9/21/2016 Food preservation ­ AccessScience from McGraw­Hill Education

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Food preservation Article by: Mendonca, Aubrey F. Department of Food Science and Human Nutrition, Iowa State University, Ames, Iowa. Potter, Norman N. Department of Food Science, Cornell University, Ithaca, New York. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097­8542.267200 (http://dx.doi.org/10.1036/1097­8542.267200)

Content

Heat Cooling and freezing Concentration and dehydration Irradiation pH control Chemical preservative Packaging Combined preservation methods Bibliography Additional Readings

The branch of food science and technology that deals with the practical control of factors capable of adversely affecting the safety, nutritive value, appearance, texture, flavor, and keeping qualities of raw and processed foods. Since thousands of food products differing in physical, chemical, and biological properties can undergo deterioration from such diverse causes as microorganisms, natural food enzymes, insects and rodents, industrial contaminants, heat, cold, light, oxygen, moisture, dryness, and storage time, food preservation methods differ widely and are optimized for specific products. Apart from application of a single food preservation method, a combination of methods may be used to improve the safety and storage stability of foods. This concept is currently referred to as hurdle technology; it is also known as food preservation by combined methods, combination preservation, or combination techniques.

Food preservation methods involve the use of heat, refrigeration, freezing, concentration, dehydration, radiation, pH control, chemical preservatives, and packaging applied to produce various degrees of preservation in accordance with the differing use patterns and shelf­life needs of unique products.

Perishability of many food materials was somewhat controlled long before the principles of modern food preservation were understood. Cheese and other fermented milk products, wine, sauerkraut and pickles, smoked meats and fish, dried and sugared fruits, and numerous other foods had their beginnings in attempts to extend the storage life of the basic commodities from which they were derived, but results were often disappointing.

Optimum food preservation must eliminate or minimize all of the factors that may cause a given food to deteriorate, without producing undue adverse effects. This can be especially difficult since the components of foods may be more sensitive to preservation treatments than the highly resistant bacterial spores and natural food enzymes targeted for destruction. Many nonbiological causes of food deterioration must be prevented also; these include oxygen, light, and loss of moisture.

http://www.accessscience.com/content/food­preservation/267200 1/6 9/21/2016 Food preservation ­ AccessScience from McGraw­Hill Education While traditional methods of food preservation, including heating, freezing, drying, refrigerating, and acidifying, are still widely used in the food industry, new food preservation methods are actively being researched. These include physical methods such as high hydrostatic pressure, pulsed electric fields, ohmic heating, and ultrasound. Such methods have been shown to be effective in preserving food with minimal effects on nutrients and sensory quality that might otherwise be destroyed by traditional food preservation methods such as heating, drying, and acidification.

Heat

Thermal processes to preserve foods vary in intensity. True sterility to ensure total destruction of the most heat­resistant bacterial spores in nonacidic foods may require a treatment of at least 250°F (121°C) of wet heat for at least 15 min, or its lethal equivalent, to be delivered throughout the entire food mass. Such a treatment would be damaging to most foods. The term commercial sterility refers to a less severe condition that still assures destruction of all pathogenic organisms, as well as organisms that, if present, could grow in the product and produce spoilage under normal conditions of handling and storage. Most of the canned food supply that is stable at room temperature is commercially sterile. This is commonly achieved in canning retorts with steam under pressure at temperatures and for times that vary, depending upon container size and chemical and physical properties of the food, which can affect heat­transfer rates and the thermal resistance of organisms. See also: Sterilization (/content/sterilization/655600)

Many foods are subjected to still less severe heating by methods that produce pasteurization to assure destruction of pathogens and extend product shelf life by inactivating food enzymes and reducing the number of spoilage organisms. Pasteurization of milk is achieved with a temperature of 145°F (63°C) for 30 min, or its thermal lethal equivalent. Since significant numbers of nonpathogenic bacteria survive, storage life is extended by refrigerating the pasteurized milk. Beer, wine, fruit juices, and other foods are commonly pasteurized, but at different temperatures. Heat blanching is a kind of pasteurization applied to vegetables to inactivate enzymes when such products are to be frozen, since frozen storage of itself does not stop enzyme activity. See also: Pasteurization (/content/pasteurization/492000)

The lethality of heat always depends upon temperature and time. Higher temperatures for shorter times can be as effective as lower temperatures for longer times, and appropriate combinations can be selected for thermal lethal equivalency. Time­ temperature combinations with equivalent microbial lethality, however, are not equal with respect to the damaging effects these can have on color, flavor, texture, and nutritive values of foods. In this regard, higher temperatures for shorter times will yield products superior to those produced with lower temperatures for longer times.

Advances in thermal processing incorporate the high­temperature short­time (HTST) principle whether pasteurization or commercial sterilization is the goal. The application of high­temperature short­time processing is more easily accomplished with liquid foods or liquids containing small particulates than with solid foods, since the former can easily be heated and cooled rapidly by passing them in thin layers through specially designed heat exchangers. This is done in the process of aseptic canning, where products prepared to commercial sterility standards are heated to temperatures as high as 302°F (150°C) for 1 or 2 s and as quickly cooled, and then sealed in previously sterilized containers within an aseptic environment.

Cooling and freezing

The slowing of biological and chemical activity with decreasing temperature is the principle behind cooling (refrigeration) and freezing preservation. In addition, when water is converted to ice, free water required for its solvent properties by all living systems is removed. Even severe freezing, however, will not destroy large numbers of microorganisms or completely

http://www.accessscience.com/content/food­preservation/267200 2/6 9/21/2016 Food preservation ­ AccessScience from McGraw­Hill Education inactivate food enzymes; these can resume rapid activity, unless inhibited by other means, when food is removed from cold or frozen storage.

Most microorganisms grow best in the range of about 60–100°F (16–38°C). Psychrotrophic bacteria thrive at low temperatures and can grow slowly at temperatures down to 32°F (0°C) and below if free water exist. Most pathogens cannot grow below 40°F (4°C). Home refrigerators commonly operate in the range of about 40–45°F (4–7°C). Some fruits and vegetables store best at temperatures of about 50°F (10°C), and commercial refrigerated storage may be optimized for specific products. Refrigerated storage life of many foods can be extended by the use of packaging that minimizes moisture loss and controls gas atmospheres within packages.

Highest­quality frozen foods depend upon very fast rates of freezing. Slow freezing leads to the growth of large irregular ice crystals capable of disruption of delicate food textures. Slow freezing also increases the time during which food constituents can react adversely with solutes that become concentrated by liquid water changing to ice as freezing progresses. Thus, rapid freezing has been the goal of advanced freezing processes.

Commercial freezing methods utilize refrigerated still air; high­velocity air, which is faster and more efficient; and high­ velocity air made to suspend particulate foods, such as peas, as in a fluidized­bed fast freezer. Indirect­contact freezing utilizes hollow flat plates chilled with an internally circulated refrigerant to freeze solid foods, or with refrigerated tubular heat exchangers that rapidly slush­freeze liquids. Immersion freezing involves direct contact of the food or its container with such refrigerants as cold brine, a glycol approved for food, or a fast­freezing cryogenic liquid, such as liquefied carbon dioxide or liquid nitrogen. Liquid nitrogen has a temperature of −320°F (−196°C). See also: Cold storage (/content/cold­ storage/148000)

Concentration and dehydration

When sufficient water is removed from foods, microorganisms will not grow, and many enzymatic and nonenzymatic reactions will cease or be markedly slowed. Free water that can enter into biological and chemical reactions is more important than total water, since some water may be bound and unavailable to support deteriorative processes. Free water exerts vapor pressure and possesses water activity (that is, provides water for bacterial growth), which must be decreased below critical levels if foods are to be preserved. Sugar syrups are concentrated foods whose water activity is below that required to support microbial spoilage. Sugar added to fruit juice will bind water, lower the juice's water activity, and, in sufficient concentration, yield a jelly that does not undergo microbial spoilage at room temperature. Concentration preservation, therefore, can be achieved by physically removing water, as by boiling or with lower­temperature vacuum evaporation, or by binding water through the addition of sugar, salt, or other solutes.

Foods preserved by dehydration contain considerably lower water activity and less total water than concentrated foods. Sun­dried cereal grains contain about 14% total water. Most dehydrated foods such as dried milk, instant coffee, and dehydrated potato flakes or granules contain less than 10% total water, and some, such as fruit juice crystals, contain less than 2%.

Most dehydration methods utilize heat to vaporize and remove water. This is most efficiently achieved when a food can be highly subdivided to produce a large surface area for rapid heat transfer into the food and rapid moisture transfer out. Liquid foods and purees commonly are atomized into a heated chamber (as in spray drying), spread thinly over the surface of a revolving heated drum from which they are continuously scraped (as in drum drying), and sometimes thickened or

http://www.accessscience.com/content/food­preservation/267200 3/6 9/21/2016 Food preservation ­ AccessScience from McGraw­Hill Education foamed and cast on belts that move through a tunnel oven. Solid foods may be diced to uniform piece size for more even drying and dried with heated moving air in cabinets, on belts, or within rotating cylinders to provide tumbling action. Fluidized­bed dryers use high­velocity air to suspend particulates for still faster drying.

The heat and oxygen sensitivity of many foods necessitates vacuum dehydration for high quality. Under vacuum, water can be removed at reduced temperature, and oxidative changes are minimized. Solid foods tend to shrink and undergo shape distortion when they are dried. This can be overcome by freeze­drying whereby foods are frozen quickly and placed in a chamber under high vacuum. Vacuum and temperature conditions are regulated to promote sublimation of water vapor from the ice phase without the ice melting. The food's structure remains rigid as it goes directly from the frozen state to dryness. Because of its gentleness, freeze­drying is also used to dehydrate liquid foods such as coffee. A disadvantage of freeze­drying, however, is that it is more costly than other drying methods. See also: Drying (/content/drying/206100); Sublimation (/content/sublimation/664000)

Irradiation

X­rays, ultraviolet light, and ionizing radiations (including gamma and beta rays) belong to the electromagnetic spectrum of radiations and differ in frequency, wavelength, penetrating power, and the effects upon biological and nonbiological systems. Ionizing radiations may be obtained from radioactive isotopes, such as cobalt­60, or from electron accelerator machines. These radiations penetrate foods and exert their major effects by producing free radicals from water and other substrates. Depending upon dose intensity, these radiations can inhibit the sprouting of tubers, destroy insects, inactive some enzymes, and kill microorganisms to the point of pasteurization or sterilization. Preservation of food using sterilizing irradiation doses is done for various purposes, such as for use by astronauts during space missions and for immune­ compromised persons who are highly susceptible to microbial infection. See also: Free radical (/content/free­ radical/271500)

Food irradiation remains highly controversial, partly because of fears that the safety of products and processes cannot be adequately regulated. In the United States, treatment of spices, raw or frozen beef, pork, or poultry meat to destroy microbial contamination is among the very few applications that are permitted. Several other countries permit wider use of food irradiation, including low­dose irradiation pasteurization to extend the storage life of highly perishable fruits and vegetables, poultry, and seafoods. pH control

Hydrogen­ion concentration affects the rate and course of a great variety of chemical reactions. Microbial growth and metabolism and the activities of food enzymes exhibit pH optima and can be controlled to various degrees beyond these optima. The natural acids of certain fruits and vegetables, acid added as a chemical, and acid produced by fermentation can inhibit or partially inhibit several pathogenic and spoilage organisms. Clostridium botulinum, the most heat­resistant pathogen found in foods, will not grow and produce toxin at a pH of 4.6 or below. Therefore, foods with a pH in this range do not constitute a health hazard from this organism, and they do not require heat processing as severe as that required for more alkaline foods. Further, acid enhances the lethality of heat, often permitting milder heating conditions.

The pH of acidic foods, however, is rarely sufficiently low to assure long­term preservation from acid alone. Many acidic and fermented foods further depend upon prior pasteurization of their ingredients, the addition of salt and other chemicals, and refrigeration. See also: pH (/content/ph/504000)

Chemical preservative

http://www.accessscience.com/content/food­preservation/267200 4/6 9/21/2016 Food preservation ­ AccessScience from McGraw­Hill Education The U.S. Food and Drug Administration and comparable agencies in various countries vigorously regulate the chemicals that may be added to foods as well as the conditions of their use. Chemical preservatives and similar substances include antimicrobials, such as sodium benzoate, sorbic acid, and sodium nitrite; enzyme inhibitors, such as sulfur dioxide, to control browning of fruits and vegetables; and antioxidants, including butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), to help control fat rancidity. New chemicals must undergo rigorous testing to be approved, and approved chemicals may subsequently be prohibited when new information relative to safety warrants such action. There is much pressure to remove chemicals from the food supply, especially where their effects can be achieved by other means.

Packaging

Preservation methods cannot be effective without adequate packaging. Packaging protects foods from contamination, moisture gain or loss, flavor loss and odor pickup, the adverse effects of light, physical damage, and intentional tampering. Packaging enables food to be stored under vacuum, inert gases, or carefully selected gases that can control respiration of fruits and vegetables, biological changes in meat, and growth of microorganisms. Packages and packing materials must be carefully chosen to withstand the stresses of heating, freezing, and other operations since many products are processed within their final containers. Ultimately, a food product's quality and storage life are determined largely by its package. See also: Food engineering (/content/food­engineering/266000); Food manufacturing (/content/food­ manufacturing/266500); Food microbiology (/content/food­microbiology/267000)

Combined preservation methods

The application of combined food preservation methods is called hurdle technology. This approach to food preservation is based on the use of a combination of several preservative factors (called hurdles) such as high or low temperature, reduced water activity, acidity, and antimicrobial food additives. These hurdles inhibit or prevent growth of food­borne microorganisms to enhance food safety and reduce food spoilage. The application of hurdle technology permits mild but very effective preservation of safe, stable, nutritious foods with desirable sensory quality because only a relatively low level of each preservation factor is usually required to improve the storage stability of foods.

Aubrey F. Mendonca Norman N. Potter

Bibliography

G. V. Barbosa­Canovas et al., Nonthermal Preservation of Foods, Marcel Dekker, New York, 1998

L. Leistner and G. Gould, Hurdle Technologies: Combination Treatments for Food Stability, Safety and Quality, Kluwer Academic/Plenum, New York, 2002

D. G. Olson, Irradiation of food, Food Technol., 52:56–62, 1998

N. N. Potter and J. H. Hotchkiss, Food Science, 5th ed., Aspen Publishers, Gaithersburg, 1998

M. Shafiur Rahman (ed.), Handbook of Food Preservation, Marcel Dekker, New York, 1999

P. Zeuthen and L. Bogh­Sorensen, Food Preservation Techniques, CRC Press, Boca Raton, 2003

Additional Readings

A. Ait­Ouazzou et al., The antimicrobial activity of hydrophobic essential oil constituents acting alone or in combined processes of food preservation, Innovat. Food Sci. Emerg. Tech., 12(3):320–329, 2011 DOI: 10.1016/j.ifset.2011.04.004 (http://dx.doi.org/10.1016/j.ifset.2011.04.004)

http://www.accessscience.com/content/food­preservation/267200 5/6 9/21/2016 Food preservation ­ AccessScience from McGraw­Hill Education R. Bhat, A. K. Alias, and G. Paliyath (eds.), Progress in Food Preservation, Wiley­Blackwell, Chichester, West Sussex, UK, 2012

A. W. Duea, The Complete Guide to Food Preservation: Step­by­Step Instructions on How to Freeze, Dry, Can, and Preserve Food, Atlantic Publishing Company, Ocala, FL, 2011

G. Oms­Oliu, O. Martín­Belloso, and R. Soliva­Fortuny, Pulsed light treatments for food preservation: A review, Food Bioprocess Tech., 3(1):13–23, 2010 DOI: 10.1007/s11947­008­0147­x (http://dx.doi.org/10.1007/s11947­008­0147­x)

M. M. Theron and J. F. R. Lues, Organic Acids and Food Preservation, CRC Press, Boca Raton, FL, 2011

National Center for Home Food Preservation (http://nchfp.uga.edu/)

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