PA6 or PET (biorientation plus heat setting

enhances dimensional stability and mechanical resist-

ance), which can be coated with PVDC to increase the

gas-barrier properties and are laminated to suitable

sealing layers (PE, EVA, or PP and ionomers when

heat resistance for pasteurization or cooking in

the package is requested). For long storage-life

Cryovac* and Darfresh* are registered trademarks of Cryovac Inc.,

a subsidiary of Sealed Air Corporation.

CHILLED STORAGE/Packaging Under Vacuum 1193

applications (pasteurized cooked ham or sausages)

where maximum gas-barrier properties are needed,

aluminum foil is also used.

0044 The total thickness of the above materials varies

between 70 and 250 mm, the higher thickness being

employed for heavy and hard products.

Thermoformed Packages

0045 These are obtained on continuous thermoforming

machines, which use rollstock materials. Two rolls

are used, one for the bottom web, which is unwound,

heated by a warm plate and formed into cavities

where the product to be packaged is subsequently

loaded, and a second one, the top web, which is

sealed onto the bottom web inside a vacuum

chamber from which the air has been removed. Ther-

moforming is widely applied to packaging of perish-

able foodstuffs because of the flexibility of the

process, the packaging speed, and the ease of auto-

mation.

0046 A wide variety of packaging materials is utilized.

Bottom webs are generally based on PA laminated or

coextruded with all types of polyolefins. PVDC or

EVOH are used when high gas-barrier properties are

needed. Some bottom webs are also heat-shrinkable

after thermoforming.

0047 Top webs are generally laminated structures similar

to those used for pouches, the thickness of which

seldom exceeds 100 mm. Metallized materials are

often used in the top web formulation.

0048 Thermoforming is utilized for packaging many

kinds of perishable products, both consumer and

industrial units, including whole ham and fresh

meat primal cuts, with the exception of the biggest

units of cheese and processed meat.

Skin Packages

0049 Skin packaging represents an evolution of thermo-

forming where the top web is softened by means of

heat and subsequently formed onto the packaged

product.

0050 The bottom web can be either flexible or rigid and

can be preformed into a tray or a product-sized cavity

into which the product to be packaged is loaded.

0051 In skin packaging, the top film conforms tightly

to the product shape with advantages in terms of

appearance, limited product crushing due to vacuum,

and the possibility of contour sealing around the

product, limiting the exudation of liquids from the

product. Because of its appealing appearance and

the high cost of the packaging materials, skin-

packaging utilization is limited to consumer or family

units of meat (both fresh and processed), fish, pre-

pared meals, and cheese.

0052Top webs are usually based on ionomeric resins or

on coextruded structures of polyolefins. PVDC or

EVOH provides the necessary gas barrier.

0053Bottom webs are generally either laminated or

coextruded structures similar to those used for

thermoforming.

0054When a rigid tray is required, polyvinylchloride

(PVC), polystyrene (PS) or PET is employed.

Influence of Vacuum Packaging on the

Storage Behavior of Chilled Foods

Fresh Meats and Poultry

0055Meats contain an abundance of nutrients necessary

for the growth of microorganisms; they are particu-

larly rich in soluble organic substances such as carbo-

hydrates, amino acids and nucleotides. Therefore,

spoilage of fresh meat and poultry during chilled

storage is mainly due to the growth of aerobic psy-

chrophyllic bacteria belonging to the Pseudomonada-

ceae family and to the Moraxella–Acinetobacter

group. Growth of these bacteria results in develop-

ment of off-odors (due to hydrogen sulfide, ammonia,

amines, and indole) and bacterial slimes, which con-

tribute to meat discoloration. (See Spoilage: Bacterial

Spoilage.)

0056Vacuum packaging in oxygen-impermeable mater-

ials limits oxygen supply to the typical aerobic spoil-

age microflora, providing conditions suitable only for

the slower-growing lactic acid bacteria, which, in

chilled conditions, require several weeks to produce

off-odors. (See Lactic Acid Bacteria.)

0057In addition, vacuum packaging has an impact on

meat color, which is mainly due to the presence in the

muscle tissue of myoglobin, a conjugated protein

where the protein moiety (globin) is bonded to a

heme group.

0058The iron atom of the hematin nucleus can form

complexes with different ligands and can be in either

the ferrous (Fe

2þ

) or ferric (Fe

3þ

) oxidation state. The

globin can be in either the native or denatured state.

Among the different complexes of heme, globin and

ligands, three are important in fresh meat.

0059oxymyoglobin, with oxygen as the ligand and iron

in the ferrous state, characterized by a bright red

color;

0060myoglobin, with water as the ligand and iron in the

ferrous state, characterized by a purplish red color;

0061metmyoglobin, with water as the ligand and iron in

the ferric state, characterized by a brown color.

0062Oxymyoglobin is the pigment normally present on

the surface of meat exposed to air, and gives the meat

its bright and attractive color.

1194 CHILLED STORAGE/Packaging Under Vacuum

0063 During storage, as a consequence of the growth of

aerobic bacteria that reduce the availability of oxygen

on the meat surface, and of the reduction in the

capability of the meat to reduce its own metmyoglobin

level, metmyoglobin tends to predominate, imparting

its brown color to the meat surface, which contributes

to consumer rejection of the product.

0064 In vacuum-packaged meat, as a consequence of

preventing oxygen access to the meat surface, the

pigment is of the myoglobin color, and the meat

appears darker than a sample exposed to air or pack-

aged in modified atmospheres. Displaying of vacuum-

packaged consumer units of fresh meat can create

problems of consumer acceptance because of the

purplish color of the meat surface. In this case, proper

consumer warning has to be given to explain the

origin of the color and the advantages of vacuum

packaging in terms of prolonged storage life. (See

Controlled-atmosphere Storage: Applications for

Bulk Storage of Foodstuffs.)

0065 The storage life at 0–2



C of fresh primal meat

vacuum packaged in bags or pouches is 4–8 weeks,

allowing full aging of meat types such as beef that

require maturation. Consumer units in the form of

meat slices have a storage life of 2–3 weeks.

Processed Meats

0066 It is useful to classify the many existing types of

processed meats into two main categories of

products:

0067 products having high-activity water, the produc-

tion processes of which often imply cooking (frank-

furters, patties, bologna, fresh sausages, cooked

ham, luncheon meats);

0068 cured products with low-activity water (raw ham,

salami, bacon).

The spoilage of the former category of products can

be due to surface drying, fat oxidation, discoloration

or to microbiological factors (sliminess, souring,

greening). The latter category of products is less sus-

ceptible to spoilage caused by bacteria. Rancidity,

discoloration and mold growth are the most common

limiting factors for storage life. (See Meat: Structure;

Spoilage: Molds in Spoilage.)

0069 All the above products can benefit from vacuum

packaging, which avoids surface drying, mold growth

and greatly slows down the oxidation of fat and

pigments. Pasteurizable packages are also commonly

used for microbial stabilization of some products

(cooked ham, patties, and frankfurters). Further-

more, cook-in-the-package technology has been

developed for production of cooked ham, allowing

optimization of the production process, hygienic

quality and yield.

0070The chilled storage life of vacuum-packaged pro-

cessed meats is very variable, depending primarily

upon product composition and packaging material

characteristics and can vary from a couple of weeks

(certain types of fresh sausages) to several months

(cured products and pasteurized or cook-in-the-

package products).

Cheese

0071Vacuum packaging is mainly applied to hard and

semihard types of cheese, for both industrial and

consumer units, and its effect in storage-life extension

is due mainly to:

0072elimination of surface drying;

0073slowing down of fungal growth;

0074limitation of oxidation of fatty substances.

0075An interesting application of vacuum packaging is

the curing in the package technology that is utilized

for certain types of cheeses (Emmenthal, Gouda,

Edam). The cheese is vacuum-packaged at an early

curing stage and cured inside the package. This tech-

nique improves yield (limiting rind formation and

water loss) and allows a certain product standardiza-

tion. Curing in the package is particularly demanding

in terms of gas transmission properties of the pack-

aging material; a good compromise is necessary be-

tween a sufficiently high carbon dioxide permeability

to allow the escape of gas formed inside the cheese

during curing and a sufficiently low oxygen transmis-

sion rate to avoid mold growth. The gas-transmission

rate requirements can vary from cheese type to cheese

type and also depend upon production conditions,

which are often variable among different dairies.

(See Cheeses: Cheeses with ‘Eyes’.)

Fish

0076Wet fish is probably the most perishable type of food-

stuff because of the high content of soluble substances

in the flesh, many of which contain nitrogen, and

triglycerides characterized by polyunsaturated fatty

acids (in fatty types of fish such as clupeids, salmonids

and scombroids). In addition, fish flesh is character-

ized by a high level of enzymatic activity, which plays

a major role in the early stages of spoilage. (See Fish:

Spoilage of Seafood.)

0077Vacuum packaging retards the growth of aerobic

spoilage bacteria and limits the oxidation of fat.

However, vacuum-packaged fish is very perishable

and has to be carefully stored at temperatures below



C, the normal storage life being 5–7 days. Pre-

packaged consumer units of wet fish, utilizing skin

packages, have recently been introduced at a commer-

cial level due to the need for a distribution system

capable of handling in a relatively easy way a product

CHILLED STORAGE/Packaging Under Vacuum 1195