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

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The level of TMA produced can be used as an ‘index

of freshness,’ as the fish decays during storage on ice.

This illustrates how a volatile compound can be

classed as desirable, or as an off-odor, dependent on

its concentration.

Enzymatic and Chemical Degradation

Mechanisms

0018 The metabolic processes in plants and animals, which

are critical in relation to their storage stability after

harvesting or after slaughter, involve a complex series

of enzyme-catalyzed chemical reactions (i.e., bio-

chemical reactions). These lead to deterioration of

food quality, and the process is normally referred to

as autolysis (i.e., self-destruction). The endogenous

enzymes causing autolysis are an integral part of the

plant or animal food – and any treatment to affect the

enzyme activity cannot be done without the possibil-

ity of affecting the food itself.

0019 Enzymatic and chemical reactions overlap in the

sense that the enzyme speeds up the chemical reac-

tion, but to such an extent that the reaction rate is not

significant in the absence of the enzyme. A ‘chemical

catalyst,’ e.g., a transition metal ion such as copper,

iron, or nickel, can cause a large increase in the rate,

but the reaction will normally proceed at a measur-

ably rate in the absence of the ‘chemical catalyst.’

Thus, the oxidation in air of fish oils is catalyzed by

copper ions, but the oxidation still proceeds measur-

ably in the absence of copper ions. In contrast, the

hydrolysis of fish oil occurs readily at room tempera-

ture when catalyzed by the enzyme lipase, but in

the absence of lipase, the hydrolysis does not occur

significantly, not even at 100



C (at pH 7).

0020 Enzymes are complex proteins, and their catalytic

action is related to the shapes taken up by the protein

chains. This shape is held together mainly by numer-

ous weak hydrogen bonds, and these can be weakened

and changed by processes such as heating, freezing, or

changing the pH. The result is a loss of catalytic

activity by the enzyme; however, for endogenous

enzymes, the effect of such treatments on the organo-

leptic qualities of the food must be considered. More

specific methods of inhibiting enzyme activity include

the use of specific enzyme inhibitors such as sulfur

dioxide and bisulfites, the use of competitive sub-

strates, or chelation of metals necessary for the

activity of the enzyme.

0021 The undesirable effects of enzyme-catalyzed reac-

tions (i.e., autolysis) can be prevented in principle,

but often, the effect on the food’s organoleptic quality

may not be acceptable.

0022 The major chemical components of foods, i.e.,

lipids, proteins, and carbohydrates, can undergo

chemical changes, whether enzyme-catalyzed or

not, which can influence the safety, organoleptic

quality, or nutritional quality of the food. The

major reactions involved are oxidation and hydroly-

sis, at least initially, but more complex chemical

changes may occur subsequently. These can result

in the production of rancidity and browning (i.e.,

discoloration) which are often undesirable (see

Figure 1).

0023In general terms, the chemical changes can:

0024produce new compounds – possibly with undesir-

able toxic or organoleptic properties

0025destroy food components of relevance to its

nutritional quality (e.g., vitamins) or organoleptic

quality (e.g., loss of color).

Some examples of enzymatic and chemical degrad-

ation mechanisms are discussed below, but reference

to Figure 1 will emphasize how these mechanisms are

interconnected, especially in cellular foods.

Enzymatic Degradation Mechanisms

0026As previously stated, fresh meat and fish deteriorate

in quality mainly for microbiological reasons when

stored at ambient or chill temperatures. Frozen

storage (e.g.,30



C) will prevent microbiological

growth, but enzyme activity may still proceed

slowly.

0027Coldwater fish often show rapid autolysis when

stored chilled or on ice, and in this case, enzymatic

degradation mechanisms are as important as micro-

biological degradation mechanisms. This arises be-

cause coldwater fish live at temperatures near zero,

and their enzymes work efficiently in this temperature

range – hence, storage in ice, or at refrigeration tem-

perature, affords little reduction in temperature. This

contrasts with tropical fish where ice storage is effect-

ive in slowing down autolysis. (See Fish: Spoilage of

Seafood.)

0028Thus, for coldwater fish (e.g., cod, haddock),

enzyme activity proceeds noticeably during chilled

storage, especially protease activity leading to protein

hydrolysis and resultant excessive softening of fish

tissue. This tissue breakdown encourages microbial

growth, especially involving the breakdown of sulfur

containing amino acids to volatile, foul smelling

thiols, and disulfides. The fish gut is the source of

very active proteases, and gutting of the fish is effect-

ive in slowing down this autolysis and so extending

the chill storage life. (See Fish: Processing.) Frozen

storage also slows down the enzyme activity, but for

gadoid fish, such as cod, haddock, and hake, enzyme

activity can convert trimethylamine oxide to

dimethylamine and formaldehyde, i.e.:

STORAGE STABILITY/Mechanisms of Degradation 5607

C−N

−O

−

C−N

H−CO

Enzyme

Trimethylamine

oxide

Dimethylamine Formaldehyde

catalyzed

The formation of formaldehyde has been related to

the toughening of such fish in frozen storage by its

ability to cross-link proteins.

0029 Meat and fish may be high in lipid content, and

for fish, the lipid is highly unsaturated and susceptible

to oxidation (see Figure 1). Lipoxygenases catalyze

oxidation in air of lipids with at least two double

bonds (See Oxidation of Food Components), and

specific lipoxygenases have been identified in the

gill tissue of fish. The initially formed hydroper-

oxides are decomposed to volatile carbonyl com-

pounds (aldehydes) with undesirable odors (see

Figure 1).

0030Even though plant foods have a low lipid content,

they have various hydrolytic and oxidative enzymes

that catalyze hydrolysis and oxidation of the lipids to

produce rancidity. The initial action of lipases in pro-

ducing fatty acids from the hydrolysis of triacylgly-

cerols may not produce the necessary volatile fatty

acids for rancidity, but the lipoxygenase enzymes ef-

fectively catalyze the oxidation of the unsaturated

fatty acids to give the carbonyl compounds, which

have the rancid odor, i.e.:

Lipids

Hydrolysis*

Autoxidation** Cross-linking

Proteins Carbohydrates

Radicals

Hydrolytic

rancidity

Oxidation of

vitamins (A,D,E)

Carbonyl

compounds

Ascorbic

acid

Polyphenols

Oxidation***

Enzymatic

browning

Ascorbic acid

degradation/

browning

Maillard**

browning

(ambient temp.)

Oxidation*

Oxidative

rancidity

Oxidation* Hydroperoxides

Hydrolysis*

Aminoacids

Sugars

Nutritional

losses

Stale

flavours

Protein

cross-linking

Texture

toughening

Hydrolysis*

fig0001 Figure 1 Enzymatic and chemical degradation mechanisms – rancidity and browning. *Enzymatic or chemical mechanism;

**chemical mechanism; ***enzymatic mechanism; *

anaerobic also, involving hydrolysis.

5608 STORAGE STABILITY/Mechanisms of Degradation

Lipases

Fatty acidsTriacylglycerols + H

Lipoxygenases

Carbonyl compounds

(unsaturated) (unsaturated)

This process occurs more readily if the plant food is

crushed or ground, allowing the substrates and

enzymes to mix. Thus, raw peanuts only go rancid

rapidly if they are ground, but if they are dry-roasted,

the enzymes are inactivated, and no oxidation occurs.

0031 The blanching process used for unprocessed plant

foods (e.g., peas, beans) prior to freezing is necessary

to prevent flavor deterioration resulting from enzyme-

catalyzed oxidation, especially lipoxygenase. (See

Freezing: Operations.) Additionally, for plant foods,

the oxidation (in air) of phenolic substrates catalyzed

by a group of enzymes referred to as the polyphenol

oxidases (PPO) is important if they are to be pro-

cessed and stored under ambient or chilled condi-

tions. This is referred to as enzymatic browning (see

Figure 1) and is typified by the surface browning of

peeled fruit and vegetables (e.g., apples and potatoes).

Processing involving chopping, slicing, and peeling

allows the substrate, air, and enzymes to mix and

promotes enzymatic oxidation of the phenolic sub-

strate to quinones. These undergo further complex

nonenzymatic reactions to form melanin pigments,

which may be pink, brown, or black.

PPO enzymes

Quinone Melanins

Polyphenol substrate + O

The process may be controlled by inactivating the

polyphenol oxidase enzyme system – but care must

be taken not to cause undesirable changes in texture

(e.g., by blanching) or flavor (if the enzyme inhibitor

sulfur dioxide is used). Addition of a competitive

substrate (e.g., 4-hexylresorcinol) for the enzyme

may be more acceptable, or simply the addition of

ascorbic acid, which is able to reduce the quinone

back to the phenolic compound, hence preventing

the conversion of the quinone to the melanins.

(See Browning: Enzymatic – Biochemical Aspects;

Enzymatic – Technical Aspects and Assays.)

0032 Plants after harvesting are still respiring, and

some plants, fruits in particular, show a large increase

in respiration during storage. This is linked to the

rapid ripening of some fruits, e.g., bananas and

pears, which can limit their desired shelf-life. The

enzyme systems involved in the ripening process can

be slowed by chilling the fruit (though for bananas,

the temperature must not be below 10



C) and also by

decreasing the oxygen concentration and increasing

the carbon dioxide concentration (i.e., the reverse of

the respiration process). However, the oxygen con-

centration must not drop below about 3–5% in order

to prevent anaerobic respiration, where alternative

enzyme systems operate to convert sugar to ethanol

(i.e., fermentation) rather than to carbon dioxide and

water. (See Ripening of Fruit.)

0033During the ripening process of plant foods, the

taste and texture develop to give an acceptable sweet-

ness and softness. In relation to the taste, the process

involved is the starch – sugar conversion, and this can

be temperature-dependent in a complex manner.

0034The overall starch – sugar (sucrose, glucose, fruc-

tose) conversion is as follows:

Phosphorylase

Glucose-1-phosphateStarch

Sucrose phosphate

Several steps

involving

glycolytic

enzymes

Glucose

Fructose

Invertase

Sucrose

The starch may be converted mainly to sucrose (a

nonreducing sugar), or the sucrose may undergo hy-

drolysis catalyzed by the enzyme invertase (sucrase)

to the reducing sugars glucose and fructose. The mix-

ture of glucose and fructose (so-called ‘invert sugar’)

is sweeter than sucrose – hence, the actual sweetness

achieved can vary in relation to the sucrose –‘invert

sugar’ ratio. An important commercial aspect of the

starch–sugar conversion is the storage of potatoes to

be used for making crisps (i.e., fried potato slices,

called ‘chips’ in the USA). If the potato has too

much of the reducing sugar, the crisp may become

too dark as a result of the Maillard browning reaction

(see Figure 1). Storage of the potatoes at 4



C (to

reduce respiration) results in ‘cold-induced’ sweeten-

ing, i.e., the reducing sugar level increases, and the

crisps produced are too dark. Storage at about 10



results in less starch–sugar conversion, and the crisps

produced are of an acceptable light brown color. The

higher storage temperature (10



C) appears to inacti-

vate the glycolytic enzymes, hence inhibiting the

sugar production. (See Potatoes and Related Crops:

Processing Potato Tubers.)

Chemical Degradation Mechanisms

0035Storage conditions under which microorganisms and

enzymes are inactive (e.g., after heat processing) can

STORAGE STABILITY/Mechanisms of Degradation 5609

still enable chemical degradation mechanisms to op-

erate, especially where radicals are involved in oxida-

tion reactions (see below). In general terms, however,

the conditions, especially temperature, required for

rapid chemical reactions are more extreme than

used for food storage, where ambient and subambient

temperatures are used. However, the long storage

times required for processed foods (often in terms of

months) can allow time for products from a slow

chemical reaction to become noticeable. One

example is the Maillard reaction, an example of

nonenzymatic browning (see Figure 1) but better con-

sidered as a ‘carbonyl-amino’ reaction. (See Browning:

Nonenzymatic; Toxicology of Nonenzymatic Brow-

ning.) The Maillard reaction occurs quickly on

heating foods containing amino compounds (e.g.,

proteins, amino acids) and reducing sugars (e.g., glu-

cose, lactose) and is responsible for the desirable

colors and flavors of baked, fried, and toasted

foods. The Maillard reaction is slow at normal stor-

age temperatures but, over extended storage periods,

can produce undesirable darkening and a stale flavor

(e.g., in dried fruits and milk powder) and toughening

of texture (through protein cross-linking) in frozen

fish.

0036 The Maillard reaction involves a reaction between a

polar carbonyl compound (aldehyde or ketone) and a

polar amino compound, as outlined below:

δ−

Melanoidins

Colored/brown

macromolecules

Polar carbonyl

compound

(e.g., glucose)

Polar amino

compound

(e.g., protein)

δ−

δ+

The opposite charges initiate a linking (as indicated

by the arrow) between the glucose and the protein,

which ultimately produces the brown melanoidins.

0037 The degradation aspects arise from the fact that

nutritionally important proteins and essential amino

acids react and so become unavailable for digestion

and assimilation. Additionally, the complex series of

reactions leading to the undesirable brown melanoi-

dins involve side-reactions leading to volatile off-

odors (stale odors) for the stored foods.

0038 The broader description of the Maillard reaction as

a carbonyl-amino reaction indicates how the car-

bonyls produced from the oxidation of lipids, poly-

phenols, and ascorbic acid can also give the Maillard

reaction with proteins/amino acids in the absence of

reducing sugars (see Figure 1). Ascorbic acid is oxi-

dized readily in air, and the dehydroascorbic acid

produced is a diketone, which can give the carbonyl-

amino reaction – this explains why some foods brown

quickly, even though their reducing sugar content is

low. The Maillard reaction (carbonyl-amino reaction)

is very slow at pH 3–3.5, but ascorbic acid can oxi-

dize at this pH and still produce darkening as a result

of its own specific degradation. Such ascorbic acid

degradation is the form of nonenzymatic browning

occuring in stored citrus fruit concentrates and spray-

dried powders, especially noticeable for lemon and

grapefruit. Obviously, the nutritional value as a

source of vitamin C is reduced. (See Citrus Fruits:

Lemons; Grapefruits.)

0039The oxidation of unsaturated lipids giving rise to

oxidative rancidity is perhaps the most important

chemical degradation mechanism for stored foods,

and especially so for refined edible oils and derived

products such as margarine and salad creams. See

Oxidation of Food Components. On exposure to air,

and especially in the presence of sunlight and transi-

tion metals (e.g., copper, iron), unsaturated lipids

are oxidized to hydroperoxides by a radical chain

reaction normally referred to as autoxidation (self-

oxidation). This process is important since the aut-

oxidation, once started, is self-perpetuating, and the

hydroperoxides produced not only decompose to

rancid odors but are good oxidizing agents, which

can cause the destruction of fat-soluble vitamins

(A, D, E) and the oxidation of sulfur side groups in

proteins leading to their cross-linking, with conse-

quent loss of nutritional quality and toughening of

texture. The autoxidation of lipids requires a radical

to be formed to initiate the radical chain reaction, this

may take a long time, and this induction period may

be followed by a surprisingly rapid oxidation, making

prediction of storage life difficult (See Storage Stabil-

ity: Shelf-life Testing). The initial radical may be de-

rived from enzyme (lipoxygenase)-catalyzed lipid

oxidation (during the extraction/processing

procedures) producing a trace of hydroperoxide.

The subsequent breakdown of this hydroperoxide to

radicals gives the autoxidation mechanism its re-

quired initial radical (i.e., an unpaired electron

system):

Hydroperoxide Alkoxyl

radical

Hydroxyl

radical

O H RO + OH

For other possible sources of this first radical, (See

Oxidation of Food Components.)

0040Once a radical has been produced (e.g., OH), the

following chain reaction can occur with the lipid

(L — H):

5610 STORAGE STABILITY/Mechanisms of Degradation

L H + OH L + H

L OO

L O O + L L OO

L + O

H H + L

The last two steps provide the repeating chain reac-

tion, and as long as air (oxygen) is present, the con-

version of the lipid to hydroperoxide will continue. In

addition to the points made above concerning the

reactions and decomposition of the lipid hydroperox-

ide, it has been observed that one particular dialde-

hyde (malondialdehyde) is produced, which, with

protein, gives the Maillard reaction in such a way

that protein cross-linking occurs.

0041 The role of transition metals in catalyzing chemical

reactions has been referred to above, e.g., the decom-

position of the lipid hydroperoxide is catalyzed

by transition metals and especially by the heme iron

pigments in meat. Other reactions of metals are

examples of chemical degradation mechanisms

when undesirable precipitates or colors are produced.

The heme pigments in meat react in the presence

of oxygen to produce either the desirable bright

red oxymyoglobin or the undesirable metmyoglobin.

(See Meat: Preservation.) Anthocyanins (phenolic

compounds) in fruits can form colored salts with

metals, one example being the formation of a pink

tin complex on the surface of pears when they

are canned. The blue–black discoloration that

can occur with some canned peaches is of a similar

origin An example of undesirable precipitate forma-

tion is the formation of magnesium ammonium

phosphate (struvite) in canned crutaceans (e.g.,

crab). (See Shellfish: Contamination and Spoilage of

Molluscs and Crustaceans.) The crystals formed re-

semble glass and hence lead to consumer rejection.

However, this can be prevented by adding calcium

disodium ethylenediaminetetraacetic acid, which

preferentially binds with the magnesium. For acidic

foods, the reaction of metals with acids to produce

hydrogen and dissolve the metal can be a problem

during storage of canned foods. (See Acids: Properties

and Determination; and Canning: Quality Changes

During Canning.) The tin plate used in steel cans can

be dissolved by acidic foods, and the dissolved tin

concentration could be a health problem – the use of

protective lacquers to prevent this is well established.

However, the recent recall (November, 2000) of a

batch of canned tomato soup because of its high

dissolved tin concentration shows that this chemical

reaction can still be a problem!

0042The complex nature of foods makes it difficult to

be sure of the degradation mechanisms operating, as

reference to Figure 2 will confirm. In some cases, all

the mechanisms proposed could be operating, as is

found for milk. Milk is highly susceptible to degrad-

ation by mechanisms that are:

0043physical (e.g., cream (fat) separation);

0044microbiological (e.g., both spoilage and pathogenic

bacterial growth);

0045enzymic (e.g., lipase-catalyzed lipolysis to fatty

acids);

0046chemical (e.g., ‘light-induced flavor’ from lipid

oxidation and methional formation catalyzed by

riboflavin in the presence of light)

(See Milk: Physical and Chemical Properties.)

0047The above list of food degradation mechanisms

provides a framework that can be used as a starting

point for appreciating the degradation mechanisms

operating in specific food systems.

See also: Canning: Quality Changes During Canning;

Emerging Foodborne Enteric Pathogens; Food

Poisoning: Classification; Freezing: Operations; Storage

of Frozen Foods; Meat: Preservation; Microbiology:

Classification of Microorganisms; Detection of Foodborne

Pathogens and their Toxins; Packaging: Packaging of

Solids; Preservation of Food; Spoilage: Chemical and

Enzymatic Spoilage; Bacterial Spoilage; Fungi in Food –

An Overview; Molds in Spoilage; Yeasts in Spoilage;

Storage Stability: Shelf-life Testing

Food

type

Storage

conditions

Degradation

mechanism

Consumer

concern

*Processed *Map

*Fabricated *Frozen *Enzymatic

(colloidal) *Aerobic *Chemical

*Fresh *Ambient *Physical *Safety

(cellular) *Chilled *Microbiological *Quality

fig0002 Figure 2 Factors of relevance to mechanisms of degradation of foods.

Modified atmosphere packaging/vacuum packaging;

Both

organoleptic and nutritional.

STORAGE STABILITY/Mechanisms of Degradation 5611

Further Reading

Adams MR and Moss MO (2000) Modern Food Micro-

biology, 2nd edn. London: Royal Society of Chemistry.

Eskin NAM (1990) Biochemistry of Foods, 2nd edn. New

York: Academic Press.

Frankel EN (1998) Lipid Oxidation. Dundee, UK: The Oily

Press.

Ho C-T and Hartman TG (1994) Lipids in Food Flavours.

Washington, DC: American Chemical Society.

Jay JM (2000) Modern Food Microbiology, 6th edn. New

York: Aspen.

Jen JJ (1989) Quality Factors of Fruit and Vegetables.

Washington, DC: American Chemical Society.

Karel M (1984) Chemical effects in foods stored at room

temperature. Journal of Chemical Education 64: 335–

339.

O’Brien J, Nursten HE, Crabbe MJC and Ames JM (1998)

The Maillard Reaction in Foods and Medicine. London:

Royal Society of Chemistry.

Powrie WD (1984) Chemical effects during storage of

frozen foods. Journal of Chemical Education 61: 340–

347.

Snowdon AL (1990) A Colour Atlas of Post Harvest Dis-

eases and Disorders of Fruit and Vegetables. London:

Wolfe Scientific.

Wong DWS (1989) Mechanisms and Theory in Food

Chemistry. New York: Van Nostrand Reinhold.

Wong DWS (1995) Food Enzymes – Structure and Mechan-

isms. New York: Chapman & Hall.

Parameters Affecting Storage

Stability

M Hole, University of Lincolnshire and Humberside,

Lincoln, Grimsby, UK,

Introduction

0001 In developed countries modern attitudes to food

retailing include the demand for even longer shelf-

lives at the point of sale and within the home. Together

with the increasing demand for convenience foods

and complete meals (which can readily be reheated

if required, even from the frozen state) the parameters

influencing the necessary storage stability must be

increasingly better understood. This is especially so

considering the trend towards short-shelf-life, chilled

foods, which have the desired ‘fresh’ qualities.

0002 The nature of the changes which influence have

been characterized in part (See Storage Stability:

Mechanisms of Degradation) as physical, chemical,

enzymatic, and microbial.

0003The relevance of such changes in influencing stor-

age stability will depend on the treatment of the food

prior to storage. It is useful to consider foods as either

fresh or processed. Processed foods (e.g., heated,

frozen, dried, salted, smoked) generally will have no

active microbial population and minimal enzymatic

activity prior to storage. In contrast, fresh foods

(i.e., not processed and stored at ambient or chill

(typically 0–5



C) temperatures) will have active

microbial populations, and some residual enzymatic

activity. There is also an increasing demand, both

by the catering sector and the individual consumer,

for minimally processed fruit and vegetables (e.g.,

sliced, peeled). Additional problems exist for this

form of fresh food in relation to its storage, due to

increased enzymatic reactions, e.g., discolorations,

and increased susceptibility to microbial growth.

0004The parameters of major importance for storage

stability are temperature, moisture content, pack-

aging and food additives (i.e., added chemicals).

Each of these parameters will be considered in turn

but it should be appreciated at the outset that the

combined use of the above parameters is usually

beneficial in practice.

0005The methods used to extend the storage life of

foods are increasingly subject to critical examination

by consumers, and this can influence their implemen-

tation by the major supermarket chains. Thus the use

of irradiation as a means of extending shelf-life is not

presently used in the UK, except for some spices. A

recent (1999) statement by the Institute of Food

Science and Technology (UK) commends irradiation

as a safe and effective method for food preservation.

Applications identified include:

0006certain fruits and vegetables: to reduce spoilage by

microorganisms, e.g. molds in strawberries

0007poultry: to reduce levels of Salmonella and other

food-poisoning organisms

0008red meats: to reduce levels of Escherichia coli

0157:H7 and other food-poisoning organisms

0009imported sea foods, e.g., warm-water prawns: to

improve their safety during storage

0010dried herbs and spices: to reduce food-poisoning

bacteria

0011The use of irradiation allows for extended storage

if foods are suitably packed to prevent contamin-

ation, although frozen storage may also be beneficial.

0012The use of irradiation in this way is gaining accept-

ance in the USA but not in the UK. In trials in the USA

irradiated fruit (e.g., strawberries) outsold the non-

irradiated form – not least because of their extended

mold-free storage period. (See Irradiation of Foods:

Basic Principles.) The major parameters for storage

stability described reflect those widely used in present

5612 STORAGE STABILITY/Parameters Affecting Storage Stability

commercial practice. Consumer opinion is very im-

portant when supermarkets consider new methods of

storage stability and this is especially the case with

genetically modified (GM) foods. Genetic modifica-

tion of foods can allow extended storage, and a gen-

etically modified tomato with a reduced ripening rate

(and hence longer storage period) was used to pro-

duce a tomato paste which was successfully sold in

UK supermarkets until quite recently, when the super-

markets made a decision not to stock GM foods in

general. (See Biotechnology in Food Production;

Gene Expression and Nutrition.)

Temperature

0013 The temperature at which food is stored is the

most important parameter which influences storage

stability – especially for fresh foods. Biological

processes, such as the respiration of fruit and vege-

tables, enzymatic activity, and microbial growth, such

as the mold on cheese or bread, can all be greatly

reduced in rate by the use of chill or refrigerated

conditions (0–5



C): whilst in the frozen state

(20



C typically) they effectively cease. Chemical

reactions are typically reduced by 50% of their ambi-

ent (20–25



C) rate by a drop of 10



C but some

reactions can still proceed even during frozen storage

if this exceeds a few months. Physical effects resulting

in textural changes are one potential disadvantage of

frozen storage, especially for intact tissue systems,

such as vegetables, fruit, meat, and fish. This has led

to the increased use of chilled storage where the

‘fresh’ image is maintained for such tissue systems.

The use of chilled storage combined with modified-

atmosphere packaging can extend the storage period

significantly. However some fruits and vegetables

suffer chilling injury, notably bananas below 12



tomatoes below 7



C, apples below 2



C, and cucum-

bers below 7



C. (See Chilled Storage: Use of Modi-

fied-atmosphere Packaging; Chill Foods: Effect of

Modified-atmosphere Packaging on Food Quality.)

0014 Reference to Table 1 shows the much greater stor-

age periods possible with frozen storage but possible

texture changes must be considered. An extreme

example concerns the strawberry which loses all its

firmness on thawing with excessive loss of water

(‘drip loss’). Frozen storage of eggs leads to the

yolk gelling and fish texture can toughen appreciably

after frozen storage. Frozen stored milk acquires a

gel-like consistency and frozen cream has large

aggregates of demulsified fat after thawing. Icecream

can acquire a ‘sandiness’ due to lactose crystalliza-

tion. The use of cryoprotectants to reduce such detri-

mental effects is described below. (See Freezing:

Principles.)

0015When water freezes to form ice there is a 9%

increase in volume and since ice is ‘pure H

O,’ the

remaining water will have a higher concentration of

solutes (so-called ‘freeze concentration’). These two

aspects can cause undesirable changes, especially tex-

tural, during frozen storage. In addition temperature

fluctuations during frozen storage will allow small ice

crystals to melt and reform as larger ice crystals which

can result in cellular damage. Sublimation of ice

crystals from the surface of the food due to the low

humidity at frozen storage temperatures (10



Cto

30



C) can result in ‘freeze-drying’ of the surface,

with the resultant discoloration. (See Freeze-drying:

The Basic Process.)

0016Despite the above limitations, frozen storage is per-

haps the most widely accepted method (in developed

countries) of long-term storage of tissue foods, such

as vegetables (with prior blanching to inactivate

enzymes), meat, and fish. For processed foods such

as bakery products and increasingly for complete

meals, frozen storage is used and can be combined

with microwave treatment for rapid regeneration.

0017Bread is an example of a processed food where

there is no cellular structure but where texture is

important – rejection due to ‘staling’ is an important

contribution to waste. Chill storage accelerates such

staling but frozen storage below 20



C will prevent

staling for up to 3 months.

0018More generally, processed foods containing starch

can suffer textural changes due to the starch polymer

chains associating. Thus starch gels will be lumpy on

thawing after frozen storage. An array of structurally

modified starches has been produced which can pre-

vent these undesirable changes occurring during

frozen storage of such processed convenience foods.

(See Starch: Modified Starches.)

0019In general the minimal changes in organoleptic

properties of appropriate frozen foods, along with

tbl0001Table 1 Temperature effects on shelf-life stability

Commodity Storagetemperature andshelf-lifedays (D),

months (M) at (



Ambient Chill Frozen

(20



Fish (lean) <1D 5–7D (0) 10–12M

Fish (oily) <1D 3–5D (0) 6–8M

Meat (red) <1D 4D (2) 6–12M

Pork <1D 4D (2) 3–6M

Milk (pasteurized) 0.5D 4D (5) NS

Bread 2–4D NS 3M

Peas 1–2D 4–8D (5) 12M

Lettuce (iceberg) <1D 10D (5) NS

Strawberries 1–2D 3D (5) NS

Cakes (nondairy) 7D NS 3M

Wrapped to prevent dehydration: see text.

NS, not suitable: see text.

STORAGE STABILITY/Parameters Affecting Storage Stability 5613

low nutritional changes (e.g., vitamin losses) and the

inherent safety from microbial growth at such low

temperatures (20



C or below) suggest the con-

tinued popularity of frozen storage where extended

storage times (months) are required. With chill stor-

age the knowledge that some pathogens can grow at



C, e.g., Listeria monocytogenes, restricts storage

times and necessitates strict time–temperature

regimes. However the increasing demands for ‘fresh-

like’ produce has led to improved storage periods at

chill conditions by utilizing specific packaging (see

below). (See Listeria: Properties and Occurrence;

Spoilage: Bacterial Spoilage.)

Moisture

0020 The moisture (water) content of fresh tissue foods can

be very high (above 70%) and this allows microbial

spoilage to limit storage stability, if the temperature is

not controlled. (See Water Activity: Effect on Food

Stability.)

0021 Foods can be made stable towards microbial

changes, and also enzymatic and chemical changes,

by reducing the water content. Figure 1 indicates how

water activity, a

(which is related to moisture con-

tent but attempts to measure how available the water

is), is related to possible changes during storage. Fresh

tissue foods have a

above 0.95, jams (30% moisture)

have a

0.80–0.75, dried fruits (20% moisture) have

0.65–0.60, pasta (12% moisture) has a

0.50,

whilst dry foods such as biscuits and cornflakes with

5% moisture or below have a

below 0.30. Figure

1 would suggest that storage stability is assured if a

is kept below about 0.3. For processed foods of such

low a

which are consumed directly (rather than

rehydrated), their texture can be of critical import-

ance. Thus the crispness of breakfast cereals, biscuits,

and potato crisps (chips in the USA) is perhaps the

major consumer expectation. Such products are

hygroscopic due to their high carbohydrate content

and can absorb water with resultant loss of the desir-

able crisp or brittle texture. Figure 1 also shows that

dry foods, if lipid containing, can readily undergo

oxidative rancidity – hence a potato crisp with oil

content of 35% and a moisture content of 2–3%

would be prone to both off-flavor production and

loss of crispness during storage. (See Drying: Theory

of Air-drying.)

0022Dehydrated foods intended for rehydration prior to

consumption, e.g., dried potato mash, will absorb

moisture with resultant clumping, which adversely

affects the texture on rehydration.

0023In contrast to dried foods, intermediate-moisture

foods (a

0.6–0.9) represent processed foods storable

without refrigeration but which can lose moisture

and acquire a dry rather than the desired moist tex-

ture – cakes and pastry products are typical examples.

(See Intermediate-moisture Foods.)

0024The undesirable ingress or loss of moisture depends

on the humidity of the surrounding atmosphere and

can be controlled by packaging. Where intermediate-

moisture foods and fresh (tissue) foods are concerned,

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Relative rate of change

12345678

Water activity

fig0001 Figure 1 Stability of food as a function of water activity (a

) at ambient temperature. a

¼ p/p

, where p and p

are the equilibrium

partial vapor pressure of water in the food and of pure water respectively. Key: 1 Lipid oxidation; 2 nonenzymic browning;

3 hydrolytic reactions; 4 loss of ascorbic acid; 5 enzymatic activity; 6 mold growth; 7 yeast growth; 8 bacterial growth.

5614 STORAGE STABILITY/Parameters Affecting Storage Stability

the high a

could allow microbial – especially mold –

growth. The composition of intermediate-moisture

foods is controlled by use of humectants (e.g.,

sugar), pH, and preservatives in order to prevent

migration of moisture and microbial growth. (See

Preservation of Food; Spoilage: Molds in Spoilage.)

0025 For fresh fruit and vegetables, where respiration

continues, water is produced and will encourage

mold growth if not removed, e.g. by storage in a dry

atmosphere. However, such water loss must be min-

imized if undesirable texture changes are not to occur

(e.g., loss of firmness of green vegetables and pota-

toes) – hence a reduction in temperature in order to

slow respiration is often used. Bread has a relatively

high moisture content (35–40%) and to maintain the

desirable firm crust (which has a moisture content of

3–5%) the moisture which migrates from the interior

towards the crust must be allowed to escape.

Packaging

0026 Along with the general function of containing the

food and preventing contamination, packaging can

be an effective barrier against transfer of microorgan-

isms, oxygen and, as already considered, moisture.

0027 Additionally, exclusion of light and increasingly

the control of the gaseous contents of the pack are

important.

0028 For sterilized foods prevention of ingress of micro-

organisms is the major requirement and metal con-

tainers (cans) have been so used for over 100 years.

Flexible pouches and rigid plastic containers – the

latter allowing direct reheating by microwave – are

increasingly used. With oxygen removal (or reduc-

tion) such sterilized foods are storable for up to 12

months at ambient temperature. Detrimental oxida-

tion of lipids, color, and flavors is accelerated by light

and hence opaque packaging, e.g., metallized films, is

beneficial. Discoloration in (metal) cans, e.g., with

high-pH protein-rich foods (meat, fish), is avoided

by the use of special lacquers on the inside surface

of the can. (See Canning: Quality Changes During

Canning; Sterilization of Foods.)

0029Where lipid oxidation is the major limitation to

storage stability, vacuum or inert gas (nitrogen) pack-

aging is beneficial. Vacuum packaging of cured meats

and nitrogen flushing of dry roasted nuts and muesli

are examples.

0030Carbon dioxide (CO

)at1toþ2



C has an

inhibitory effect on microbial growth and this

allows modified-atmosphere packaging (MAP),

coupled with chill storage, to give ‘fresh’ food an

extended storage life, which is commercially import-

ant for foods such as fresh meat, fish, fruit, and

vegetables, and for fresh baked products but not for

dairy products (Table 2). For fresh red meat, storage

with high levels of oxygen (70–85%) is needed to

maintain the desirable red color (otherwise the

brown color typical of frozen stored meat is formed).

Other examples are shown in Table 2, where the

avoidance of oxygen for fatty fish should be noted,

along with the high nitrogen concentrations to pre-

vent pack collapse due to the high solubility of carbon

dioxide in fish flesh. This latter aspect gives rise to

increased ‘drip loss’ which is absorbed into a pad

placed beneath the fish.

0031For fresh fruit and vegetables, their continuous

respiration results in changes in the oxygen and

carbon dioxide concentrations in the pack and the

permeability of the packaging film to these gases

must be considered.

0032The continuing and rapid respiration after harvest-

ing of fruits, especially climacteric fruits such as

bananas and apples, presents particular problems in

extending their storage lives. The use of relatively low

oxygen concentrations (2–3%) and high carbon diox-

ide concentrations (approximately 5%), allied with

chill temperatures (0–5



C), can reduce the rate of

respiration and give a useful extension of the storage

period. The required gas mixture can be achieved

tbl0002 Table 2 Examples of shelf-life extension using modified-atmosphere packaging

Commodity Gasmixture (%) Shelf-life

days (



Oxygen Carbondioxide Nitrogen

Fish (lean) 30 40 30 10 (0) 5(þ2)

Fish (oily) 60 40 10 (0) 5(þ2)

Meat (red) 75–85 25–15 21 (0) 7(þ2)

70 20 10 21 (0) 7(þ2)

Poultry 90 10 11 (þ2)

Meat (cooked) 100 10 (þ2)

Baked products (nondairy) 20–70 30–80 3 months (ambient)

Pasta (fresh) 50 50 3–4 weeks (0 to 5)

Strawberries 1–3 4–6 95–91 7 (þ5)

Lettuce (iceberg) 2–3 5–6 93–91 21 (þ5)

STORAGE STABILITY/Parameters Affecting Storage Stability 5615

rapidly by gas flushing, and with permeable plastic

packaging it can be maintained if the permeability to

oxygen (from outside) and to carbon dioxide (gener-

ated inside the pack) is correctly allowed for. It is

possible to achieve the required carbon dioxide and

oxygen concentrations by simply packaging in air but

equilibration then occurs more slowly. Recognition of

the role of ethene (ethylene) in initiating the ripening

process has suggested the use of ‘active packaging’

where ethene is removed by oxidation with potassium

permanganate, normally included in a sachet within

the pack. Such an approach has been used in Japan

with kiwi fruit but is not presently (2002) used in

the UK.

0033 Genetic modification to prevent ethene formation,

and hence ripening, is an alternative possibility and

has been demonstrated with tomatoes. Ripening can

then be initiated as required by the introduction of

exogenous ethene. Such an approach would reduce

dependence on MAP and refrigeration, resulting in a

simpler and cheaper storage system. However, the

present adverse consumer opinion concerning GM

foods would be a major problem in applying this

approach.

0034 Some novel gas mixtures for use in MAP have

recently been shown to have additional beneficial

effects for fruit and vegetables. These include the use

of very high oxygen levels (70–100%) and the gases

argon and nitrous oxide. High-oxygen MAP can

inhibit enzymatic discolorations, prevent anaerobic

fermentations, and inhibit microbial growth, includ-

ing aerobic microbial growth. The inhibition of

enzymatic discolorations is particularly useful for

minimally processed fruit and vegetables (e.g., peeled,

sliced), such as apples and potatoes, susceptible to

enzymatic browning, and offers an alternative to the

use of sulfites.

0035 The use of argon and nitrous oxide MAP is effective

in reducing fungal growth, reducing ethene forma-

tion, and generally maintaining a higher-quality prod-

uct, especially by comparison to the use of nitrogen.

0036The application of edible semipermeable coatings

(e.g., semper-fresh) to fruits, especially exotics, has

doubled shelf-lives at ambient temperatures. The

coating, which can be applied to the ripe fruit at

harvest, is edible, invisible, tasteless, and odorless

and seals the fruit in its own modified atmosphere,

which, while inhibiting oxygen inflow (and hence

respiration) allows carbon dioxide to escape and

hence prevents anaerobic effects which lead to

unpleasant tastes. This approach could usefully be

applied to minimally processed fresh produce, e.g.,

peeled and sliced fruit.

Food Additives

0037The technical functions of such chemicals, as distinct

from any cosmetic, nutritive, or processing-aid

functions, can be categorized as:

0038control of microbial changes, e.g., preservatives,

control

0039control of enzymatic changes, e.g., pH control,

sulfites (SO

)

0040control of chemical changes, e.g., antioxidants,

sulfites

0041control of physical changes, e.g., emulsifiers,

stabilizers, cryoprotectants

Traditional ingredients such as salt, sugar, and

vinegar also serve as preservatives but are not legally

defined as food additives. Hence the traditional use of

salt was to reduce a

and so restrict microbial

growth. However the food additive preservatives are

effective at very low concentrations, as shown in

Table 3. Although high-acid foods (pH below 4) will

restrict microbial growth, the preservatives in Table 3

allow control of microbial growth at up to pH 6.5.

The majority are weak acids for which the presence of

the undissociated acid is necessary for antimicrobial

activity. Such preservatives allow storage at ambient

temperatures for extended periods, e.g., benzoic acid

in fruit squashes and nitrite in cured (canned) meat.

Nisin is unusual in being a polypeptide and an

tbl0003 Table 3 Examples of chemical food preservatives

Preservative Typical foodapplication Maximumlevel (mgkg

1

) Organism affected

Benzoic acid and benzoates (E210–213) Low-sugar jams, marmalades 800 Yeasts and molds

Sorbic acid and sorbates (E200–203) Cheese 1000 Yeasts and molds

Low-fat spreads 2000

Propionic acid and propionates (E280–283) Bread 3000 (mg kg

1

flour) Molds

Sulfur dioxide, sulfites, hydrogen sulfites

and metabisulfites

(E220–228)

Fruit juice 350

Jams 100 Microorganisms

Dried fruit 2000

Nitrites and nitrates (E249–252) Cured meats 150 (nitrite <50) Bacteria

Nisin (E234) Cheese (processed) 12.5 Bacteria

Enzymes are inhibited also.

5616 STORAGE STABILITY/Parameters Affecting Storage Stability