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

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antibiotic. It occurs in milk and is effective against

some bacteria, but not yeasts or molds, and is used in

processed cheese. (See Curing; Nisin.)

004 2 The development of low-sugar preserves necessi-

tates the addition of a preservative (often sorbic

acid) to prevent mold growth and a similar situation

exists for low-fat spreads where the usual 80% fat of

margarine can be reduced to below 30%, with cor-

responding increase in a

and susceptibility to mold

growth.

004 3 Table 3 indicates that propionic acid, usually in the

form of calcium propionate, is used to control mold

growth in bread. However some bakers have chosen

to replace this preservative by vinegar, a solution of

acetic acid in water. This change was due to adverse

consumer perceptions of food additives or ‘chemicals’

(which could be identified by their E-numbers) in

contrast to vinegar which is classed as an ingredient

and does not carry an E-number. However the preser-

vative action of vinegar is due to the acetic acid

present – a weak acid like propionic acid.

004 4 Enzymatic activity varies widely with pH and is

reduced by lowering the pH to below 4 but this acidic

environment (e.g., use of vinegar) must be acceptable

in terms of flavor – otherwise blanching may be

preferred. Treatment with sulfur dioxide or sulfites

is also effective – a particular application is preven-

tion of darkening of potato slices/chips during chill or

frozen storage. Such use for prevention of enzymatic

browning of apple slices is restricted due to flavor

taints but acid dipping (e.g., citric acid, lemon juice)

is effective. Peroxidases in general cause oxidative

changes to color and flavor and their control by heat

treatment, pH control, or sulfites must be adopted to

the particular food system. (See Enzymes: Functions

and Characteristics.)

004 5 Chemical changes limiting storage stability are

principally lipid oxidation and nonenzymatic (Mail-

lard) browning. The latter is particularly relevant for

dried milk, dried egg white, and dried fruits. Oxida-

tive rancidity of lipids is a radical oxidation process

and phenolic antioxidants (e.g., tocopherols, butyl-

ated hydroxyanisole (BHA), butylated hydroxy-

toluene (BHT) ) are effective at concentrations below

200 p.p.m. In practice combinations of a phenolic

antioxidant and ascorbic acid are often used since

they have a synergistic effect. (See Antioxidants: Nat-

ural Antioxidants; Synthetic Antioxidants; Browning:

Nonenzymatic; Oxidation of Food Components.)

004 6 The maintenance of the correct physical state of a

food can include maintaining the free-flow properties

of a powder (e.g., table salt or icing sugar) and

the stabilization of oil–water emulsions (e.g., salad

cream), whilst humectants (sugar, glycerol) help to

prevent moisture loss and hence dry textures. The

texture of a food often depends on it being a colloidal

system (emulsion, gel, foam) and additives are neces-

sary to stabilize such systems, especially if long

storage times at low temperatures are involved.

(See Stabilizers: Types and Function; Applications.)

0047Emulsifiers, like the naturally occurring lecithin in

egg yolk, help to form stable mixtures (emulsions) of

oil and water which are found in salad creams,

mayonnaise, margarine, icecream – the last has air

incorporated whilst freezing to form a solid foam.

To aid emulsion stabilization, e.g., in salad creams,

thickening agents (referred to as stabilizers) are used

to increase viscosity and so hinder oil and water

separation. Such stabilizers are natural polysacchar-

ides like starch, seaweed extracts such as alginates

and carrageenans, plant extracts such as gum arabic,

carob gum (locust bean gum) and pectins, and micro-

bially produced xanthan gum. Blends of these

so-called hydrocolloids are widely used to create

and to maintain texture in chilled and frozen pro-

cessed foods, especially dairy products ranging from

icecream to gelled milk products and yogurt. (See

Emulsifiers: Organic Emulsifiers; Phosphates as

Meat Emulsion Stabilizers; Uses in Processed Foods.)

0048Less obvious applications are the use of the emulsi-

fiers to retard bread staling, the use of a suspending

agent, often carboxymethyl cellulose, to maintain the

water-insoluble carotene pigment as a fine dispersion

in orange squashes (and so prevent the ‘orange scum’

at the top of the bottle!), and the use of glyceryl

monostearate in dried potato mash to insure rehydra-

tion without stickiness developing.

0049Additives can usefully be incorporated into prod-

ucts to minimize textural changes during their frozen

storage. The use of hydrocolloids in this connection

has been referred to above, and they are often referred

to as cryoprotectants in this connection. Thus the

addition of cryoprotective alginates to icecream

helps to prevent ice-crystal formation (and hence

‘grittiness’) during storage. The manufacture of sur-

imi (frozen, stabilized washed fish mince) depends on

the incorporation of a cryoprotectant mixture con-

sisting of 4% sucrose, 4% sorbitol, and 0.3% poly-

phosphate. This prevents protein denaturation during

frozen storage, and so maintains the heat gelation

properties of the fish proteins required for the pro-

duction of products such as shellfish analogs (crab

sticks), fish sausage, noodles, and burgers. Recently

so-called ‘antifreeze proteins’ have been isolated from

cold-water (e.g., Antarctic) fish and shown strongly

to inhibit ice-crystal formation in icecream products

and to reduce ice-crystal formation in frozen meat,

with resultant reduced drip loss on thawing. Presently

the high cost of these cryoprotectants precludes their

commercial application.

STORAGE STABILITY/Parameters Affecting Storage Stability 5617

Shelf-Life Extension

005 0 It is clear from the examples mentioned that methods

adopted to insure shelf-life extension must consider

the state of the food at the point of storage and select

the best combinations of the relevant parameters.

0051 Organoleptic reasons and concepts of naturalness

are increasingly important factors in determining the

methods used to insure the required storage stability.

Thus the increased use of MAP allied with the use of

chill rather than frozen storage is related to its ‘fresh’

appeal, in contrast to frozen food which has a ‘pro-

cessed’ image. Heat sterilization and sealing under

reduced air content, as typified by traditional canning,

or frozen storage at 20



C or below allows safe

storage for many months but flavor/texture changes

may arise.

0052 The sous-vide or nouvelle carte processes have been

promoted as maximizing flavor, texture, and nutrient

content,aswellasbeingbacteria-free.These processes

involve vacuum packing in heat-resistant packaging,

cooking the vacuum-packed product under vacuum

in a moist steam oven or pressure cooker at less than

110



C, rapidly cooling, and storing at 1–4



C for up

to 3 weeks. This represents the combination of chill

storage, relevant packaging, and exclusion of oxygen

to extend shelf-life. Where low-acid foods are

concerned, the additional use of preservatives is

suggested and overall a maximum storage time of 2

weeks is recommended.

0053 When chill storage is used, the shelf-life is meas-

ured in days and there is less of a margin for storage

abuse than with frozen storage. For foods which are

chill-stored and which are not reheated (above 70



before consumption, the recommended chill storage

temperature and time must be carefully observed.

This places more emphasis on accurate sell- and use-

by dates by the processor, careful stock control by the

retailer, and increased awareness by the consumer.

Seealso: Antioxidants:NaturalAntioxidants;Synthetic

Antioxidants; Biotechnology in Food Production;

Canning:QualityChangesDuringCanning; Chilled

Storage:UseofModified-atmospherePackaging;

Curing; Drying:TheoryofAir-drying; Freeze-drying:The

BasicProcess; Freezing:Principles; Irradiation of

Foods:BasicPrinciples; Oxidation of Food

Components; Preservation of Food; Spoilage:

BacterialSpoilage;MoldsinSpoilage; Stabilizers:Types

andFunction;Applications; Sterilization of Foods;

Storage Stability:MechanismsofDegradation

Further Reading

Ahuenainen R (1996) New approaches to improving the

shelf life of minimally processed fruit and vegetables.

Trends in Food Science and Technology 7: 179–187.

Coultate TP (1996) Food – The Chemistry of its Com-

ponents, 3rd edn. London: Royal Society of Chemistry.

Debeaufort F, Quezada-Gallo JA and Voilley A (1998)

Edible films and coatings: tomorrow’s packaging: a

review. Critical Reviews in Food Science 38: 299–313.

Feeney RE and Yeh Y (1998) Antifreeze proteins, current

status and possible food uses. Trends in Food Science

and Technology 9: 102–106.

Fennema OR (ed.) (1996) Food Chemistry, 3rd edn. New

York: Marcel Dekker.

Institute of Food Science and Technology (UK) (1999) Pos-

ition statement: the use of irradiation for food quality

and safety. Food Science and Technology Today 13:

177–179.

IFST (UK) Current Hot Topics (1999) Position Statement:

Genetic Modification of Food. http//ifst.org/hottop

10.htm.

Jeremiah LE (1996) Freezing Effects on Food Quality. New

York: Marcel Dekker.

King GA and O’Donaghue EM (1995) Unravelling senes-

cence: new opportunities for delaying the inevitable in

harvested fruit and vegetables. Trends in Food Science

and Technology 6: 385–389.

Rockland LB and Beuchat LR (1987) Water Activity:

Theory and Application to Food. New York: Marcel

Dekker.

Taub IA and Singh RP (1998) Food Storage Stability. Boca

Raton, Florida: CRC Press.

Vesmeiren L, Deulieghere F, van Beest M, de Kruijf N and

Debeuere J (1999) Development in active packaging

of foods. Trends in Food Science and Technology 10:

77–86.

Shelf-life Testing

M Hole, University of Lincolnshire and Humberside,

Lincoln, Grimsby, UK

Introduction

0001The shelf-life of a food is perhaps the most important

detail a consumer needs to know in relation to their

purchase and home storage of foods. This informa-

tion is embodied in the ‘best before’ and ‘use by’ dates

indicated for the food, and which are required by law

to be clearly shown on the food packaging. The ‘use

by’ date is particularly critical in relation to the safe

consumption of a purchased food. Such foods have

relatively short shelf-lives, often in terms of days, and

include fresh and chilled meats and dairy products. In

contrast, ‘best before’ dates are used for foods where

quality, rather than safety, is the practical concern –

thus, a potato crisp, which has absorbed moisture so

that it has lost its crispness, is of undesirable quality

but does not pose a food safety issue.

5618 STORAGE STABILITY/Shelf-life Testing

0002 If the manufacturer should propose an unrealistic

shelf-life, leading to a ‘use by’ date that is too long,

then the likely consumer complaints due to adverse

health and/or quality aspects, possibly requiring

product recalls, would do great damage to the manu-

facturer’s reputation. The subsequent commercial

consequences for the food manufacturer would be

serious, and it is obvious that the correct determin-

ation of the shelf-life is most important – for both the

manufacturer and the consumer. The consumer can

obtain very useful information on the purpose of ‘use

by’ dates from the Foodsense leaflets provided by the

Ministry of Agriculture, Fisheries and Food.

0003 In unprocessed and unpreserved foods, the factors

that limit the shelf-life are mostly related to the

growth of spoilage microorganisms, whilst, in non-

perishables, chemical and physical factors become

more important, e.g., development of off-odors and

off-flavors due to rancidity in fried snack foods, mois-

ture migration in biscuits, and staling of bread and

cakes. The factors that limit the shelf-life of a selec-

tion of foods are indicated in Table 1.

0004 In order to give an indication of the shelf-life of

a product, some type of test must be available to

the manufacturer. Storage studies would normally

be performed as new products are developed, with

regular testing for the shelf-life limiting factors.

However, with some long-term preservation methods,

it may be more convenient to have accelerated shelf-

life studies. This would enable production to com-

mence quite rapidly after the initial development of

the product.

Experimental Design

0005With any experimental procedure, results will only be

valid if the initial sampling and testing stages are

carefully planned and controlled. If any statistical

analysis of the result is required, then the testing

procedure should be designed to be appropriate to

the statistical method being applied. The samples

taken should be representative of the whole batch

and, in careful shelf-life studies, samples should in-

clude any extremes of product storage conditions. It is

also important to test at regular intervals during pro-

duction, especially after any change in ingredients or

methods of production. The design of any procedure

for testing shelf-life should include consideration of

the following factors:

0006elucidation of the major types of quality loss for the

product (see Table 1 for examples);

0007knowledge of the factors that control the initial

quality during manufacture, e.g., use of food addi-

tives;

0008the environmental conditions of storage, e.g., tem-

perature, equilibrium relative humidity (ERH), and

light;

0009the type and properties of the packaging to be used,

e.g., permeability to oxygen, light and moisture;

0010the kinetics of the reaction or reactions leading to

loss of quality in the actual food (stored in the usual

way).

Acceptability Criteria

0011The acceptance of a food product depends to some

extent on the individual preferences of the consumer;

however, all consumers have a right to expect whole-

some foods that will not cause disease, are of attract-

ive appearance, and are of a good (unchanged)

nutritional value. The major attributes of acceptabil-

ity can be described as: absence of pathogenic micro-

organisms; low levels of spoilage organisms; absence

of off-flavors; no deterioration in color or overall

appearance; no loss of desired texture; and high (un-

changed) nutritional value. It is the function of the

manufacturer to decide upon suitable acceptability

limits for each type of product, and this should form

part of the specification of that product.

tbl0001 Table 1 Factors that limit the shelf-life of foods

Type of food Acceptability criteria

Bread Mold growth

Loss of moisture, staling

Breakfast cereals Rancidity development

Moisture gain/loss of crispness

Vitamin loss

Dried pasta Moisture gain (or loss)

Color loss

Adsorption of undesirable flavors

Fried snack foods Oxidative and hydrolytic rancidity

Fresh poultry Pathogen growth/microbial decay

Bruising

Frozen poultry Sensory quality changes

Color changes/rancidity

Syneresis, moisture loss

Fresh meat Bacterial growth

Loss of color

Fresh fish Bacterial decomposition

Frozen fish Lipid oxidation

Protein denaturation (toughening)

Frozen vegetables Color

Dairy products and milk Bacterial growth

Lipid hydrolysis/flavor changes

Ice cream Textural changes

Oxidation of lipid

Evaporated milk Loss of vitamins

Salad dressing Breakdown of emulsion

Color/flavor changes

Canned foods Taste/flavor changes

Chilled foods Bacterial growth

Flavor changes

STORAGE STABILITY/Shelf-life Testing 5619

Methods of Testing

0012 It is vital that the tests carried out need to relate

specifically to the safety aspect concerned or, in

quality terms, relate to the limiting quality criteria.

0013 The main methods of testing the shelf-life of a food

include chemical/physical tests, microbiological enu-

merations and sensory evaluations. Due to the com-

plex nature of most composite foods, many chemical

tests, e.g., to establish nutrient levels, are very lengthy,

often involving complex extraction procedures and

use of sophisticated and expensive equipment. Micro-

biological and sensory tests are generally regarded as

easier to perform and are more adaptable for use in

nonspecialized laboratories.

0014 Although the ultimate test of a product’s accept-

ability or attractiveness lies with the consumer, the

consumer cannot necessarily judge whether foods are

safe.

Microorganisms

0015 The first criterion of any testing is to insure that the

product is safe – both from food-poisoning micro-

organisms and from any toxins. Food-poisoning

organisms, e.g., Salmonella typhimurium, may be

detected by growth of the organism on selected

media, but growth is usually relatively slow, and a

result would not be obtained until 3–4 days after the

test had been initiated. Rapid methods of measuring

microbial concentrations are now available, and a

range of automated instrumental methods can be

used, e.g., the ‘Bactometer’.(See Food Poisoning:

Tracing Origins and Testing; Microbiology: Detec-

tion of Foodborne Pathogens and their Toxins.)

Chemical Toxins

0016 A variety of tests can be utilized to test for the range of

toxins found in foods. For example, the presence of

toxic metals, e.g., mercury or lead, may be deter-

mined by the use of atomic absorption spectrometry,

and the highly toxic aflatoxins, produced as second-

ary metabolites from the growth of the mold Asper-

gillus flavus on cereals, may be detected by using

the fluorescent properties of the compounds, e.g.,

use of high-performance liquid chromatography

with fluorescence detection. (See Heavy Metal Toxi-

cology; Mycotoxins: Occurrence and Determination.)

Flavor Changes

0017 From Table 1, it is apparent that the shelf-life of

‘microbiologically safe’ foods is often limited by

either changes in flavor or development of off-odors

and off-flavors, e.g., production of volatile carbonyl

compounds from lipid oxidation leading to rancid

flavors. These changes in flavor may be monitored

by sensory means or from utilization of analytical

techniques such as headspace gas chromatography.

With this technique, the volatiles arising from a food-

stuff are swept into a chromatography column con-

tained in a thermostatically controlled oven. A carrier

gas moves the flavor compounds through the column

where selective separation occurs, depending on the

polarity of the packing material (stationary phase) of

the column andthe molecular structure of the volatiles.

Eventually, the separated compounds are detected and

are registered as a peak on a trace. The identity of the

peaks may be established by comparison of their

retention times with standards or from coupling the

gas chromatography with a mass spectrometer. The

contribution of a specific volatile to any off- or

undesirable odor may be found by smelling the indi-

vidual components as they emerge from the column.

Increasing use is made of ‘electronic noses’ (e.g.,

‘Aromascan’) for the instrumental characterization

of aroma. In contrast to gas chromatography, these

electronic noses detect the overall aroma, giving

rise to an aroma fingerprint that can be used to indi-

cate the shelf life limit. (See Sensory Evaluation:

Aroma.)

0018It is well established that a wide range of analytical

tools are available to the food analyst for determin-

ation of the (molecular) components of foods that

may be responsible for any shelf-life deterioration,

e.g., nutrients (essential amino acids and vitamins).

In some foods, determination of the factors that

actually limit shelf-life may involve very complex

analyses; thus an associated factor, which changes at

the same rate but does not affect shelf life directly,

may be used. The reader is referred to the bibliog-

raphy for a more thorough review of methods used in

the chemical analysis of foods.

Color

0019Changes in the color of foods are often major factors

limiting shelf-life. Instrumental testing of surface

color, or of the color of a well-homogenized sample,

may be achieved with a colorimeter, which gives

results in terms of the Hunter L, a and b values.

That is, a measure of the degree of ‘whiteness,’‘red-

ness,’ and ‘blueness’ of a sample. Otherwise, com-

parisons of the food color with acceptable colors are

useful, but this is essentially a sensory method of

analysis. (See Colorants (Colourants): Properties and

Determination of Natural Pigments; Properties and

Determinants of Synthetic Pigments; Sensory Evalu-

ation: Appearance.)

5620 STORAGE STABILITY/Shelf-life Testing

Texture

0020 Sensory methods are also used to provide information

on the texture of foods. Attributes such as firmness,

crispness, juiciness, and chewiness are fairly typical of

those used. In addition, the texture of certain foods,

e.g., biscuits and apples may be measured using phys-

ical (instrumental) tests and equipment such as the

Instron. This particular piece of equipment may be

modified to undertake a variety of penetration, exten-

sibility, or shearing tests. (See Sensory Evaluation:

Texture.)

Sensory Methods

0021 A wide variety of sensory evaluation tests are avail-

able for use in testing the acceptability of foods. The

tests may be classified into those that are affective

(i.e., ask for an opinion or preference) and those

that are nonaffective (i.e., require a score to be

assigned for a particular attribute). However, for

shelf-life studies, it is usually more appropriate to

use scoring tests. For the results to be meaningful, it

is important to have a suitably designed taste panel/

sensory test giving results that can be statistically

validated. This also requires the establishment of a

null hypothesis, e.g., no difference between the

samples presented, in order to establish the signifi-

cance of the results. It is usually recommended that at

least 10 persons are required to act as panellists if the

results obtained are to be valid, the actual number

depending on their level of training (if untrained, then

more panellists will be required). Sensory panels are

sometimes regarded as time-consuming and expen-

sive to set up, but they do have an advantage over

instrumental methods in that several attributes of the

food can be measured in one session. (See Sensory

Evaluation: Sensory Characteristics of Human

Foods; Food Acceptability and Sensory Evaluation;

Practical Considerations; Descriptive Analysis.)

Accelerated Shelf-life Testing

0022 Ideally, a food would be tested under the conditions

of intended storage in order to measure the length of

time over which it is acceptable, whether from a

safety or quality aspect. For a food with a shelf-life

of a few days or weeks (e.g., chilled meat), this is

no problem, but for foods with shelf-lives of months,

or years, the manufacturer would like a quicker

method, i.e., one that gives a result in a shorter

time than the shelf-life under the intended storage

conditions.

0023 Since storage temperature is usually the main

factor influencing the shelf-life, most accelerated

methods simply use a higher temperature than the

intended storage temperature and hope to relate the

shorter shelf-life at the higher temperature to the

shelf-life at the intended storage temperature. The

theoretical basis is related to the Arrhenius relation-

ship, one form of which is:

ln k ¼ cst:

Ea

where cst., Ea (the activation energy), and R (uni-

versal gas constant) can be taken as constants.

Hence:

ln k /

where T is the temperature (degrees Kelvin), and k

is the rate constant, which, in this context, can be

related to the shelf-life time, i.e.:

lnðshelf-lifeÞ/

A graph of ln(shelf-life) against 1/Twill give a straight

line (see Figure 1). Thus, if the shelf-life is measured

(Kelvin)

In (shelf-life)

fig0001Figure 1 Arrhenius plot showing extrapolated shelf-lives for

two foods with different rates of quality deterioration.

STORAGE STABILITY/Shelf-life Testing 5621

for a series of temperatures higher than the intended

storage temperature, extrapolation will give the shelf-

life at the lower temperature. The method has limita-

tions and can be costly, but this approach is often

used. Some examples are given below; other examples

will be found in the bibliography. Frozen foods often

have shelf-lives of many months, or years, and accel-

erated shelf-life testing can be usefully applied. How-

ever, the elevated temperature in this case must be

realistic – a temperature at which the food was not

frozen would not be realistic. Thus, the shelf-life at an

intended storage temperature of 30



C might be

usefully indicated from shelf-life measurements

made at up to 5



C. This would ensure that the

food matrix was similar (i.e., frozen) at the elevated

temperatures. This illustrates an important point; the

nearer the elevated temperatures are to the intended

storage temperature, the more realistic the extra-

polated shelf-life will be.

0024 Where foods are being reformulated and the effect

of different formulations on shelf-life is needed,

accelerated methods are very useful in indicating

relative shelf-lives. This allows a new shelf-life to be

proposed from a knowledge of the original shelf-life.

It has been stressed that the shelf-life determinations

are only relevant if based on measurements of the

actual limiting factor, for example, odor. This aspect

can become critical when accelerated shelf-life testing

is carried out, if the limiting factor changes at the

higher temperature. An example (discussed by

Labuza – see Further Reading) is provided by the

storage of dehydrated mashed potato. At ambient

temperatures, the limiting factor is off-odors from

lipid oxidation, but above about 40



C, the limiting

factor for organoleptic acceptance is discoloration due

to Maillard browning (See Browning: Nonenzymatic).

Hence, if accelerated shelf-life testing is carried out

above 40



C, sensory analysis will give results that

extrapolate to an incorrect shelf-life for ambient tem-

perature storage. More specifically, results obtained

above 40



C extrapolate to a shelf-life above 1000

days at 20



C, whilst measurements made at 20



(where odor is the limiting factor) indicate a shelf-

life of about 500 days.

0025 Thus, the food system must be carefully studied to

insure that the accelerated shelf-life testing is done as

realistically as possible. The sensitivity of a food to

temperature changes is indicated by the slope of the

‘Arrhenius graph’ (See Figure 1) – the more sensitive

food having the larger gradient. An alternative

approach is the use of the Q

value, where

rate at ðTþ10Þ



rate at T



shelf-life at T



shelf-life at ðTþ10Þ



The more sensitive the food is to temperature change,

the higher the Q

value, as illustrated in Table 2.

For an initial shelf-life of 100 days at T



C, a food

with Q

¼2 would have a half-life of 50 days at

(Tþ10)



C, whilst if Q

¼4, the half-life would be

25 days at (T þ10)



Prediction of Shelf-life

0026The use of accelerated shelf-life testing avoids the

need for prolonged storage studies – and in reality is

a predictive method based on measurements made at

higher temperatures than the actual storage tempera-

ture. The possible problems with accelerated shelf-life

testing can be minimized if predictive methods using

data from the actual storage conditions can be used.

Thus, if the rate of deterioration (i.e., the kinetics) is

known (at the actual storage temperature), initial

values of the loss of quality, for example, can be

used to indicate the quality loss, and hence shelf-life,

for extended storage times.

0027The loss of quality for most foods can be repre-

sented by:



¼ kA

where A is the quality factor measured, e.g., extent of

nonenzymatic browning, t is the time, k is a constant

that depends on the temperature and water activity,

n is a power factor giving the order of the reaction

and dA/dt is the rate of change of A with time (nega-

tive, loss of A; positive, production of undesirable end

products). In practice, zero-order (n ¼0) or first-

order (n ¼1) kinetics are relevant for quality changes

in foods.

Zero-order Kinetics

0028The order of the reaction, n, defines whether the rate

is dependent on the value of A. Many food systems

tbl0002Table 2 Q

values for selected foods

Type of food Q

value Criteriaforendofshelf-life

Fresh cod 4.4 Bacterial growth

Sterilized milk 1.71 Flavor change

Pasteurized milk 2.64 >10

CFU

1

Pasteurized egg 5.37 Flavor change

Spray-dried egg 1.21 60% loss of vitamin A

Margarine 1.91 25% loss of vitamin A

Canned kidney beans 1.7 20% loss of thiamin

CFU, colony-forming units.

5622 STORAGE STABILITY/Shelf-life Testing

are assumed to behave as zero-order reactions (n ¼0).

Thus, the rate of loss is constant under constant

storage conditions. Thus:



¼ k,

which may be integrated to give

A ¼ A

 kt or A

¼ A

 kt

where A

is the initial quality value at time t, A

is the

value of A at the end of the shelf-life and t

is the shelf-

life in days, months, or years.

0029 A may be defined analytically or by taste-panel

evaluation. If A

is assumed to be 100% quality and

just acceptable quality, then:

k ¼

 A

100%

Thus, based on this knowledge, it is possible to pre-

dict the shelf-life of a food at a single given tempera-

ture, if the value of the quality change at any given

time is known.

0030 For example, if it is known that a certain food has

lost 25% of its quality in 50 days when held under

constant conditions, then the rate of loss:

k ¼

 A

100  75

¼ 0:5% per day:

The amount of shelf-life left as a function of time may

also be represented by a graphical plot (Figure 2).

0031 Reaction types in foods that are thought to give

zero-order kinetics include enzymatic degradation,

nonenzymatic browning, and lipid oxidation (rancid-

ity). Loss of quality of frozen foods in general follows

zero-order kinetics.

First-order Kinetics

0032Many foods deteriorate by first-order kinetics (n ¼1),

which results in an exponential decrease in the rate of

change as the quality decreases. Thus, the rate of

quality loss is directly dependent on the amount left:



¼ kA:

Integration gives:

ln ðA

Þ¼kt

A semi-logarithmic plot of A

against time (t) gives

a straight line with slope k.

0033The types of deterioration that follow first-order

kinetics include microbial growth (fresh meat and

fish), microbial production of off-flavors, vitamin

losses (canned and dried foods), and loss of protein

quality (dried foods). (See Canning: Quality Changes

During Canning; Fish: Spoilage of Seafood; Meat:

Preservation.)

0034A knowledge of the kinetics of food deterioration

has been incorporated into computer packages,

which can be applied to rapidly predict shelf-lives

for more complex situations.

0035Such computer-based predictive models were de-

veloped initially to predict the growth of pathogens

in foods in order to predict shelf-lives where the food

would be safe to eat. The ‘Food Micromodel’, de-

veloped by the Ministry of Agriculture, Fisheries

and Food in the early 1990s is such a model and can

be applied to predict the growth of a wide range

of microorganisms, both pathogenic and spoilage

organisms, under a wide range of storage conditions.

The approach is particularly relevant for chilled

foods, and the ‘Forecast’ service provided by Camp-

den and Chorleywood Food Research Association

(CCFRA) allows the prediction of the growth of

common spoilage bacteria for a range of storage con-

ditions (pH, temperature, salt content, etc.). Another

example of a computer-based predictive model (also

from CCFRA) is the prediction of mold-free shelf-life

using the ERH CALC

package. This model allows

rapid calculation of the ERH (or water activity) and

hence the growth of molds from a knowledge of

the ingredients and storage conditions of bakery

products.

Time–Temperature Indicators (TTIs)

0036Since temperature is such an important factor in de-

termining shelf-life, it would be useful to know the

full thermal history of a food after processing. Any

thermal abuse during distribution and storage would

alter the shelf-life, and hence the ‘use by’ date of the

food.

0 50 100 150 200

100

120

Time (days)

Quality changes (%)

fig0002 Figure 2 Zero-order reaction plot showing residual shelf-life

against time (k ¼ 0.5% per day).

STORAGE STABILITY/Shelf-life Testing 5623

0037 Time–temperature indicators are available that

give a full thermal history of the food, and such

sensors attached directly to the packaged food would

be most useful in this connection. If they were linked

to the computer-based models for shelf-life predic-

tion, any temperature abuse would result in a new

shelf-life prediction.

Home Storage

0038 There is an obvious need for the manufacturer’s stor-

age instructions to be followed in order that the ‘use

by’ date is valid. The food industry can arrange the

correct storage conditions up to the point of sale, but

thereafter, it is necessary for the consumer to insure

correct home storage, especially for chilled foods. The

possible lack of correct temperature control during

home storage indicates the need for some margin of

error to be incorporated, especially into the ‘use by’

date.

See also: Fish: Spoilage of Seafood; Food Poisoning:

Classification; Tracing Origins and Testing; Heavy Metal

Toxicology; Meat: Preservation; Microbiology:

Detection of Foodborne Pathogens and their Toxins;

Mycotoxins: Occurrence and Determination;

Preservation of Food; Water Activity: Effect on Food

Stability; Sensory Evaluation: Sensory Characteristics

of Human Foods; Food Acceptability and Sensory

Evaluation

Further Reading

Campden and Chorleywood Food Research Association

(CCFRA) (1997) Evaluation of Shelf Life for Chilled

Foods, Technical Manual No. 28. Chipping Campden,

UK: CCFRA.

Campden and Chorleywood Food Research Association

(CCFRA) (1999) ERH CALC

– Version 2. Chipping

Campden UK: CCFRA.

Fennema OR (ed.) (1996) Food Chemistry, 3rd edn. New

York: Marcel Dekker.

Institute of Food Science and Technology (IFST) (1993)

Shelf Life of Food – Guidelines for its Determination

and Prediction. London: IFST.

Labuza TP (1984) Application of chemical kinetics to

deterioration of foods. Journal of Chemical Education

61: 348–358.

Ministry of Agriculture, Fisheries and Food (MAFF) (1998)

Understanding Food Labels. London: Foodsense, Food

Safety Directorate.

Man CMD and Jones AA (1994) Shelf Life Evaluation of

Foods. London: Blackie Academic and Professional.

Nielson SS (1998) Food Analysis, 2nd edn. Gaithersburg,

MD: Aspen.

Singhal RS, Kulkarni PR and Rege DV (1997) Handbook of

Indices of Food Quality and Authenticity. Cambridge,

UK: Woodhead.

Smith G (1993) Encyclopaedia of Food Science, Food Tech-

nology and Nutrition. London: Academic Press.

Thorne S (1992) Mathematical Modelling of Food Process-

ing Operations. London: Elsevier Applied Science.

Stouts See Beers: History and Types; Wort Production; Raw Materials; Chemistry of Brewing; Biochemistry of

Fermentation; Microbreweries

STRAWBERRIES

R B Smith and L J Skog, University of Guelph,

Vineland Station, Ontario, Canada

A Dale, University of Guelph, Simcoe, Ontario, Canada

Background

0001 The strawberry belongs to the genus Fragaria, and is a

member of the rose family, Rosaceae. The cultivated

strawberry, F.  ananassa L. Duchesne, is a hybrid

between F. chiloensis Duch and F. virginiana Duch.

The ananassa designation is due to the resemblance to

the pineapple in flavor, odor, and shape. F. chiloensis

was introduced into Europe from Chile and F. virgini-

ana from eastern North America. The fruit of the

strawberry is grown and enjoyed worldwide. This

article discusses the global distribution, varieties,

commercial importance, morphology, nutritional

composition, handling, storage, and industrial uses

of this crop.

5624 STRAWBERRIES

Global Distribution

0002 The strawberry plant is highly adapted to a wide

range of climatic and soil conditions. Commercial

plantings of strawberries are found on every con-

tinent (except Antarctica) and from the subarctic

(Finland) to the tropics (Ecuador). However, most of

the world’s production is in the northern hemisphere

(Table 1). Strawberries are planted on soil types

ranging from desert sands requiring extensive irriga-

tion to heavy clay soils requiring underdrainage and

the addition of organic matter. In subarctic areas, the

plants may be exposed to temperatures as low as

50



C in winter and as high as 40



C in summer. In

desert areas, temperatures may be above 50



C in the

day and near 0



C at night. Where harsh winter con-

ditions prevail, plants may be protected from extreme

temperatures by covering with straw or other organic

mulch. Mist irrigation is sometimes used to cool the

fruit and leaves under conditions of high summer heat.

000 3 The geographical areas with the highest total pro-

duction and the highest production per hectare are

located where there is a Mediterranean-type climate.

These areas include California, Spain, southern Italy,

and southern France. In these areas strawberries can

be produced virtually all year and day-neutral var-

ieties, i.e., those in which fruit bud initiation is inde-

pendent of day length, are grown more extensively.

The June-bearing varieties, which predominate in

most other growing areas, have a requirement for a

short day (some as little as 8 h).

Varieties

0004The commercial strawberry is a heterozygous octo-

ploid with characteristics inherited mainly from two

genetically variable wild species. The first crosses

between F. chiloensis and F. virginiana were made in

Europe in the early 1760s. Breeding work with other

species has not been successful, primarily owing to

genetic incompatibility.

0005Variability in the wild species has been used by

breeders who use parents with desired traits when

crosses are made. When seeds from a cross are

planted, the resultant plant is a clone. Thus, breeders

select individual plants with desired characteristics

rather than selecting a population of plants.

0006The general objectives of strawberry-breeding pro-

grams are as follows: (1) to enhance yields; (2) to

improve fruit quality, which includes factors such as

flavor, firmness, keeping quality or other characteris-

tics; (3) to produce plants adapted to specific environ-

mental or cultural conditions; (4) to develop varieties

resistant to diseases and insects; and (5) to develop

varieties with specific requirements, such as suitabil-

ity for mechanical harvesting and processing, or extra

firmness for long-distance shipping. New strawberry

varieties are continuously being introduced, thus

worldwide they number in the hundreds. Some of

the more important varieties are listed in Table 2.

Commercial Importance

0007The Food and Agriculture Organization (FAO) lists

strawberry production statistics for 64 countries.

Seven countries – Spain, Japan, Korea, Turkey,

Poland, Mexico, and the USA – account for more

than 66% of the world’s 2.7  10

kg annual produc-

tion (Table 1). This worldwide production would

have a farm-gate value of over $5.4  10

(US) based

on their value in North America. This return would

represent $2 kg

1

, averaged for fresh-market berries.

0008In countries such as Poland and Mexico, a large

portion of the crop is processed and sold for foreign

currency. In the European Community (EC), straw-

berries are an important cash crop as this trading

block produces over 7.0  10

kg annually. Much of

the fresh crop produced in the USA and the EC

is shipped to northern areas for fresh-market con-

sumption. Growers harvesting for fresh-market

consumption receive a higher price per unit of fruit

than those harvesting for processing, but fresh market

harvesting costs are higher.

0009Yields in the state of California are over 40 000 kg

1

– a level of production two to six times that of

other areas in the USA or in other countries. Califor-

nia produces 80% of the US crop, and 65% of the

tbl0001 Table 1 Global distribution of production by geographical area,

including a listing of countries producing 5.0  10

kg or more

Global distribution by geographical area and countries withinany one

area producing 5.0 10

kg or more

World total

Africa

North and Central America

Mexico

USA

South America

Asia

Japan

Korea, Republic of Russian Federation

Turkey

Europe

France

Germany

Italy

Poland

Spain

Oceania (includes Australia and New Zealand)

From Food and Agriculture Organization of the United Nations (1998)

ProductionStatistics. FAOSTAT,Availableathttp://apps. Fao. org

(February 2000).

STRAWBERRIES 5625

North and Central American crop, making it an

important cash crop in that state.

0010 In some countries, particularly around large cities,

consumers are invited by growers to pick their own

fruit. This has become a very important means of

harvesting and marketing the strawberry crop. Pick-

your-own farm operators do not have problems find-

ing labor to harvest their berries. However, there are

other problems, such as in-field consumption, crowd

control, incomplete harvesting, and grower–public

interaction.

Morphology

0011 The strawberry is an accessory fruit with an aggregate

of achenes (the true fruits which contain the seed),

attached in an orderly fashion to the epidermis of the

receptacle (the floral axis to which the various flower

parts are attached). The large, fleshy receptacle (con-

sisting of pith, cortex, and a vascular system) is the

succulent, edible portion of the fruit. It expands in

response to hormones produced by the ovules after

fertilization has occurred.

0012 The fruit of the commercial strawberry has a red-

colored exterior and an interior color ranging from

white to dark red. Fruit shape depends on such factors

as variety, environmental conditions under which it

was grown, and planting location. Fruit size, which

varies considerably even within cultivars, depends on

environmental factors and position of the fruit within

a cluster. The primary berry is the largest and has the

most achenes. Secondary and tertiary berries become

progressively smaller and have fewer achenes. The

fruit ranges between 2 and 5 cm in length depending

on variety and other factors. Descriptors used to

identify the shapes of strawberries include globose,

globose conic, conic, long conic, oblate, necked

(a long neck), long wedge, and short wedge.

Chemical and Nutritional Composition

0013A list of some of the nutrients in strawberries is pre-

sented in Table 3, with those for apples and oranges

included for comparison. The strawberry contains the

tbl0002 Table 2 Strawberry varieties grown in quantity in different geographical areas of the world

Geographicalarea Varieties

California (some of the same varieties in Mexico) Selva, Chandler, Commander, Oso Grande, Camerosa, Seascape, Lido,

Key Largo, E26, Coronado, San Miguel, Catalina

North America (except California and Mexico) Totem, Bounty, Kent, Earliglow, Honeoye, Governor Simcoe, Glooscap, Selva,

Camerosa, Sweet Charlie, Annapolis, Jewel, Cavandish, Delmarvel, Veestar,

Redcrest, Hood, Chandler

South America Camerosa, Chandler, Pajaro, Selva, Oso Grande, Sweet Charlie, Tudla

South Asia Hokowase, Reiko, Toyonoko, Nyoho, Sachinoka, Tochiotome, Suhong, Rachel,

Douglas, Pajaro, Cruz, Senga Sengana, Chandler, Oso Grande, Selva,

Sequoia

Western Europe (excluding Spain, Italy, France) Senga Sengana, Dania, Elsanta, Elvira, Karola, Symphony, Pegasus, Honeoye,

Evita, Bolero, Tango, Selva, Zefyr, Bounty

Spain, Italy, France Pajaro, Camerosa, Chandler, Oso Grande, Tudla, Eris, Tethis, Clea, Miranda,

Marmolada, Idea, Addie, Miss, Selva, Seascape, Elsanta, Gariguette,

Dar Select, Eros

Eastern Europe/northern Asia Senga Sengana, Zarya, Gorella, Elsanta, Kokinskaja Rannaja, Festivalnaja

Romashka, Zenit, Nadezhda, Rannyaya Plotnaya, Lurck Vira, Istochnik, Desna

Oceania Chandler, Pajaro, Parker, Selva, Camerosa

Data from Hancock JF (1999) Strawberries. Wallingford, UK: CABI Publishing.

tbl0003Table 3 Nutrient composition (per 100 g fresh weight) of

strawberries compared to that of apples and oranges

Nutrients andunits Strawberry Apple Orange

Water (g) 91.57 83.93 86.75

Food energy (kcal) 30 59 47

Food energy (kJ) 127 245 197

Protein (N  6.25: g) 0.61 0.19 0.94

Total lipid (g) 0.37 0.36 0.12

Cholesterol (mg) 0 0 0

Carbohydrate (g) 7.02 15.25 11.75

Fiber (g) 0.53 0.77 0.43

Calcium (mg) 14 7 40

Magnesium (mg) 10 5 10

Phosphorus (mg) 19 7 14

Potassium (mg) 166 115 181

Ascorbic acid (mg) 56.7 5.7 53.2

Folic Acid (mg) 17.7 2.8 30.3

Vitamin A (mg) 8.1 1.5 6.3

Data from USDA (1982) Composition of Foods: Fruits and Fruit Juices, Raw,

Processed, Prepared. US Department of Agriculture, Human Nutrition

Information Services, Agriculture Hand-book, pp. 8–9. Washington, DC: US

Government Printing Office.

5626 STRAWBERRIES