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

Подождите немного. Документ загружается.

significant with cryogenic tunnels, especially

following a change in throughput or product type.

The reason for this is poor control. A quantity of

the cryogen is also required to cool the equipment

down prior to start-up and to maintain temperature

during idle periods. With efficient plant usage

this need not account for more than an additional

2%, over and above that used for actual product

freezing.

Maintenance

0018 Cryogenic freezers have few moving parts so mainten-

ance is far easier than for mechanical systems. There

are obvious benefits for long rail journeys in using dry

ice for which no further attention is required once the

cargo and refrigerant have been loaded.

Weight Loss

0019In several freezing applications, the advantages listed

in Table 2 are sufficient to overcome the penalties of

cryogenic systems. The potential saving in weight loss

in freezing unwrapped products in comparison with

air-blast freezing can be substantial. For products of

great value such as shellfish this is of particular eco-

nomic significance, since the lost weight/water has the

same value per unit weight as the product.

Flexibility

0020The flexibility factor has other ramifications. Mech-

anical systems are limited in their output of cold –

both above and below the norm. Overloading affects

quality; underloading increases unit costs. With cryo-

genic systems the ‘cold’ is on tap. This is particularly

important with a new product or a seasonal product

when demand is unpredictable and variable.

0021Cryogenic systems are also often flexible in the type

of product that can be fed through them. In a conven-

tional blast freezer, the maximum air velocity inside

the freezer limits the amount of heat given up by the

evaporator coils. In a cryogenic tunnel, the rate at

which refrigerant is added is more or less independent

of the gas velocity. This gives cryogenic tunnels

considerable flexibility in responding to changes in

production rate.

Size

0022Cryogenic freezers tend to be more compact than

those using mechanical refrigeration, in some cases

needing only half the floor space for the same

throughput. Carbon dioxide systems are extremely

compact, for example 1360 kg of product per hour

can be produced using an area of 3.6 3.6 m. Also

the equipment supplying the refrigeration itself takes

up substantially less room than mechanical refriger-

ation equipment.

Weight

0023Low weight is a particularly attractive aspect for air-

line catering applications, and other transport appli-

cations.

Noise

0024Cryogenic systems operate relatively quietly com-

pared with the noise and vibration caused by mech-

anical compressors. With the growing concern over

noise pollution, this may become more of an issue. In

tbl0004 Table 4 The cryogen requirements for freezing various

foodstuffs

Foodstuff Liquidnitrogen

(kg cryogen kg

1

foodstuff)

Carbon dioxide

(kg cryogen kg

1

foodstuff)

Meat and meat products 0.7–1.2

Beef fat 0.82

Beef lean 1.05

Veal 0.95

Pork 0.73

Lamb 0.88

Beefburgers 0.6–0.8 0.92

Meat pies 0.91

Poultry 1.1–1.3 1.03

Fish 1.03

Shellfish – scampi 1.0–1.3 1.27

Fruit and vegetables 1–1.5

Beans (green) 1.29

Peas 1.10

Asparagus 1.34

Strawberries 1.30

Fruit pies 1.11

Pastry products 0.4–1.1

Precooked foods (warm) 1.6

Icecream 0.7 1.02

Bread 0.66

Theoretical requirements assuming foodstuff temperature reduction from

20 to 18



tbl0003 Table 3 Major factors affecting cryogen consumption ratio

Factors

Moisture content

Temperature in/out

Residence (freezing) time

Belt loading ratio

Production rate

Operator discipline

Length of production run

Efficiency of freezer

2728 FREEZING/Cryogenic Freezing

road transport, overnight parking of mechanically

refrigerated vehicles is sometimes difficult in noise-

sensitive areas, and cryogenic systems overcome this

problem.

Safety

0025 In comparison with nitrogen, carbon dioxide is safer.

Carbon dioxide is sensed by humans at levels less than

0.5%, although much higher concentrations of nitro-

gen are tolerable it is not sensed and a potential

hazard stems from fainting due to hypoxia (shortage

of oxygen to the brain).

Synergism

0026 The rapid freezing rates possible with cryogenic freez-

ing may be employed to enhance mechanical systems.

An example is the crust freezing of delicate product,

such as fish fillets, using cryogens before freezing in

spiral freezers to prevent mechanical damage as the

product travels through the spiral. Crust freezing can

also be employed to reduce dehydration. However,

legislation can complicate this issue. In countries

where practices to compensate for weight loss, such

as misting of burgers, are allowed, the economics

don’t favor crust freezing.

0027 Crust freezing can also be used to reduce frosting

problems. Moisture loss during freezing can cause

frosting of refrigeration coils, reducing the efficiency

of the systems and requiring defrosting. Crust freez-

ing traps the water, preventing moisture migration.

Equipment

0028 Cryogens are typically employed as sprays in tunnel

or batch cabinet systems, or direct immersion tunnels.

Operation of cryogenic systems is relatively straight-

forward. The two basic functions to regulate are: (1)

conveyor speed to give the required retention time,

and (2) refrigerant flow rate to give the desired

product temperature.

Tunnel Freezers

0029 Essentially a tunnel freezer is a countercurrent heat

exchanger. The cryogen is sprayed at one end and the

gas passed countercurrent to product traveling on a

belt in the opposite direction. This enables the exit

temperature to be only 20



C below the product inlet

temperature. Thus product can be precooled before

being subjected to the full force of the refrigeration

effect. Approximately 50% of the product’s heat can

be extracted before it reaches the cryogen spray.

Straight or spiral tunnels are utilized. As a rule,

carbon dioxide tunnels tend to be longer than liquid

nitrogen tunnels for the same capacity.

0030A tunnel freezer operates by conveying the product

through an enclosed space within which the cryogen

is discharged. Economy of cryogen consumption is

improved by insulation and isolation of the freezing

tunnel, and by control of the cold gas within the

tunnel. The product passes through three distinct

zones in the tunnel: the precool zone, the freezing

zone, and the postcool zone. In the precool zone, the

product is cooled using gas and fans force gas verti-

cally past the product. In the freezing zone, sprays

discharge cryogen directly on product and belt, while

in the postcool zone, which is a gas-cooled section,

with more fan circulation, some temperature stabil-

ization takes place.

Rotary Freezers

0031Rotary freezers have also been developed to IQF

freeze products such as shrimps, cubed chicken

meat, or meat balls. A fine mist of cryogen is used to

freeze the surface of the product and prevent individ-

ual pieces sticking to one another before they enter a

rotating drum, which keeps them separated during

the freezing process.

Immersion Freezers

0032Immersion freezers are the simplest and most inex-

pensive way of using cryogens to freeze foodstuffs.

Immersion freezers are best suited to high volumes of

product where crust freezing is required or where the

product is sufficiently robust to withstand the thermal

shock of a high temperature differential. Very fast

heat transfers are possible, but the system is wasteful

in that only the latent heat of the liquid is used and

cold vapor is lost.

Cabinet Freezers

0033Batch cabinet freezers utilizing cryogens have been

developed for freezing prepared and precooked

food, particularly for airline meals. Liquid cryogen

is injected at time intervals into a cabinet containing

the food on racks; circulation fans are usually utilized

to provide even and efficient freezing. Freezing times

vary from 5 min to 1 h, depending on the product and

its temperature.

Pellet Freezers

0034Liquids and semiliquids are often frozen into pellets.

This can be carried out using contact freezing be-

tween belts or in molds, or cryogenic freezing using

liquid nitrogen and a forming device. Belt freezers

employ a similar contact method of freezing to plate

freezers. Single-band and double-band freezers are

designed to freeze thin layers, usually of liquid or

FREEZING/Cryogenic Freezing 2729

semiliquid products such as vegetable pure

es, fruit

pulps, egg yolk, sauces, soup, etc. In double-band

systems the product is frozen between two endless

belts, of which the top is flat and the lower belt

corrugated. The product is spread into the corruga-

tions; the top belt encloses the exposed surface, thus

freezing the product as IQF pellets.

Combined Freezers

0035 A cost-effective alternative to 100% cryogenic freez-

ing is a combined freezer. The principle is to use a

cryogenic freezing unit for initial freezing of the outer

surface of the product, followed immediately by

mechanical freezing to reduce the temperature of the

bulk of the product. One advantage of this type of

approach is that the cryogenic stage can often be

retrofitted.

Freezing Rate

0036 Most food freezing systems rely on convection as the

principal means of heat removal. The rate of heat

removal from the product depends on the surface

area available for heat flow; the temperature differ-

ence between the surface and the medium; and the

surface heat transfer coefficient. Much higher coeffi-

cients are achieved in cryogenic systems than in most

conventional refrigeration (Table 5).

0037 The rate at which heat can be conducted away

from the surface is not the sole criterion that

governs the time taken to freeze a product. Heat

must also be conducted from within the product

to its surface before it can be removed. Most food-

stuffs are poor conductors of heat and this imposes a

severe limitation on attainable freezing times for

either large individual items or small items frozen in

bulk.

0038 Cryogens are particularly suited to food products

that have a high surface-area-to-volume ratio and in

which the thermal diffusivity of the food does not

restrict the transfer of heat from the product to the

freezing medium. Typical examples are fish fillets,

shellfish, pastries, burgers, meat slices, sausages,

pizzas, and extruded products.

Product

0039Typical product freezing times in nitrogen systems are

shown in Table 6.

0040Rapid freezing has been claimed to improve taste,

texture, aroma, nutritive value and appearance,

bacteriological, enzymic, oxidative and chemical

degradation, and to reduce drip loss upon thawing.

Ice Crystal Formation

0041The most apparent influence of the freezing rate on

product quality is related to the size of the ice crystals,

which are formed during the freezing process.

0042It has long been shown by photomicrographs that

the faster meat and fish are frozen, the smaller the ice

crystals produced, and that above a certain rate these

are predominantly intracellular, when by implication

the damage to cell walls should be minimal. However,

extracellular ice crystal formation is not necessarily

disruptive because muscle cell wall membranes are

elastic, unlike those of vegetable tissue. Nevertheless,

largely on the strength of the photomicrographs,

plausible advocacy of rapid freezing equipment

throughout the 1930s and 1940s resulted, and these

views remain.

Quality

0043The debate about the relationship between freezing

rate and quality has gone on for many years. Many

trade publications indicate that a considerable

improvement in quality can be obtained by ultrarapid

freezing with cryogens. However, it is doubtful

whether the rate of freezing has any detectable effect

on drip or on palatability of meat after thawing and

cooking over a wide range of freezing rates. Recent

tbl0005 Table 5 Published surface heat transfer coefficients for

different cooling processes

Method Heat transfer coefficient (Wm

2

1

)

Still air 5–10

Air blast, slow 15–20

Air blast, fast 40–50

Fluidized bed 100–4000

Contact, plate 500

Immersion, still 50–200

Immersion, agitated <1000

Cryogenic, R 12 1000

Cryogenic, liquid nitrogen 1500

tbl0006Table 6 Freezing times of nitrogen freezers

Product Freezing time (min)

Fruit and vegetables 0.5–6

Meat and meat products 3–20

Meat patties, hamburgers 3–5

Pastry products 4–8

Precooked foods (warm) 4–8

Large, bulky meat products.

2730 FREEZING/Cryogenic Freezing

studies with beefburgers show no differences in eating

quality between those frozen to 18



Cin22or

2.7 min. For fish, it has been concluded that freezing

times of up to 26 h do not significantly influence the

quality of the product.

0044 A number of authors have classified foods

according to their sensitivity regarding freezing rate

into four groups:

0045 Products that remain practically unaffected by

variations in freezing rate, i.e., products with a

high content of dry matter, such as peas, meat

products with a high fat content, and certain

ready meals.

0046 Products which require a minimum freezing rate

(0.5–1



C min

1

), but are relatively unaffected by

higher freezing rates, i.e., fish, lean meat, and

many starch- and flour-thickened ready meals.

0047 Products whose quality improves when freezing

rates are increased (3–6



C min

1

), e.g., many

fruits, egg products, and flour-thickened sauces.

0048 Products in which high freezing rates are advanta-

geous for the product quality, but where tempera-

ture tensions in the product result in a destruction

of the tissue, e.g., fruits and vegetables such as

raspberries, tomatoes, and cucumbers.

0049 Some work has indicated that there is an interaction

between freezing rate and cooking method. When

meat was cooked after thawing, no differences were

found between freezing rates (13, 2, or 0.04 cm h

1

)

in terms of flavor, juiciness, tenderness, thaw loss, or

TBA (thiobarbituric acid) values. However, meat that

had been cooked from frozen was found to show a

favorable effect of faster freezing rates.

0050 With mushrooms, while the quantity of the drip

loss is almost independent of the freezing rate, the

color of the lost juice changes conspicuously. It

contains more coloring and flavoring ingredients the

slower the freezing.

0051 Among the products that obtain a decisive and

lasting quality improvement by rapid freezing are

carrots, corn on the cob, mushrooms, egg products,

sauces, cream, shrimps, and prawns.

Appearance

0052 Freezing rate does affect the appearance of many food

products. For example, fast freezing of poultry tends

to produce a lighter-colored product as the small ice

crystals scatter the light more than larger crystals.

This lighter color is preferred by many consumers.

In fish, rapidly frozen fillets have a dense white,

opaque appearance, in comparison with the dark,

translucent, vitreous product produced by very slow

freezing.

Drip Loss

0053When ice crystals grow they may puncture the cell

walls so the juice bleeds out. This causes ‘drip loss’

when the product is thawing. The larger the average

crystal size is, the greater the number of punctured

cells and the greater the drip loss. The loss of juice can

result in a loss of firmness and flavor.

0054The drip loss from some food (i.e., strawberries)

can be clearly related to freezing rate. Slow freezing

can produce drip losses of 20%. Fast freezing without

surface cracking can reduce losses to approximately

5%. With meat and meat products freezing consider-

ably increases drip. However, other factors inherent

in the animal and its processing before freezing have

more effect on the magnitude of drip than differences

in freezing rate.

0055There are many products that are situated between

these two types.

Dehydration–Weight Loss

0056Lower weight loss, during the freezing of unwrapped

food, is one of the main advantages of cryogenic

freezing. Weight loss during freezing of beefburgers

can be as high at 3% in a poorly designed air blast

freezing system compared with 0.4% in a liquid

nitrogen tunnel.

0057However, technological advances in conventional

freezing systems can substantially reduce – if not

completely remove – this saving. In a modern spiral

freezer operating at 30



C air temperature, the

weight loss will be approximately 1.2%. In an

air impingement freezer operating at a 46



C air

temperature, the weight loss from a burger can

be <0.4%

Cracking

0058Some products, delicate soft fruits and light pastry

products, may crack when they are submitted to

very high freezing rates or very low temperatures.

Research suggests that crust freezing produces a

shell that prevents further volume expansion, when

the internal portion of the unfrozen material under-

goes phase transition. If the internal stress is higher

than the frozen material strength, the product will

crack during freezing. Products with high void spaces,

which allow internal stresses to dissipate, show a

lessened chance of freeze-cracking. Precooling pre-

vents freeze-cracking because it reduces the differ-

ences in temperature between the product and the

freezing medium, and reduces the difference between

freezing time at the center and at the surface. When

the phase change of the core region occurs before the

surface becomes brittle, food products can support

FREEZING/Cryogenic Freezing 2731

the internal pressure and freeze-cracking does not

occur.

Summary

0059 Cryogenic systems can be used effectively to freeze,

transport, and store a wide range of foodstuffs, and a

wide range of equipment and techniques is available

using either nitrogen or carbon dioxide. While there

are many practical, as well as economic, disadvan-

tages to cryogenic refrigeration systems, there are

many situations in which low capital or maintenance

cost and/or rapid freezing rates make these techniques

worthwhile.

See also: Freezing: Principles; Operations; Storage of

Frozen Foods; Structural and Flavor (Flavour) Changes;

Nutritional Value of Frozen Foods; Transport Logistics

of Food

Further Reading

Anonymous (2001) Rapid Cooling – Above and Below

Zero. Paris: International Institute of Refrigeration.

Bald WB (ed.) (1991) Food Freezing: Today and Tomorrow.

London: Springer-Verlag.

George RM (1993) Freezing processes used in the food

industry. Trends in Food Science and Technology 4:

134–138.

Heap RD and Mansfield JE (1983) The use of total loss

refrigerants in transport of foodstuffs. Australian Re-

frigeration, Air Conditioning and Heating 37: 23–26.

Kennedy CJ (ed.) (2000) Managing Frozen Foods. Cam-

bridge: Woodhead Publishing.

Mallett CP (ed.) (1993) Frozen Food Technology. London:

Blackie Academic & Professional.

Storage of Frozen Foods

GLo

ndahl, Helsingborg, Sweden

T Nilsson, Enebyberg, Sweden

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 The quality of frozen foods depends on the quality

and nature of the product to be frozen, on the pro-

cessing of the product prior to freezing, the freezing

process, and the packaging of the product.

0002 During storage, reactions of a physical as well as

chemical and biochemical nature will cause changes

in the food which gradually lead to a loss of quality.

The changes are cumulative and the shelf-life of

frozen food is limited by, and dependent on, the

nature and speed of the reactions.

0003Some of the most important changes during stor-

age, which will influence the quality, are as follows:

0004loss of water

0005loss and transfer of volatile compounds

0006oxidative changes in fats

0007denaturation of proteins

0008color changes

0009transfer of aromatic compounds from product to

product

These changes, which will be dealt with in more detail

in the following sections, are in most cases interact-

ing, and their influence on the overall quality of the

food is complex in its nature. It is therefore difficult to

find a single objective method to determine the shelf-

life of a product. However, much work has been

directed with this goal in mind. In some cases, valu-

able information can be gained from such tests, e.g.,

determining the rancidity of fatty products, loss of

vitamins, weight loss, color changes, etc. However,

in most cases organoleptic tests are the most common

methods used to determine the shelf-life for frozen

products stored under various conditions.

0010All changes during storage are temperature-

dependent. In general, the lower the temperature,

the slower the speed of the deteriorative changes.

(See Storage Stability: Mechanisms of Degradation;

Parameters Affecting Storage Stability.)

Loss of Water, Dehydration

0011During storage, the water vapor pressure over the

surface of the product is higher than that of the sur-

rounding air. Since the surface of the cooling coils

always has a lower temperature than the air in the

storage room, ice will form on the coils. Thus a less

than saturated relative humidity is maintained in the

air, and also a difference in vapor pressure between

the surface of the product and the surrounding air. As

a consequence, sublimation of ice from the product

will occur and continue as long as this pressure differ-

ence is maintained.

0012Not only does the loss of water mean a loss of

weight of the product, which may have economical

consequences, but also, in many cases, drying may

severely impair the quality of the product. The water

evaporates from the surface of small ice crystals. Such

crystals have, in relation to volume, a larger area than

bigger ones and are therefore prone to evaporate and

disappear first. The contact surface between the prod-

uct tissue and the surrounding air (oxygen) is thereby

considerably enhanced, which will increase the

2732 FREEZING/Storage of Frozen Foods

oxidative reactions in the product. In some cases,

combinations of oxidative reactions in the proteins

and drying will lead to a condition known as freezer

burn. This is a severe quality change in the surface of

the product. (See Drying: Theory of Air-drying.)

Prevention of Water Loss and Drying

Packing

0013 If the product is packed in a tight-fitting, water-

vapor-proof material, evaporation cannot take place.

However, it is important that the packing material is

tight-fitting; otherwise, sublimation will take place

within the package. The packing material will follow

the temperature changes caused by temperature fluc-

tuations in the room faster than the product itself.

When the temperature is lowered, evaporation from

the product will form ice on the inside of the packing

material, and when the temperature conditions are

reversed the ice will be deposited on the surface of

the product. If there are big variations in the storage

temperature, ice formation within the package can be

quite important. Small spaces within the package and

a storage temperature as even as possible are measures

to avoid this condition. If the temperature differences

between the surface of the cooling equipment (coils)

and the storage temperature are small, sublimation

from the product will be low. Good insulation and

efficient cooling sytems are effective tools to achieve

this. Temperature abuses, which will allow warm air

or warm products to enter the room, should be

avoided.

Glazing

0014 Glazing is the term used for the application of a thin

layer of ice on the surface of a frozen product either

by dipping the product in water or by spraying

(Table 1). This is an effective method of preventing

drying of the product. The sublimation is less pro-

nounced since it takes place from an even surface

which occupies a much smaller area than the ice

crystals in the product. In addition, the loss of water

comes from added water; the amount can be calcu-

lated to prevent any sublimation from the product

during the intended storage period.

0015The lower the storage temperature, the lower

the sublimation losses as the water vapor pressure

decreases with the temperature. At 10



C, it is

2.0 mmHg over ice as compared to 0.3 mmHg at

30



Loss and Transfer of Volatile Compounds

0016Water is not the only substance that will escape from

a frozen food during storage. Many substances which

cause the typical smell and aroma of a food are also

volatile at frozen storage temperatures and will evap-

orate from the surface of the product and enter the

surrounding air. As a result, some of the aroma char-

acteristics of the product are diminished or even lost

during storage, and this will affect the quality of the

product. (See Sensory Evaluation: Aroma.)

0017A more severe quality consequence of this phenom-

enon, however, concerns other products stored in the

same environment. If a ‘foreign’ aroma compound is

deposited on a product and dissolved in either the fat

or water phase of the product, it can severely affect

the quality of the product. Meat, dairy products,

including icecream, and other foods with a high fat

content are most susceptible to pick up aroma from

other products, e.g., fruits, especially citrus fruits

and strawberries, some vegetables, fish, etc. This

phenomenon is well known during chilled storage,

but it is also important during cold storage. It has

led in some cases to considerable economic losses.

0018The only effective method of preventing aroma

transfer during cold storage is packing the product

in a material which will act as a barrier for the com-

pound in question. Since the compounds responsible

for the damage are not easily identified, choice of

packing material must in most cases be based on

experience and tests. A low storage temperature will

reduce the release of volatile compounds and subse-

quently diminish the problem, but cannot prevent it.

A suitable packing material is therefore the only

method left. (See Flavor (Flavour) Compounds: Struc-

tures and Characteristics.)

Oxidation of Fat

0019One of the most important quality changes in frozen

food during storage is the formation of off-flavors

caused by rancidity of fats. Rancidity is caused by

oxidative changes of polyunsaturated fatty acids.

The higher the proportion of polyunsaturated fatty

acids, the more prone the fat is to undergo oxidative

changes. Some species of fish (e.g., herring, salmon)

tbl0001 Table 1 Storage life: acceptability for frozen shrimps (Pandalus

borealis)

Product Acceptability

(monthsat 18



Block-frozen, vacuum-packed 6–7

IQF, Polybag 3–4

IQF, vacuum-packed 6–7

IQF, vacuum-packed, þ4% glaze 8–9

IQF, vacuum-packed, þ8% glaze >10

IQF, individually quick frozen.

FREEZING/Storage of Frozen Foods 2733

are rich in fat with a high content of polyunsaturated

fatty acids, and oxidative changes (rancidity) are

more pronounced in these species than in lean ones

(e.g., cod). In lean species, oxidative reactions take

place, but to a much lesser degree. One compound

causing off-flavors –‘cold-store flavor’–has been

identified as cis-4-heptenol, which results from the

oxidation of o-3 polyunsaturated fatty acids. (See

Oxidation of Food Components.)

0020 Off-flavors are the most important quality factors

resulting from oxidation of fatty acids but, in add-

ition, color changes in the tissue can occur as a result

of reactions between proteins and the oxidation prod-

ucts.

0021 Rancidity changes in fat are also caused by lipolytic

enzymes, such as lipases and phospholipases. Since

these enzymes are active at low temperatures, lipoly-

tic changes with an accummulation of free fatty acids

can take place during cold storage. Interactions be-

tween fatty acids and other components in the food

can cause changes in fat-rich food which will affect

the quality. Normally, however, oxidative changes in

fat are the most important factors influencing the

quality of fat during frozen storage.

0022 Oxidation of fatty acids takes place not only in fat

from fishes, but also in fat from other animals. In beef

the fat contains only a small amount of polyunsatur-

ated fatty acids, and rancidity caused by oxidation

plays a minor role. In pork, however, the fat contains

a much higher amount of polyunsaturated fatty acids,

and pork meat is consequently much more prone to

oxidative changes during frozen storage. The com-

position of the fat in pork depends to a large extent

on the composition of the fat in the feed, and the

keeping quality of different pork lots can differ con-

siderably even if stored under identical conditions.

0023 Oxidation of fat is enhanced by the presence of

certain catalytical factors, e.g., special metal ions,

heme products, or salt, but the process is slowed

down or inhibited by so-called antioxidative sub-

stances. If the contact area between oxygen and the

tissue is increased, as is the case when ice crystals

disappear due to drying, oxidation is considerably

enhanced. (See Antioxidants: Natural Antioxidants.)

0024 Oxidation of fatty acids can be inhibited by pre-

venting oxygen (air) from coming into contact with

the fat. As has been shown in numerous investiga-

tions, close-fitting packing in an oxygen-proof mater-

ial, e.g., vaccum-packing, will minimize the oxidative

reactions and considerably prolong the shelf-life of

the product. The same result can also be achieved by

substituting air within the package by an inert gas,

e.g., nitrogen. In addition, glazing gives good protec-

tion against oxygen reaching the fat.

Changes in Proteins

0025During frozen storage, changes in the quality of

frozen food of both vegetable and animal origin

take place as a result of alterations and reactions of

the proteins. Many of these protein alterations are

still poorly understood, while others have been

thoroughly studied in numerous investigations.

Animal Proteins

0026A well-known characteristic of frozen fish and

mammals is a decrease in the water-binding capacity

of muscle tissue, resulting in a loss of juice at thawing

(drip). In addition, an increase in the firmness of the

meat flesh is often observed. These changes, which

are more pronounced in fish, also take place in

meat (beef and pork) but to a lesser degree. (See

Fish: Processing; Meat: Preservation; Water Activity:

Effect on Food Stability.)

0027The most important changes take place in the myo-

fibrillar proteins, especially in the myosin globular

head of the molecule, and lead to a decrease in its

solubility, while the thin filaments show a much

higher stability during storage at low temperatures.

The alterations have been shown to be temperature-

dependent and occur more rapidly at higher storage

temperatures. Even if the rate of naturally occurring

metabolic changes in the muscle tissue is minimized

during frozen storage, some reactions play a role

during storage. The activity of the enzymatic calcium

adenosine triphosphatase ((Ca

)-ATPase) complex,

which is of importance for the quality of the meat, is

highly temperature-dependent. If the freezing process

is slow, a great loss of the enzymatic activity takes

place compared to a fast freezing rate. During stor-

age, loss of activity of the enzyme complex continues

at a rate dependent on the storage temperature. If the

meat has been frozen in prerigor state and kept at a

low storage temperature, remaining enzymes are able

to cause a very fast reaction when the temperature is

raised during thawing. The condition can lead to a

phenomenon known as thaw rigor, in which a sudden

contraction of muscle fibers leads to a big drip loss.

0028The changes during freezing and frozen storage in

the myofibrillar proteins of muscles from both fish

and mammals are of the same biochemical nature.

However, the effects on the quality of the product

during frozen storage are different. The changes in

fish muscles are generally more pronounced than

those in meat. During frozen storage of fish, an in-

creased firmness and dryness will result in a tougher,

drier product, while beef and pork normally lose

much less of their tenderness and juiciness during

frozen storage.

2734 FREEZING/Storage of Frozen Foods

Vegetable Proteins

0029 Vegetables and fruits are harvested when ripening of

the product is at its peak with regard to flavor, texture,

and color. Since these properties are related by enzym-

atic reactions in the cells of the living plant, which will

continue even at low temperatures after harvest, the

quality of the product will soon deteriorate if the

enzymes in question are not inactivated. In vegetables

this is normally achieved by different methods of heat

treatment (blanching). Lipoxygenases, catalases, and

peroxides are enzymes involved in the deteriorative

processes. Of these, peroxide is the most heat-

resistant and, therefore, determination of peroxide

activity is widely used to check the effect of blanching

processes. (See Ripening of Fruit.)

0030 If some enzymatic activity is left, or if a reaction

takes place, changes will occur during storage which

may affect the quality of the product. One example is

the change of chlorophyls to pheophytines. The desir-

able green color of some vegetables changes to a

yellow or gray color, severely affecting the quality

of the product. Enzymatic reactions during frozen

storage can also cause off-flavors of the product.

Hygienic Considerations

0031 At normal storage temperatures for frozen food, i.e.,

18



C or lower, all microbiological activity, includ-

ing growth, has ceased. Some organisms will die or be

injured during freezing and storage, the number

depending on factors such as the species involved

and the nature of the food. From a practical point of

view this is of little importance. When the tempera-

ture rises at thawing, surviving organisms will resume

their activity, including growth. The hygienic quality

of frozen food should therefore be regarded as equal

to the condition in the product immediately before

freezing.

See also: Antioxidants: Natural Antioxidants; Fish:

Processing; Meat: Preservation; Oxidation of Food

Components; Ripening of Fruit; Sensory Evaluation:

Aroma; Storage Stability: Mechanisms of Degradation;

Parameters Affecting Storage Stability; Water Activity:

Effect on Food Stability

Further Reading

Durand MP (2001) History of the application of cold to

food. Bulletin Academie Nationale du Medicine 185(2):

269–272.

International Institute of Refrigeration (1986) Recommen-

dations for the Processing and Handling of Frozen

Foods, 3rd edn. Paris: International Institute of Refriger-

ation.

Jul M (1984) The Quality of Frozen Foods. London:

Academic Press.

ndahl G and Danielsson C (1972) Time Temperature

Tolerances for Some Fish and Meat Products. Proceed-

ings of the International Institute of Refrigeration,

Warsaw, Commission C2.

Ludovico-Pelayo L, Hume A and Love RM (1984) Seasonal

variations in flavour change of cold-stored trout. In:

Zeuthen P, Cheftel JC, Eriksson C et al. (eds) Thermal

Processing and Quality of Foods, pp. 659–663. Essex:

Elsevier Science Publishers.

Phillips KM, Simpkins AH, Amama KR et al. (2001) Long-

term stability of nutrients in a mixed food control

material. Fresenius Journal of Analytical Chemistry

370(2–3): 297–302.

Wagner JR and An

on MG (1987) Protein Denaturation in

Bovine Muscle during Frozen Storage. Proceedings of

the XVIIth International Congress of Refrigeration,

Wien, pp. 717–722. Paris: International Institute of

Refrigeration.

Structural and Flavor (Flavour)

Changes

C James and S James, University of Bristol,

Langford, UK

Background

0001The main aim of freezing is to extend the storage life

of the raw material or food product beyond that

achieved at temperatures above the initial freezing

point of the material. In a number of cases, freezing

is used to produce a product, i.e., icecream, sorbet, ice

lollies, etc., that cannot be produced in any other way.

0002If the temperature of the food is reduced to below

10



C within a few hours and maintained below

that temperature, microorganisms, bacteria, molds,

or yeasts will not grow. Microbially, the food will

always remain as safe to eat as it was before freezing,

irrespective of the length of frozen storage. Some

microbiologists consider 5



C to be a practical

maximum temperature to prevent the growth of

microorganisms on food.

0003The textural quality of the food will change with

time and conditions of storage. More importantly, the

flavor will deteriorate with time. Ultimately, these

changes will result in an unacceptable product.

FREEZING/Structural and Flavor (Flavour) Changes 2735

Textural Changes

0004 Most of the textural changes that occur as a result of

freezing and frozen storage only become apparent

when the food is thawed. A few changes can be ob-

served while the food is still frozen. The most notice-

able of these is called ‘freezer burn.’

Changes Apparent in Frozen Product

0005 Freezer burn Moisture is lost from the surface of a

food when the vapor pressure at its surface is higher

than that in the surrounding environment. Freezing

considerably reduces the vapor pressure at the surface

of a food, but it is still likely to be higher than that in a

frozen storage room or display cabinet. Ice from the

exposed surface of unwrapped frozen food sublim-

ates directly into the surrounding environment. The

rate of the subsequent desiccation depends mainly on

the rate of air movement over the surface and its

humidity, and extreme desiccation results in a layer

of spongy, dry dark material developing on the

surface. This is known as ‘freezer burn.’

0006 In-package frosting Tight packaging in a heavy-

weight, moisture-resistant wrapping prevents freezer

burn. Loose packaging reduces freezer burn but can

allow another problem, ‘in-package frosting,’ to de-

velop. In-package frosting, occurs when moisture

from the surface of the food freezes on the inner

surface of the package, and the main cause of this is

poor temperature control in the store or cabinet.

0007 Light surfaces Rapid freezing of products such as

poultry joints produces very small reflective ice crys-

tals at the surface. These give the product a very light

appearance that is very desirable in some markets.

Poor temperature control during storage causes the

small crystals to grow, merge, and lose their lightness.

Changes Apparent on Thawing

0008 Most of the textural changes caused by freezing are

not noticeable until thawing. During the freezing pro-

cess as the water turns to ice, it expands, and the ice

crystals formed cause the cell walls to rupture. Con-

sequently, the texture of the product becomes much

softer when the product thaws.

0009 These textural changes are more noticeable in

fruits and vegetables that have a higher water content

and especially those that are eaten raw. For example,

when a frozen tomato is thawed, it turns into mush

and liquid. This explains why celery, lettuce and

tomatoes are not usually frozen and why frozen

fruit are best served before they have completely

thawed. In the partially thawed state, the effect of

freezing on the fruit tissue is less noticeable.

0010In the case of vegetables, blanching (exposure of

the vegetable to boiling water or steam for a brief

period of time) has a marked softening effect, but

further changes during freezing or frozen storage are

comparatively small. However, if the vegetable is

frozen in the raw state or is not correctly blanched,

the enzymic action on pectic substances may lead to

unacceptable changes in texture.

0011Textural changes due to freezing are not as appar-

ent in products which are cooked before eating be-

cause cooking also softens cell walls. These changes

are less noticeable in high-starch vegetables, such as

peas, corn, and lima beans.

0012Consumers expect to purchase frozen meat at a

unit price lower than that of a similar chilled product.

This price differential results from the belief that the

quality of frozen meat is inferior to that of chilled

meat. There is little in the published scientific litera-

ture to substantiate this view. Some publications

show that freezing, frozen storage, and thawing

have beneficial effects on the textural properties of

pork loins. Taste panels have found the meat to be

slightly tenderer with improved juiciness and an

easier breakdown of particles, as evidenced by the

samples being less cohesive, easier to chew, and

releasing more moisture during mastication.

0013Meat which is frozen just after slaughter before

the rigor process is complete and, after being

thawed rapidly, contracts vigorously and exudes

much drip. The resulting meat is very tough, and

this toughness does not disappear with extended

cooking. The phenomenon is called ‘thaw rigor’ or

‘thaw shortening’ and can be avoided by waiting

some time prior to freezing. A long thawing process,

in which the meat remains in the 5to2



C range

for a long period, alleviates the effect. A similar

phenomenon is observed in fish filleted prerigor and

frozen individually.

0014Freezing rate The effect of variations in freezing

rate on the tenderness of ground beef patties has

been investigated using conditions which achieved

freezing times to 18



C of 24, 48, 72, and 97 h. In

one study, immediately after freezing, patties frozen

in 24 h were less tough than those frozen in 96 h

(Table 1). This difference was still found after 6

months storage. After 18 months, patties frozen in

48 h were tenderer than those frozen at faster or

slower rates. Slight reductions in flavor and juiciness

were also found in patties frozen in 96 h. Taste panel

and sensory scores for the tenderness of ground beef

patties frozen in air at 43



C were higher than for

those frozen at 20



C. Freezing rates were not

detailed, and patties were stored for 2 weeks at

30



2736 FREEZING/Structural and Flavor (Flavour) Changes

0015 Frozen storage Three factors during storage, stor-

age temperature, degree of fluctuation in the storage

temperature, and type of wrapping/packaging in

which the food is stored, are commonly believed to

have the most influence on frozen storage life.

0016 Storage temperature has a marked effect on the

behavior of ice crystals that could be detrimental to

the ultimate quality of the food. It has been demon-

strated that frozen tissue stored for 180 days at

20



C has small ice crystals of irregular shape in

the extracellular spaces. Storage for the same period

at 3



C results in the development of large rounded

ice formations with a concomitant compression of the

muscle fibers. It is thus desirable that frozen meat

should be stored at a sufficiently low temperature to

prevent the growth of ice crystals in the extracellular

spaces. Such growth occurs if the temperature of the

frozen tissue is allowed to rise above its eutectic point.

(The eutectic point is the lowest freezing point of all

combinations or constituents of the food.) However,

quoted values for the eutectic point of meat range

from ‘probably just below 20



C’ to 52



0017 Generally, fluctuating temperatures in storage are

considered to be detrimental to the product. It has

been found that temperature fluctuations produced

during storage and transport of frozen food lead to

activation of the recrystallization processes, causing

enlargement of the ice crystals and diminishing ad-

vantages obtained in quick freezing of products.

However, it has been reported also that repeated

freeze–thaw cycles do not cause any essential change

in the muscle ultrastructure and that several freeze–

thaw cycles during a product’s life cause only minor

quality damage (in terms of juice loss) or possibly no

damage at all. A slight but significant improvement

has been found in samples that have been frozen and

unfrozen several times when tested by a taste panel.

0018 If the temperature cycles are so severe that the

surface temperature of the food rises above 3



during the process, this could create a potential

microbiological hazard.

0019 Minor temperature fluctuations in a stored product

are generally considered to be unimportant, especially

if they are below 18



C and are only of the magni-

tude of 1–2



C. Well-packed products and those that

are tightly packed in palletized cartons are also less

likely to show quality loss. When ground pork and

beef are stored under fluctuating conditions of

between 18 and 23



C for a 6-month period, the

well-packed samples show no deterioration. How-

ever, poorly packed samples are severely affected by

the temperature swings.

0020The proteins of fish differ from those of meat in

their higher susceptibility to damage in frozen stor-

age. Frozen storage results in an increasing tendency

to lose moisture on thawing and an increased firm-

ness, leading to toughness and dryness on cooking.

These changes are the result of denaturation and

cross-linking reactions predominantly of the myo-

fibrillar proteins.

0021Drip Freezing, always tends to decrease the water-

holding capacity and hence increases drip. When

meat is frozen quickly, the water, released by the

fibrils as the meat has gone into rigor, and that

which is still held are frozen simultaneously. Conse-

quently, there is no change in their relative positions

or amounts. At slower freezing rates, however, the

water balance is altered, the extracellular water freez-

ing first. As freezing continues, the existing ice crys-

tals grow at the expense of water from the

intrafibrillar space. This can result in salt crystalliza-

tion and pH changes which can cause protein de-

naturation. This has been well documented for

frozen fish, which is much more susceptible to freez-

ing damage than meat.

0022A number of scientific investigations have defined

the effect of freezing rate on drip production in meat,

demonstrating that the optimal conditions for freez-

ing portioned meat are those that achieve freezing

rates between 2 and 5 cm h

1

to 7



C. ‘Sow freezing’

up to 0.39 cm h

1

resulted in a decreased solubility of

myofibrillar proteins, increase in weight loss during,

freezing thawing, and cooking, lower water-binding

capacity, and tougher cooked meat. ‘Very quickly

frozen’ meat (> 4.9 cm h

1

) had a somewhat lower

tbl0001 Table 1 Effect of freezing rate on tenderness of ground-beef patties (ranging from 1, extremely tough, to 8, extremely tender)

Storage time Freezing rate (hours to 18



24 48 72 96 SE

Just before freezing 6.1

5.6

5.8

6.0

0.03

Just after freezing 5.1

4.7

d,e

4.8

d,e

3.7

0.25

6 months 4.9

4.9

4.2

4.3

0.10

18 months 4.5

e,f

5.2

4.7

d,e

4.1

0.09

a, b, c, d, e, f

The same letters indicate no statistical differences.

From Berry BW and Leddy KF (1989) Effects of freezing rate, frozen storage temperature and storage time on tenderness values of beef patties. Journal of

Food Science 54: 291–296.

FREEZING/Structural and Flavor (Flavour) Changes 2737