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

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placebo run-in period, in a double-blind placebo-

controlled study. Patients were tested for mood,

cognition, and food cravings, using an interactive

computer-telephone system, during the luteal phase

of the menstrual cycle. The beverage, but not the

placebos, significantly decreased self-reported depres-

sion, anger, confusion, and carbohydrate craving, and

improved memory word recognition.

Isoflavones (Plant Estrogens)

0035 These are often derived from soya and are suggested

as a natural hormone replacement for postmenopau-

sal women. However, many are also advocating their

use for PMS. There is little evidence for a beneficial

effect on PMS, and in fact supplementing with soya

protein and other sources of plant estrogens is now

controversial, many experts believing that they can

also have a harmful effect. They should certainly not

be taken if a woman has had breast cancer or a family

history of the disease.

L-Tryptophan

0036 Serotonin reuptake inhibitors have been shown to be

beneficial in the treatment of PMS. The efficacy of

l-tryptophan, which acts specifically on serotonergic

neurons, was investigated in a RCT. Results suggested

a significant (p ¼0.005) therapeutic effect of l-

tryptophan relative to placebo for the cluster of

mood symptoms comprising the items of dysphoria,

mood swings, tension, and irritability. These results

suggest that increasing serotonin synthesis during the

late luteal phase of the menstrual cycle has a benefi-

cial effect in patients with PMS.

Low-fat Vegetarian Diets

0037 A low-fat vegetarian diet has been associated with

increased serum sex-hormone binding globulin con-

centration and reductions in body weight, dysmenor-

rhea duration and intensity, and PMS duration.

The symptom effects might be mediated by dietary

influences on estrogen activity.

0038 The UK PMS society states, ‘a healthy diet, particu-

larly one which is low in fat and high in fibre can

relieve PMS,‘ and they recommend the following:

0039 Eating three main meals per day with three smaller

snacks in between, all of which should contain

starchy foods. This is particularly important in

the luteal phase of the cycle.

0040 Cutting down on sugar and sugary food, as these

provide sudden bursts of energy rather than a

steady release, which will help combat fatigue and

mood swings.

0041Eating fiber helps avoid premenstrual constipation,

but it is important to drink plenty of water or

sugar-free drinks (six to eight glasses a day) to

prevent bloating associated with PMS.

0042Having one pint of milk a day of either skimmed or

semi-skimmed, or an alternative source of calcium

such as low-fat cheese or yogurt,

0043Reducing salt intake.

0044Eating five portions of fruit and vegetables per day.

0045Reducing alcohol intake to no more than 2 units per

day (1 unit ¼half a pint (284ml) of lager or beer, or

one small glass of wine, or one measure of spirits).

0046Limiting tea and/or coffee to no more thtrun an 5

cups per day, as excess caffeine can make premen-

strual symptoms worse.

See also: Calcium: Properties and Determination;

Carbohydrates: Metabolism of Sugars; Cocoa:

Chemistry of Processing; Production, Products, and Use;

Essential Fatty Acids; Herbs: Herbs and Their Uses;

Magnesium; Thiamin: Properties and Determination;

Physiology; Tocopherols: Properties and Determination;

Physiology; Vegetarian Diets; Vitamin B

: Properties

and Determination; Physiology

Further Reading

Barr SI, Janelle KC and Prior JC (1995) Energy intakes are

higher during the luteal phase of ovulatory menstrual

cycles. American Journal of Clinical Nutrition 61: 39–43.

Budeiri D, Li Wan Po A and Dornan JC (1996) Is Evening

Primrose Oil of value in the treatment of PMS? Con-

trolled Clinical Trials 17: 60–68.

Dalton K (1984) The Premenstrual Syndrome and Proges-

terone Therapy. London: William Heineman.

Freeman EW (1997) Premenstrual syndrome: current per-

spectives on treatment and aetiology. Current Opinions

in Obstetrics and Gynecology 9: 147–153.

Horrobin DF and Manku MS (1989) Premenstrual syndrome

and premenstrual breast pain: disorders of essential fatty

acid (EFA) metabolism. Prostaglandins, Leukotrienes and

Essential Fatty Acids Review 37: 255–261.

O’Brien PMS (1993) Helping women with premenstrual

syndrome. British Medical Journal 307: 1471–1475.

Proctor ML and Murphy PA (2002) Herbal and dietary

therapies for primary and secondary dysmenorrhoea.

Cochrane Menstrual Disorders and Subfertility Group.

Cochrane Database of Systematic Reviews. Issue 1.

Schellenberg P (2001) Treatment for the premenstrual

syndrome with agnus castus fruit extract: prospective,

randomised, placebo controlled study. British Medical

Journal 322: 134–137.

Speroff L, Glass RH and Kase NG (1994) Clinical Gynae-

cologic Endocrinology and Infertility. Baltimore, MD:

Williams & Wilkins.

Wyatt KM, Dimmock PW, Jones PW and O’Brien PMS

(1999) Efficacy of vitamin B-6 in the treatment of pre-

menstrual syndrome: systematic review. British Medical

Journal 318: 1375–1381.

PREMENSTRUAL SYNDROME: NUTRITIONAL ASPECTS 4765

PRESERVATION OF FOOD

M F Sancho-Madriz, California State Polytechnic

University, Pomona, CA, USA

Introduction

0001 Food preservation consists of the application of sci-

ence-based knowledge through a variety of available

technologies and procedures, to prevent deterioration

and spoilage of food products and extend their shelf-

life, while assuring consumers a product free of

pathogenic microorganisms. Shelf-life may be defined

as the time it takes a product to decline to an un-

acceptable level. Deterioration of foods will result in

loss of quality attributes, including flavor, texture,

color, and other sensory properties. Nutritional

quality is also affected during food deterioration.

Physical, biological, microbiological, chemical, and

biochemical factors may cause food deterioration.

Preservation methods should be applied as early as

possible in the food production pipeline and therefore

include appropriate postharvest handling before pro-

cessing of both plant and animal foods (Figure 1).

Processing techniques usually rely on appropriate

packaging methods and materials to assure continuity

of preservation. Handling of processed foods during

storage, transportation, retail, and by the consumer

also influences the preservation of processed foods.

0002 Selection of technology and procedures for food

preservation depends on factors inherent to the prod-

uct, common pathogenic and spoilage microorgan-

isms, and cost. Product-inherent factors include

customary ways of consuming the particular food,

sensitivity to heat or other principle used to inactivate

microorganisms, and other physical and chemical

characteristics of the food.

Rationale and Goals of Food Preservation

0003 Many food products are seasonal, and from year to

year the yields of agricultural production vary

depending on weather, pests, and other factors.

During the production season, especially when the

yields are high, the offer of fresh products often

exceeds the demand. The products that are not con-

sumed in the fresh market can be used in processed

form for long periods of time and by people in regions

located far from the production areas. Processed

foods not only reduce the waste of food but also

offer the consumer many convenient features that

make preparation at home easier and faster. Food

preservation plays an important role in special cir-

cumstances like wars or space missions far from

earth. In the nineteenth and twentieth centuries

many new developments in food preservation oc-

curred during war times, in both industry and govern-

ment-owned laboratories. More recently, the

development of foods suitable for space missions

that spend a significant amount of time away from

earth has posed new challenges to food scientists.

0004If the food production system is pictured as a long

pipeline that carries food from producers to con-

sumers, one will discover a number of leaks in the

pipeline that prevent large amounts of food reaching

their final destination. Those leaks or losses represent

different forms of food deterioration and spoilage,

and are likely to happen from the beginning of food

production (Figure 2).

0005Food preservation aims to prevent and reduce the

loss of food in the production system and extend its

shelf-life. While developed nations have been very

successful in reducing food losses and increasing effi-

ciency of the production system, food deterioration

remains an important issue in developing nations and

very poor countries where undernutrition of the

population is common. In many instances the food

problems of those countries could be significantly

reduced by the implementation of adequate post-

harvest handling and food preservation methods

without increasing food production. (See Food

Security.)

0006Appropriate postharvest handling of food products

is essential in food preservation, especially for prod-

ucts sold in the fresh market or as minimally pro-

cessed items. Exposure to sunlight and heat, high

Physical

deterioration

Chemical and biochemical

deterioration

ConsumerFood producer

Biological and microbiological deterioration

fig0001Figure 1 Losses in the food production pipeline.

Food preservation = Adequate postharvest handling

+ food-processing technology + packaging technology

+ adequate transport and storage

fig0002Figure 2 Main components of food preservation.

4766 PRESERVATION OF FOOD

relative humidity, rain, dirt, insects, and rodents in-

creases food deterioration by accelerating chemical

reactions in the food and favoring microbial growth.

Fast cooling of produce after harvest, storage under

temperature- and humidity-controlled conditions,

protection of the products by physical barriers, and

minimizing the time between harvest and processing

are all forms of reducing postharvest losses.

Factors that Cause Deterioration and Spoilage of

Food

0007 The factors that cause food deterioration may be

grouped under physical, biological and microbio-

logical, and chemical and biochemical factors. Most

of the time more than one factor is responsible for the

spoilage of a food product and some factors may

favor or enhance the effect of others. Light and

other forms of radiation, heat, cold, moisture loss,

or gain, and the application of force that may alter

the structure of the food are examples of physical

factors that cause food deterioration. Chemical and

biochemical factors include reactions of food com-

ponents with oxygen or with each other, and reac-

tions catalyzed by enzymatic activity. Maillard

browning is caused by a chemical reaction between

proteins and carbohydrates and it results in changes

in color, flavor, and odor of the food product. This

type of browning is desirable in baked products and

roasted coffee amongst others, but it is detrimental in

foods where browning is not desired. Oxidative ran-

cidity is a chemical change in unsaturated fats and oils

and it produces off-flavors and odors. These and

other chemical reactions affect sensory qualities of

food and can significantly alter the nutritional value

of products. (See Browning: Enzymatic – Biochemical

Aspects; Oxidation of Food Components; Spoilage:

Chemical and Enzymatic Spoilage.)

0008 Biological factors include birds, rodents, insects, and

parasites. (See Insect Pests: Insects and Related Pests;

Problems Caused by Insects and Mites.) These may

consume or destroy the food and contaminate it with

pathogenic or spoilage microorganisms. Bacteria,

yeasts, and molds are different types of microorganisms

that may cause food deterioration and foodborne ill-

ness. (See Aflatoxins; Microbiology: Classification of

Microorganisms; Mycotoxins: Classifications; Occur-

rence and Determination; Toxicology; Spoilage:

Bacterial Spoilage; Fungi in Food – An Overview;

Molds in Spoilage; Yeasts in Spoilage.) Some viruses

may also be transmitted through food but they are not

associated with the deterioration of products. Not all

microorganisms are detrimental to food; some are

indeed very useful and have been utilized since ancient

times for the production of fermented products like

wine, yogurt, and cheese. Fermented cassava and corn

are typical in some African and Latin American coun-

tries. (See Fermented Foods: Origins and Applications;

Dietary Importance.)

0009Because consumers assume that processed foods

are safe, the goals of food preservation must include

not only measures for the prevention of quality de-

terioration, but also destruction of pathogenic micro-

organisms that may cause foodborne illness. A food

product that does not show evidence of deterioration

may not necessarily be free of pathogenic microor-

ganisms. (See Food Poisoning: Classification; Food

Safety.) A preventive strategy for assuring safe food

products is the Hazard Analysis Critical Control

Point (HACCP) system. HACCP considers biological,

physical, and chemical hazards. (See Hazard Analysis

Critical Control Point.)

Factors that Affect Microbial Growth

0010In addition to the common nutrients found in most

foods, microorganisms require certain conditions to

survive and grow. The control and optimization of

those factors is a fundamental principle of food pre-

servation. The main factors are discussed below.

0011pH The degree of alkalinity or acidity of a water

solution is based on the concentration of hydrogen

(Hþ) or hydroxyl (OH) ions. The term pH is by

definition the negative logarithm of the hydrogen ion

concentration. The pH scale ranges from 0 to 14, and

pure water has a pH of 7. Each microorganism will

grow only within an optimum pH range. (See pH –

Principles and Measurement.)

0012Water activity Water activity is a measure of the

availability of water in a food product. Solutes dis-

solved in water bind the water molecules and make

them not available for microorganisms to use. Water

activity may be defined as the ratio of the vapor pres-

sure of water in food to the vapor pressure of pure

water at the same temperature. Microorganisms re-

quire a minimum level of water activity in order to

grow and thus water activity reduction can be used

to control microbial growth. (See Water Activity: Prin-

ciples and Measurement; Effect on Food Stability.)

0013Oxygen Microorganisms that require the presence

of free oxygen for their growth are known as aerobes,

whereas oxygen inhibits the growth of anaerobes.

When they can tolerate the presence of oxygen to a

certain level they are known as facultative anaerobes.

0014Temperature An optimum temperature range is

required for microbial growth. Bacteria have been

classified into groups depending on their required

PRESERVATION OF FOOD 4767

temperature range. The temperature range for

psychrotrophs is 14–20



C, for mesophiles is 30–



C, and for thermophiles is 50–66



C. Some

thermophiles can grow at temperatures up to 77



Traditional Methods of Food Preservation

0015 The following section provides a brief description of

traditional food preservation methods currently used

by the food-processing industry. For more details on

each method see the indicated articles within this

encyclopedia or the suggested publications for further

reading. (See Traditional Food Technology.)

Acidification

0016 The use of acid to preserve foods, whether by fermen-

tation of the product or by addition of acid foods or

acids, is a very common method to preserve foods

that does not require a high level of technology or

special processing equipment. The acid acts as a pre-

servative by controlling microbial growth and pH

meters are used to determine if the food has reached

the target pH level. (See Acids: Properties and Deter-

mination; Natural Acids and Acidulants; pH – Prin-

ciples and Measurement.) Acidified foods, sometimes

called pickled foods (see Pickling), have been defined

by the US Food and Drug Administration (FDA) as

low-acid foods to which acid(s) or acid food(s) is (are)

added to produce a product that has a finished equi-

librium pH of 4.6 or less and a water activity greater

than 0.85. The pH value under 4.6 is necessary to

prevent the growth of Clostridium botulinum; how-

ever, an additional preservation method such as pas-

teurization, refrigeration, or chemical preservatives is

used to destroy or control the growth of deterioration

microorganisms. (See Clostridium: Botulism.) Foods

have different degrees of buffering capacity against

acids and therefore different foods require different

amounts of acid to reach a desired pH level; the

higher the buffering capacity, the higher the amount

of acid needed to acidify the product. Other changes

in fermented foods such as metabolites produced by

some microorganisms also have an effect on micro-

bial inactivation. (See Beers: History and Types;

Cheeses: Types of Cheese; Fermented Foods: Origins

and Applications; Fermented Milks: Dietary Import-

ance; Wines: Types of Table Wine; Yogurt: The Prod-

uct and its Manufacture.)

Thermal Processing

0017 The development of canning as a preservation

method for foods can be traced back to Nicolas

Appert (1749–1841), a French chef who published

his procedures in 1810. Appert was awarded 12 000

francs for his proposal for preserving foods by heating

them in hermetically sealed containers. Not only did

Appert not have scientific training but also micro-

organisms were not known at the time he developed

his method, long before Louis Pasteur laid the foun-

dations for the science of bacteriology.

0018The degree of microorganism destruction achieved

by thermal processing varies, depending on the

specific temperature and time of the thermal treat-

ment. Thermal process design is based on a target

microorganism and desired number of logarithmic

reductions in microorganism concentration. Blanch-

ing, pasteurization, commercial sterilization, and

sterilization are different kinds of thermal processing.

Blanching is a mild heat treatment utilized mostly for

enzyme inactivation but it also serves other functions.

Although it reduces the number of containing micro-

organisms on the surface of foods, it is not intended as

a sole preservation method. Pasteurization involves

heating the product under atmospheric pressure with-

out exceeding the boiling point of water (100



C).

Milk and other low-acid foods (pH > 4.6) are pasteur-

ized to eliminate pathogenic microorganisms and

extend their shelf-life in combination with refriger-

ation or other preservation methods. In acid or

acidified foods pasteurization is used to destroy spoil-

age microorganisms and sometimes to inactivate

enzymes.

0019The US FDA defined commercial sterility of ther-

mally processed food as the condition that renders the

food free of microorganisms capable of reproducing

under normal nonrefrigerated conditions of storage

and distribution, and viable microorganisms (includ-

ing spores) of public health significance. According to

the FDA definition, commercial sterility may be

achieved by the application of heat alone or combined

with water activity control. (See Canning: Principles;

Heat Treatment: Ultrahigh Temperature (UHT)

Treatments; Pasteurization: Principles; Pasteurization

of Liquid Products; Pasteurization of Viscous and

Particulate Products.)

Concentration by Evaporation

0020Concentration by evaporation consists of partially

removing water from liquid foods by the application

of heat. Native Americans used natural evaporation

in wooden or bark vessels to manufacture maple

syrup using sap. Water removal causes a reduction

in water activity. The rate of heat transfer into the

food and the rate of mass transfer of vapor from the

food are the factors that determine the rate of evapor-

ation. This method of food preservation has a high

level of energy consumption and is therefore more

expensive. It offers the convenience of a concentrated

product that the consumer can dilute at home and it

4768 PRESERVATION OF FOOD

reduces the cost of transportation and packaging. For

example, a can of concentrated orange juice weighs

less than the single-strength juice and it requires a

smaller package. Destruction of heat-sensitive vita-

mins and loss of aroma are important issues in evap-

oration; however, the addition of vitamins and the use

of aroma-recovery systems reduce the effect of those

factors. (See Evaporation: Basic Principles; Uses in the

Food Industry.)

Dehydration

0021 In drying or dehydration, water is removed from the

food by hot air or heated surface driers. Examples of

the former include cabinet, tunnel, conveyor, and

fluidized bed driers, and the latter include drum and

vacuum shelf driers. As water in the surface evapor-

ates it is replaced by water from inside the food by

several mass transfer mechanisms, resulting in a re-

duction of the water content and water activity of the

product. In addition to preserving the food, dehydra-

tion reduces the weight and bulk of the food,

lowering transportation and packaging costs. Despite

added convenience, dehydration also has a significant

effect on the sensory properties of food. Some dehy-

drated products such as prunes or raisins are con-

sumed in dehydrated form or used as ingredients in

recipes; however, other dehydrated products such as

dry milk or vegetables in a soup mix are reconstituted

with water before consumption. (See Drying: Drying

Using Natural Radiation; Fluidized-bed Drying;

Spray Drying; Dielectric and Osmotic Drying; Hy-

giene.)

0022 Freeze-drying or lyophilization is a method that

accomplishes dehydration of the food by sublimating

the water. When water sublimates it goes directly

from a solid to a gas without passing through the

liquid phase. Freeze-dried foods exhibit superior sens-

ory and nutritive qualities when compared with prod-

ucts dehydrated by other methods. This method can

be used to dehydrate high-value solid and liquid foods

such as shrimp, strawberries, coffee, and juices, and

it is also used by the pharmaceutical industry. (See

Freeze-drying: The Basic Process.)

Refrigeration or Chilled Storage

0023 Chilled storage in refrigerated chambers at tempera-

tures above freezing is a widely used food preserva-

tion method. Refrigeration temperatures usually

range from 0 to 7



C in commercial and household

refrigerators. The low temperatures lower the rate of

metabolic reactions in unprocessed fruits and vege-

tables and other chemical reactions in foods. Micro-

bial growth is usually slowed at refrigeration

temperatures because metabolic reactions of micro-

organisms are enzyme-catalyzed and their rate

depends on temperature. Refrigeration will preserve

perishable foods for days or weeks, depending on the

food. This method of preservation has very mild

effects on sensory and nutritive attributes of products;

however, it does not prevent food deterioration in the

same degree and for as long as freezing or most other

preservation methods. Refrigerated foods require low

storage temperatures during transportation, retail,

and home storage and their use is limited in rural

areas of developing nations. (See Chilled Storage:

Principles; Attainment of Chilled Conditions; Quality

and Economic Considerations; Microbiological Con-

siderations.)

Freezing

0024Foods were frozen in ancient times using ice and

snow. Frozen storage cabinets were developed in the

nineteenth century. Nowadays, food is frozen using

several types of industrial mechanical refrigerators

through cooled surfaces, and cooled liquid or air.

Cryogenic freezing uses liquid nitrogen or carbon

dioxide in solid or liquid form in direct contact with

the food. Frozen food storage requires temperatures

that maintain the food in frozen condition, usually

18



C or less, and will preserve foods for months or

years if properly packaged. A proportion of the water

in the food is frozen and the concentration of solutes

in unfrozen water increases, lowering the water activ-

ity. Freezing usually stops microbial growth but it

does not destroy bacteria and molds. The parasite

Trichinella spiralis and fish parasites are killed during

frozen storage. Freezing temperatures significantly

reduce the rate of chemical reactions in foods. Freez-

ing has a low effect on nutritive quality of the food

but sensory qualities, especially texture, may be

affected by the formation of ice crystals. Fish and

seafood, meats, fruits, and vegetables have been sold

in frozen form for a long time. Baked goods and other

prepared foods have become popular for their con-

venience, especially because of the use of microwave

ovens in households. (See Freezing: Principles.)

Salting, Sugaring, and Curing

0025The addition of large amounts of salt or sugar to food is

an old method of food preservation. Fish preservation

by salting was used in Mesopotamia, Egypt, China,

and the Mediterranean basin since ancient times. Meat

was also preserved in old Mesopotamia by the addition

of salt. Salt has also been used to preserve butter,

cheese, and milk curds. Jams and preserves are the

most common products preserved by adding sugar.

When salt or sugar is added water moves from inside

the cells to the outside solutes by osmosis, causing a

partial dehydration of the cell, known as plasmolysis,

that interferes with microorganism multiplication.

PRESERVATION OF FOOD 4769

Both salting and sugaring have a significant effect in

lowering water activity in food.

0026 Curing is a method to preserve meats that also

changes the flavor, color, and tenderness of the

product. With more effective preservation methods

available, the main purpose of curing is to produce

characteristic products with unique flavor and to pre-

serve the red color of meat after cooking. The main

ingredients for curing or picking meat are sodium

chloride, sodium nitrate and/or sodium nitrite,

sugar, and spices. Sodium nitrate and nitrite have

been linked with the formation of nitrosamines, com-

pounds shown to be carcinogenic in animal studies.

Nitrates and nitrites are naturally present in some

vegetables and other foods and therefore cured

meats may not be the main source of these com-

pounds in human diets. (See Curing; Jams and Pre-

serves: Methods of Manufacture; Chemistry of

Manufacture.)

Smoking

0027 Smoking is an old method of food preservation and it

continues to be used today for fish and meats. The

smoke is obtained by burning hickory or a similar

wood and it contains formaldehyde and phenolic

compounds that have antimicrobial properties. The

heat also dries the food, increasing preservation.

Smoking of foods is currently used more for its unique

flavor properties than for its preservative action.

Chemical Preservatives

0028 A number of chemical substances are used to inhibit

the growth of microorganisms in foods. Sodium

benzoate, sodium and calcium propionate, sorbic

acid, ethyl formate, and sulfur dioxide are examples

of commercially used food preservatives. Preserva-

tives must undergo a review process that includes

data on toxicological aspects prior to their approval.

The allowed amount of a preservative in a product

varies depending on the substance and the food. The

pH level is also an important consideration as preser-

vatives have an optimum pH range for their antimi-

crobial activity. Antioxidants prevent oxidative

rancidity of fats and oils. Examples of commonly

used antioxidants are butylated hydroxyanisole

(BHA), butylated hydroxytoluene (BHT), tertiary

butylated hydroquinone (TBHQ), and propyl gallate.

Ascorbic acid (vitamin C) and tocopherols (vitamin

E) are also used as antioxidants. (See Antioxidants:

Natural Antioxidants; Synthetic Antioxidants; Syn-

thetic Antioxidants, Characterization and Analysis;

Role of Antioxidant Nutrients in Defense Systems;

Preservatives: Classifications and Properties; Food

Uses.)

0029Increased interest in natural antimicrobials has

been noted in recent years due to toxicological con-

cerns about synthetic preservatives and market trends

that show increased popularity of foods with no arti-

ficial substances. Several naturally occurring sub-

stances have been identified and studied for their

antimicrobial properties.

Role of Packaging in the Preservation of

Foods

0030Adequate food packaging is essential and it must be

used in combination with all food preservation

methods. Packaging must protect the processed food

against chemical attack, physical damage and con-

tamination with microorganisms, insects, and

rodents. Hermetic packaging refers to containers

that are completely sealed against the ingress of

vapors and gases and therefore also resistant to bac-

teria, yeasts, molds, and dust. Glass and metal cans

are the most common hermetic containers. Nonher-

metic containers may allow gas exchange but should

provide protection against microbial contamination.

Other packaging materials are plastics, paper, paper-

board, and combinations of more than one material

by lamination. Combinations are also achieved by

simultaneous extrusion of two or more layers of dif-

ferent polymers to form a single film. Packaging must

also be nontoxic, act as a barrier to moisture loss or

gain, protect against oxygen and odor absorption,

protect the food against ultraviolet light, be tamper-

resistant or tamper-evident, and be compatible with

the food. Food packages have other important func-

tions in marketing the product and educating con-

sumers on product use and handling. Food packages

often carry the date of manufacturing or an expir-

ation date, and some include a code that allows

companies to trace the product in case of a recall.

Expiration dates inform the consumer of the approxi-

mate shelf-life of the product. Shelf-life depends on

the type of food, the processing method, packaging,

and storage conditions. (See Packaging: Packaging of

Liquids; Packaging of Solids.)

0031Active packaging is an innovative concept that has

received a lot of attention in recent years. Active

packaging may be defined as a type of packaging

that changes the condition of the packaging to

extend the shelf-life or improve safety or sensory

properties while maintaining the quality of the food.

Major active packaging techniques involve the ab-

sorption of oxygen, ethylene, moisture, odors, or

carbon dioxide; they may also involve the release of

antimicrobial agents, carbon dioxide, antioxidants,

and flavors.

4770 PRESERVATION OF FOOD

Controlled- or Modified-Atmosphere Storage and

Packaging

0032 Modification of the gas composition in storage rooms

or inside the food packaging reduces the rate of

respiration of fresh fruits and vegetables and also

inhibits microbial and insect growth. Lowering

respiration rates reduces biochemical and enzymatic

activities that promote ripening and senescence. Gas

modifications include oxygen reduction and increase

in carbon monoxide or nitrogen. This method may

also be combined with chilled storage, resulting in

extended shelf-life and high quality of the products.

(See Chilled Storage: Use of Modified-atmosphere

Packaging; Packaging Under Vacuum.)

Aseptic Packaging

0033 Aseptic processing and packaging is a sophisticated

food preservation method where the food is sterilized

or commercially sterilized outside the container

and then placed in previously sterilized containers

and sealed in an aseptic environment. Aseptic systems

use ultrahigh-temperature (UHT) sterilization, a fast

heating treatment at temperatures higher than pas-

teurization temperatures. Paper and plastic pack-

aging materials are sterilized, formed, filled, and

sealed in a continuous operation at the end of the

processing line. Aseptic packaging is also used with

metal cans, large plastic or metal drums, or large

flexible pouches. Packages for aseptic processing

may be sterilized using heat, chemicals, irradiation,

or a combination of these methods.

Nontraditional or Alternative Methods of

Food Preservation

0034 Several alternative methods for food preservation are

currently used or considered for future use. Some

methods have been approved by regulatory angencies

only for certain products and after a review of avail-

able scientific information on kinetics of microbial

inactivation, toxicological aspects, and the effects of

the treatment on the food products. The US FDA has

identified several research needs applicable to

alternative technologies. The following are brief

descriptions of alternative food-processing methods

evaluated by the FDA.

Irradiation

0035 Irradiation is a preservation method where food is

exposed to radiation. Irradiated food does not

become radioactive. Machine or radionuclide radi-

ation sources are used. Machine sources include

electron acelerators and X-ray generators, and radio-

nuclide sources include radioactive materials that

give off ionizing gamma-rays. Bacteria, molds, yeasts,

and insects are inactivated by irradiation. In 1963 the

US FDA first approved irradiation for use on wheat

and wheat flour. FDA requires that foods that have

been irradiated bear both a logo and a statement that

the food has been irradiated. The safety of irradiated

foods has been studied extensively. (See Irradiation of

Foods: Basic Principles.)

Microwave and Radiofrequency Processing

0036Electromagnetic waves of certain frequencies gener-

ate heat in foods by dielectric and ionic mechanisms.

Microwave and radiofrequency heating have the ad-

vantage that they require less time than conventional

heating, particularly for solid and semisolid foods.

Industrial microwave pasteurization has been used

for at least 30 years, but not radiofrequency process-

ing systems. Food does not heat uniformly during

microwave processing, and this remains an important

issue when considering this technology. Several

methods have been used to improve the uniformity

of heating but equipment design can significantly

influence processing parameters and establishing

general conclusions has proved difficult.

Ohmic and Inductive Heating

0037In ohmic heating electric currents are passed through

the food in order to heat it. This technology is also

known as Joule heating, electrical resistance heating,

electroheating, and electroconductive heating. In-

ductive heating is a process that induces electric

currents within the food by the use of oscillating

electromagnetic fields generated by electric coils.

Microbial death kinetics data have been published

for ohmic heating but not for inductive heating. The

main advantage of ohmic heating is its ability to heat

materials in a fast and uniform fashion, including

products with particulates. Potential future uses of

ohmic heating are in dehydration, evaporation,

blanching, and extraction.

High-Pressure Processing (HPP)

0038HPP is also known as high hydrostatic pressure

(HHP) or ultrahigh pressure (UHP) processing.

During HPP, liquid or solid foods are subject to pres-

sures between 100 and 800 MPa, at temperatures

below 0



C to above 100



C and for times ranging

from a millisecond pulse to more than 20 min. Tem-

peratures ranging from 45 to 50



C during treatment

appear to increase the inactivation of pathogens and

spoilage microorganisms. Temperatures in the range

of 90–100



C, together with pressures from 500 to

700 MPa, have been used to inactivate spore-forming

bacteria. The effect of HPP works instantaneously

PRESERVATION OF FOOD 4771

and uniformly throughout the mass of food, inde-

pendent of size, shape, and food composition. The

temperature of the food increases by approximately



C per 100 MPa of applied pressure. Water activity

and pH are critical product factors in microbial in-

activation by HPP. Process factors that affect HPP

include pressure, time at pressure, time to achieve

treatment pressure, decompression time, treatment

temperature, product initial temperature, vessel tem-

perature distribution at pressure, packaging material

integrity, and concurrent processing aids.

Pulsed Electric Fields (PEF)

0039 This method involves the application of pulses of high

voltage (20–80 kV cm

1

) to foods placed between

two electrodes. Most PEF systems are at the labora-

tory phase with not many commercial systems in use.

Application of PEF is restricted to foods that can

withstand high electric fields, have low electrical

conductivity, and do not contain or form bubbles.

Although various theories have been proposed to ex-

plain the microbial inactivation by PEF, the most

studied are electrical breakdown and electroporation.

Process factors that affect microbial destruction by

PEF include electric field intensity, pulse width, treat-

ment time, and temperature.

Ultraviolet (UV) Light

0040 This type of processing involves the application of

radiation from the UV region of the electromagnetic

spectrum. Microbial inactivation occurs by DNA mu-

tations upon absorption of the UV light and exposure

must be at least 400 J m

2

in all parts of the product.

Critical factors during UV light processing include

the transmissivity of the product, the radiation path

length, the geometric configuration of the reactor and

the power, the wavelength, and physical arrangement

of the UV source(s). UV light is used for bottled-water

processing and for sanitizing food contact surfaces.

There has been an increased interest in using UV light

as a preservation method for fruit juices to replace

pasteurization and other thermal treatments that

have more impact on sensory attributes. (See Ultra-

violet Light.)

Other Alternative Technologies

0041 High-voltage arc discharge, pulsed light technology,

oscillating magnetic fields (OMF), ultrasound, and

pulsed X-rays are other types of technology that

have been explored for their potential to inactivate

microorganisms and may show promise for use in

food preservation; however, more extensive research

is needed at this time.

See also: Antioxidants: Natural Antioxidants; Browning:

Enzymatic – Biochemical Aspects; Canning: Principles;

Chilled Storage: Principles; Microbiological

Considerations; Use of Modified-atmosphere Packaging;

Packaging Under Vacuum; Clostridium: Botulism;

Curing; Drying: Drying Using Natural Radiation;

Fluidized-bed Drying; Spray Drying; Dielectric and

Osmotic Drying; Hygiene; Evaporation: Basic Principles;

Uses in the Food Industry; Fermented Foods: Origins

and Applications; Food Poisoning: Classification; Food

Safety; Food Security; Freeze-drying: The Basic

Process; Freezing: Principles; Hazard Analysis Critical

Control Point; Heat Treatment: Ultra-high Temperature

(UHT) Treatments; Irradiation of Foods: Basic

Principles; Mycotoxins: Classifications; Oxidation of

Food Components; Pasteurization: Principles;

Pickling; Preservatives: Classifications and Properties;

Spoilage: Bacterial Spoilage; Molds in Spoilage; Yeasts

in Spoilage; Storage Stability: Mechanisms of

Degradation

Further Reading

Borgstrom G (1968) Principles of Food Science, vol. 1.

Food Technology. London: Collier-MacMillan.

Buffler CR (1993) Microwave Cooking and Processing.

New York: Van Nostrand Reinhold.

Connor JM and Schiek WA (1997) Food Processing, An

Industrial Powerhouse in Transition, 2nd edn. New

York: John Wiley.

deMan JM (1999) Principles of Food Chemistry, 3rd edn.

Gaithersburg, MD: Aspen.

Fellows PJ (2000) Food Processing Technology. Cambridge,

UK: Woodhead Publishing.

Food and Drug Administration (2000) Code of Federal

Regulations, Title 21. Washington, DC: US government.

Food and Drug Administration (2000) Kinetics of Micro-

bial Inactivation for Alternative Food Processing Tech-

nologies: Executive Summary. Washington, DC: US

government.

Heldman DR and Hartel RW (1997) Principles of Food

Processing. New York: Chapman and Hall.

Jay JM (1998) Modern Food Microbiology, 5th edn.

Gaithersburg, MD: Aspen Publishers.

Potter NN and Hotchkiss JA (1995) Food Science. New

York: Chapman and Hall.

Rhaman MS (ed.) (1999) Handbook of Food Preservation.

New York: Marcel Dekker.

VanGarde SJ and Woodburn M (1994) Food Preservation

and Safety: Principles and Practice. New York: Chap-

man and Hall.

4772 PRESERVATION OF FOOD

PRESERVATIVES

Contents

Classifications and Properties

Food Uses

Analysis

Classifications and Properties

B L Wedzicha, University of Leeds, Leeds, UK

Background

0001 Preservatives are substances added to food to inhibit

microbial spoilage. Chemical food spoilage by en-

zymatic and nonenzymatic mechanisms may be con-

trolled by specific additives, e.g., antioxidants and

antibrowning agents, which are described in the rele-

vant articles. Substances such as common salt, sugar,

vinegar, or spices, which are effective antimicrobial

agents under appropriate conditions, are not re-

garded as food preservatives. Reactions of food pre-

servatives, which are not part of the chemistry of food

preservation but nevertheless affect food quality, will

also be considered. The chemical reactions that lead

to cured meat color and flavor are reviewed else-

where.

Need for Preservatives

0002 The use of preservatives to extend the life of foods

and improve their safety has been recognized for

thousands of years. Traditional methods of preserving

foods have relied on fumigation by sulfur dioxide,

curing with brines containing nitrites and nitrates,

and smoking, in which phenolic antimicrobial agents

are added to the food. These processes not only

ensure that stored food is of acceptable microbio-

logical quality but also frequently provide it with

unique organoleptic characteristics.

0003 Food preservatives ensure the safety and organo-

leptic quality of food when sold in the type of

marketing environment now found in the developed

countries. A particular problem associated with

multiuse packs of food, e.g., beverages and spreads,

is that an initially sterile, pasteurized or hygienically

assembled, food becomes infected with bacteria,

fungi, and yeasts, and begins to spoil. The instability

might arise because the food has been formulated in

such a way that it has little or no natural protection

against microorganisms. For example, a low-fat

spread contains significantly larger water droplets

than butter and can, therefore, support microbial

growth more readily. A preservative is particularly

useful in such a product. A relatively short increase

in storage life can be of significant economic and

practical advantage. This is well illustrated with ad-

vantages gained by extending the shelf-life of bread

by a few days.

0004Food preservatives are often used in conjunction

with other methods of food preservation, e.g., some

preservatives allow a milder thermal treatment to be

applied for the preservation of food with consequent

improvement in textural and nutritional properties.

Structures of Food Preservatives

0005Substances commonly used to preserve foods are

listed in Table 1. These represent a wide range of

molecular structures and tendency to form ions. The

carboxylic acids sorbic, benzoic, formic, acetic, lactic,

tbl0001Table 1 Substances recognized as suitable for use as food

preservatives

Sorbic acid and Na

,Ca

2þ

salts E200–E203

Benzoic acid and Na

,Ca

2þ

salts E210–E213

Ethyl, methyl, and propyl p-hydroxybenzoate

and Na

salts

E214–E219

Sulfur dioxide, Na

and

2þ

sulfites and hydrogen sulfites

(bisulfites), Na

and K

disulfites

(metabisulfites or pyrosulfites)

E220–E227

Biphenyl E230

Orthophenylphenol E231

Thiabendazole E233

Formic acid and Na

,Ca

2þ

salts E236–E238

and K

nitrite and nitrate E249–E252

Acetic acid and Na

and K

,Ca

2þ

salts E260–E263

Lactic acid E270

Propionic acid and Na

,Ca

2þ

salts E280–E283

Hydrogen peroxide

Nisin

Those that are approved for use within the EC have their E-number shown

alongside. The presence of any substance in this list does not imply that its

use is permitted throughout the world. Biphenyl, orthophenylphenol, and

thiabendazole are used for surface treatment.

PRESERVATIVES/Classifications and Properties 4773

and propionic are monobasic. Aqueous solutions of

sulfur dioxide are commonly referred to as sulfurous

acid, despite the fact that H

does not exist in

measurable amounts. It is established that SO

does

not interact significantly with water molecules, and

the term ‘molecular SO

‘ is more appropriate. Aque-

ous solutions of SO

behave as solutions of a dibasic

acid. The hydrogen sulfite ion is also in equilibrium

with the disulfite ion. Significant amounts of disul-

phite ion are formed in concentrated systems. When

sodium or potassium disulfite is used as a food pre-

servative, the salt is hydrolyzed to the hydrogen

sulfite ion.

0006 The nitrites are salts of nitrous acid, which is also

the source of the nitrosating species, N

, and

oxides of nitrogen, particularly NO.

0007 Regardless of the chemical form of a preservative

when it is added to food, its ionic state is determined

largely by the pH of the food and the pK

of the acid.

The pK

values of those preservatives that have an

acid group capable of ionizing within, or close to, the

pH range of foods are given in Table 2. The fraction,

x, of an acid that remains undissociated at a given pH

may be found using

x ¼ð10

ðpHpK

þ 1Þ

1

Since significant changes in x occur in the range

(pK

– 2) to (pK

þ2), the proportions of ionized

and unionized forms of the acids listed in Table 2

change substantially over the pH range of foods.

However, the hydroxybenzoate esters do not ionize

significantly in the pH range of foods and are, there-

fore, uncharged.

0008 An essential feature of food preservatives is that

their molecules should have an appropriate balance

of lipophilic and hydrophilic behavior, so as to be

capable of traversing relatively nonpolar membranes

and yet be sufficiently soluble in the aqueous environ-

ment of the microorganisms. This applies to the

undissociated carboxylic acids, which will conse-

quently tend to partition between the aqueous and

oil phase of food. The ionized acids are relatively

insoluble in nonaquous environments. A partition

coefficient, P, is defined by

P ¼ C

oil

where c refers to the concentration of undissociated

acid in each phase. Values of P vary from 0.17 and 2.5

for propionic and sorbic acids, respectively, to 26

and 88 for the ethyl and propyl esters of p-

hydroxybenzoic acid. The latter two preservatives

therefore show a marked bias for the oil phase. Nu-

merous surfactants exist in solution as micellar struc-

tures, which are also found in the lipid bilayers of cell

membranes. The interior of such structures is mark-

edly hydrophobic, and carboxylic acids are able to

partition into micelles. When expressed in the same

way as the oil–water partition coefficient, but using

mole fractions instead of concentrations, values of the

micellar partition coefficients for benzoic and sorbic

acids are in the range 1000–2000. The presence of an

oil phase and/or surfactants in solution can signifi-

cantly reduce the availability of food preservatives.

Mechanism of Antimicrobial Action

Carboxylic Acids and Esters

0009The antimicrobial action of a food preservative

depends on the concentrations of both the ionized

and unionized forms, and the specific efficacy of

each. In general, the undissociated acid is the better

antimicrobial agent, but the presence of the ionized

form cannot be neglected, particularly when, at high

pH, its concentration may be much larger than that of

the undissociated acid. The minimum concentration

of an undissociated acid required to inhibit micro-

organisms is generally one to two orders of magni-

tude smaller than that of the corresponding anion.

0010There is no general theory to explain the mechan-

isms whereby these carboxylic acid and ester pre-

servatives exert their effect. Transport of these

preservatives across cell membranes is passive, but

accumulation in the cell causes a reduction in the

protonmotive force through the membrane and a

reduction in pH within the cell. These contribute to

the cessation of cell growth or cell death. Certain

preservatives could also exert specific effects on meta-

bolic enzymes. Sorbic acid is alleged to react with the

sulfhydryl groups of fumarase in catalase-positive

bacteria, molds and yeasts, and aspartate, succinic,

and yeast alcohol dehydrogenase. It is suggested that

this might apply to sulfhydryl-containing enzymes in

general. Benzoate inhibits enzymes in oxidative phos-

phorylation and at various stages in the tricarboxylic

acid cycle.

0011Microorganisms are able to metabolize some food

preservatives when present at sublethal concentra-

tions. A significant example is the conversion of

tbl0002 Table 2 pK

values of commonly used food preservatives

Sorbic acid 4.76

Benzoic acid 4.18

Sulfur dioxide 1.86

Formic acid 3.75

Nitrous acid 3.40

Acetic acid 4.76

Lactic acid 3.08

Propionic acid 4.88

4774 PRESERVATIVES/Classifications and Properties