STORAGE OF FOOD

STORAGE OF FOOD. Civilizations are built upon a stable and reliable source of food that is provided by a combination of current production, imports, and the preservation of seasonally abundant crops. Preagricultural, nomadic people followed herds of migrating animals or periodically visited traditional locations to slaughter animals and gather fruits, vegetables, and grains as they matured during the year. Locally cultivated crops became the predominant source of food once agriculture became established and farmers tilled specific plots of land. Food that was locally abundant for only a short period of time had to be stored against times of scarcity. A poor harvest or the appropriation of food by marauding brigands could produce local famine. However, the most common and recurring cause of famine resulted from the farmer's inability to store enough food to last from one harvest to another. Summer was often not an idyllic time of plenty for primitive agrarian societies, but a time when many went hungry. It is not surprising then that the fall harvest festivals were such joyous times, for they heralded the end of the seasonal famine and ushered in a time of plenty.

Primitive storage techniques were well developed in prehistoric times. Early storage methods included the selection and growth of naturally dormant crops, and the drying, parching, smoking, and salting of meats, fruits, and vegetables. Mature grains, nuts, roots, and tubers have a period after harvest when they do not sprout and can easily be stored with the simplest of technologies and protective structures. Many cultivars of temperate fruits and vegetables such as apples, pears, and cabbage were selected for their natural storability. More perishable foods (meat, fruits, and vegetables) were sliced into thin sections and dried and/or smoked, or pickled in brine. Some of these techniques, such as sun drying (e.g., raisins), smoking (e.g., ham, fish), and pickling (e.g., dill pickle, pickled pig's feet) are still used today. The storage of food became more sophisticated as the population increased and more concentrated as demand for fresh fruits and vegetables increased. Storage was also used to provide planting materials or propagules for the next season.

The growth of large urban centers and the establishment of large standing armies at the end of the eighteenth century provided an impetus for the development of better storage methods. Canning was developed in 1809 in response to a competition sponsored by Napoleon to provide a better supply of food for his armies. A tin-plated metal canister (from which the term "can" is derived) was filled and a lid hand-soldered in place after the can was heated in a water bath for a specified time. Canned food has a very long storage life, especially when stored under low temperatures. Food in century-old cans that were discovered at an arctic base was not spoiled, although it did contain viable bacteria and was unappetizing as a result of flavor and textural changes. Like canning, many storage technologies such as fermentation remained an unpredictable art until Pasteur identified microorganisms as the scientific cause of spoilage and decay in the 1860s.

With the Industrial Revolution, storage became increasingly important as the population increased dramatically and people moved into dense urban areas. As an affluent middle class developed, commodities that had once been luxuries available only to the nobility became widely available because of increasingly better storage and transportation technologies. For example, meat from Australia and Argentina appeared in European butcher shops, while tropical fruit such as bananas, citrus, and pineapples became available in grocery stores. Increased concern for a healthy diet and a better understanding of the link between diet and health prompted public demand for higher quality food.

Food quality encompasses a remarkable number of attributes. Although all food is made up of the same elementary constituents (carbon, hydrogen, oxygen, nitrogen, potassium, sulfur, etc.), the atoms it contains are arranged into molecules of diverse size, shape, and function. The idea that foods differed in molecular structure developed in the 1830s as food was found to contain the three major components of carbohydrates, proteins, and fats. Since then, more discovery of minerals, vitamins, and amino acids has increased to more than fifty, the essential nutrients found in food. Animals are incapable of many of the elementary syntheses performed by plants and therefore rely on them for many complex molecules such as vitamin C and the twenty-four essential amino acids. Many foods are consumed because of their pleasant aroma, taste, or texture, others because of their presumed medicinal properties. Storage technologies are devised and selected to preserve those specific characteristics that are most important to the consumer.

Limits to Storage Life

The storage life of food is usually limited by the loss of acceptable visual appearance, palatability, or food value, and these criteria of quality are often lost in the reverse order listed. Food provides both nutrition and quality to our diet. Often a commodity can be physically preserved longer than the traits for which it is being stored. For example, the flavor, texture, and nutrition of many fruits and vegetables are reduced before visual appearance of spoilage. Many storage technologies have been devised to retain an acceptable appearance or taste while ignoring changes in food value. This is understandable since

appearance and palatability can be easily assayed by examining and eating the commodity, while measuring food value requires sophisticated laboratory assays.

Food spoilage results from three main causes: chemical changes from ripening and senescence processes, growth of unwanted microorganisms such as bacteria and fungi, and insect and rodent pests. However, many of the processes that cause food spoilage in one commodity are often necessary for high quality in another. Ripening of bananas and tomatoes harvested at a mature but unripe stage is necessary, but excessive ripening leads to poor quality. Insects are needed for the pollination of many crops, but they can also eat the plant and spread disease. Microorganisms cause spoilage, but yeast ferment wine, beer, and sauerkraut, while bacteria are needed for production of pickles and yogurt.

Often more than one series of reactions affects storage life. For example, the major factor influencing the storage life of fresh meat is microbial growth and fat oxidation. Meat is normally marketed after rigor mortis and a period of aging. The most common preservation methods for fresh meat are cooling (three to six weeks at 32°F [0°C]) and freezing (nine to fifteen months at <4°F [<20°C]). Fish is far more perishable than meat because it is an excellent substrate for microbial growth. Fish lipids are largely unsaturated and therefore very susceptible to oxidation.

Chemical deterioration. Many chemical reactions contribute to the loss of storage life. The majority are enzymatically driven while others are chemical reactions that occur because of the close proximity of reactive molecules within food. Maillard browning involves color, flavor, and odor changes that result from a chemical reaction between proteins and carbohydrates. The rate of this spontaneous reaction is rapid at baking temperatures, as in the browning of bread, much slower at room temperature, as in the browning of applesauce, and very slow at refrigerator temperatures. Removal of water during dehydration concentrates the reactants and accelerates Maillard browning.

Hydrolysis is the splitting of molecules, usually polysaccharides such as complex sugars, starches, and pectins with the chemical addition of water. During the processing of some fruits, the hydrolysis of sucrose into its components of glucose and fructose greatly affects the sweetness of the product. Storage of potatoes at too cold a temperature (32°F; 0°C) promotes the hydrolysis of starch to sugar. Sugars accumulating during this "sweetening" process turn dark brown when heated (i.e., Maillard browning), making the potatoes unsuited for the production of potato chips and French fries. Modification of amylopectins by hydrolysis contributes to textural changes and the formation of gels.

Oxidative rancidity is a chemical change in the unsaturated bonds of a fat or oil that produces chemicals giving food off-odors and off-flavors. Exclusion of oxygen and light combined with the addition of antioxidants retards rancidity. Vitamins C and E, which are antioxidants, can be destroyed by oxidation. Vitamin C, or ascorbic acid, is used as a reducing agent to prevent oxidative browning of cut fruits and vegetables. Both enzymatic and nonenzymatic reactions contribute to lipid oxidation. Blanching destroys enzymes responsible for rancidity reactions in fruits and vegetables. One of the functions of packaging is to exclude oxygen and light from processed foods. Cooked meats can become rancid within a day, but proper packaging or freezing can delay the process by several months. However, not all oxidation is detrimental; oxidative bleaching of pigments in flour during storage results in whiter flour.

Microbial contamination. Bacteria and fungus are everywhere in our environment, and most foods provide an excellent substrate for their growth. Packaging, whether natural (banana peel, seed coat, or egg shell) or artificial (glass bottle, metal can, or foil pouch) protects the enclosed food from microbial contamination. Some foods contain natural antimicrobial chemicals (for example, the tannins in unripe fruit), while other antimicrobial compounds, such as the fungicide in wax coatings, can be applied to food. Many foods are sterilized, pasteurized, or fumigated before packaging and storage to control microbes. Storage conditions of low temperature and humidity retard microbial growth. However, once these protective barriers are breached, microbial growth is often unchecked and rapidly destroys the commodity.

Food can be a vector and provide a growth medium for many pathogenic microbes (e.g., Campylobacter jejuni, Escherichia coli, Listeria monocytogenes, Salmonella, Shigella, Vibrio cholerae ). However, common spoilage organisms (e.g., Botrytis, Colletotrichum, Erwinia, Fusarium, Penicillium, Rhizopus ) are not human pathogens, although some bacteria (e.g., Clostridium botulinum, Staphylococcus aureus ) and fungi (e.g., Aspergillus flavus ) can produce potent toxins.

Insect and rodent pests. The major insect pests in stored food are moth (Lepidoptera) larva and beetle (Coleoptera) larva and adults. Insects have been controlled in stored food using physical methods for thousands of years. Neolithic farmers in the Nile Delta kept seeds cool and dry by storing them in clay jars buried in the ground. Exposure to high temperatures has been used since the sixteenth century to control insects in stored grain. Under many circumstances the easiest, most rapid, and most economical method of controlling insects is with insecticides, often in the form of fumigants. Fumigation has been the method of choice since the late 1940s, but the unwanted side effects of some of the chemicals used (for example, toxicity to other species, depletion of stratospheric ozone, acquired resistance by the pest) have promoted interest in physical control measures. Many physical conditions such as temperature, relative humidity, moisture content, and atmospheric composition can be manipulated to affect insect survival. The structure containing the commodity (granaries, elevators, bags, and packaging), forces on the commodity (compression and impaction), and irradiation are also used in controlling insect infestation.

In general, insect survival in stored food is dependent on temperature, relative humidity, and gas composition. Temperatures lower than 55°F (13°C) or higher than 95°F (35°C) dramatically reduce survival. Sublethal temperatures also have effects; for example, at 68°F (20°C), most insects survive but stop feeding. Additional reductions in survival occur when stressful temperatures are combined with reduced moisture or oxygen levels. Drying affects insect population directly by reducing survival and indirectly by causing cracks that increase the susceptibility of the commodity to insect attack. The respiration by large numbers of insects produces heat and moisture, favoring both mold and further insect growth.

Physical movement of grain increases insect mortality. Moving grain by auger or pneumatic conveyer increases mortality of larva and adults. Impacts from dropping grain six meters resulted in up to 90 percent mortality. Moving can directly kill or injure insects by crushing them, or it can prevent feeding and mating by disrupting the microecological niches required by some insects. However, handling can also increase the amount of cracked seeds and grain dust, which are the preferred diet of some insects.

Biological control with insect pathogens, predators, or parasites is an important component of integrated pest management strategies. Unlike other treatments, biological control agents can reproduce, so one inoculation may be sufficient to establish lasting control. In contrast, many physical treatments such as cold, heat, fumigation, and irradiation have no lasting effect, and reinfestation must be controlled by proper storage and packaging.

Over the centuries, feral rodents have adapted to living in or near houses and farms. The most common rodents are the brown or Norway rat (Rattus norvegicus ), the black rat (Rattus rattus ), and the house mouse (Mus musculus ). Food storage facilities provide ideal conditions for rodents to multiply very rapidly with food, shelter, and a lack of predators. Common signs of a rodent problem are fecal droppings, tail and footprints in dust, droppings and urine stains in feeding areas, gnawing marks on wood, plastic, metal, pipes, and food containers, spilled food, smear marks from rodents' fur (distinctive dirty marks, particularly along skirting boards and around doors), rat holes, nesting sites, rat runs, and the rodents themselves. Constant gnawing is necessary to keep rats' teeth ground down; their gnawing of electrical wires has caused structural fires.

Rodents eat a lot of food, particularly in grain stores, but far more food has to be discarded because it has been contaminated with hairs and droppings. Apart from salmonella food poisoning, rats may carry as many as thirty-five diseases including rabies, plague, typhus, leptospirosis, rat bite fever, and hantavirus. Disease may be spread by eating food contaminated by rodent droppings or urine, contact with rat urine, parasites that live on rodents, and rodent bites.

There are biological factors, behavioral factors, and environmental factors to be considered in designing a rodent control program. Rats normally range no more than 150 feet from the nest; a male mouse will control an area of ten to twenty feet from the nest. Rats will migrate on their own; mice are often carried in boxes or crates into new locations. Rodents begin reproducing at a very young age, have large litters, and breed year around. A 90 percent population reduction may be replaced in as little as nine months. Rodents have a well-developed sense of taste and can remember foods that made them sick in the past. They are very cautious and wary of new objects introduced into their surroundings (such as bait boxes or traps) and of changes in the surroundings. Rodents can be controlled by using poisons and traps. Positioning of control measures can be optimized by using tracking powder to locate runs and nesting areas.

Storage Technologies

Storage technologies manipulate the extrinsic factors of temperature, water activity, and oxygen availability to control the rate of quality loss by stored commodities. Examples of these treatments include drying, salting, brining, fermentation, canning, cooling, freezing, altering storage gases, and ionizing radiation.

Temperature. Temperature is the key factor influencing the storage life and safety of fresh and processed food. Living organisms are best adapted to a narrow range of temperature. As the temperature diverges from this optimum range, the organism is first retarded in growth and then killed (Table 2). Most of the physiological changes that shorten the storage life of fruits and vegetables, as well as many of the reactions in meat, eggs, and dairy products, are enzymatic. Other reactions are purely chemical. The effect of temperature is usually much greater for enzymatically driven reactions than for purely chemical reactions. Each 50°F (10°C) drop in temperature

halves the rate of most chemical reactions (i.e., a Q₁₀ or temperature quotient of 2). In contrast, biological reactions often have Q₁₀ values between 3 and 5, meaning that the reaction is decreased to a third or fifth by an 18°F (10°C) fall in temperature.

Thermal processing. Food is not stored at elevated temperatures, but high temperatures such as those involved in canning and pasteurization are often used to arrest microbial growth, denature enzymes, and alter the structure of food prior to storage. Temperatures above 140°F (60°C) inhibit and kill growing microorganisms. Resting stages require much higher temperatures.

Canning is the heating of prepared food in hermetically sealed metal, glass, or foil containers to a specific temperature for a specified time to destroy diseasecausing microorganisms, denature enzymes, and prevent spoilage. Low-acid foods such as meats and vegetables are heated to 240–265°F (116–129°C), while acidic foods, such as fruits and tomatoes, are heated to about 212°F (100°C). The length of heating depends on the type and size of the container, the food being canned, and the method of heating. Flat foil packets may require less than ten minutes, while large metal containers may require over forty minutes. The aseptic container keeps out oxygen and contaminating microorganisms. Canned food is very stable and can be stored for many months at ambient temperatures; however, storage life is extended when the food is stored at low temperatures. Once opened, canned food is prone to spoil rapidly and should be stored under refrigeration.

Pasteurization is the heating of a liquid such as milk, wine, or beer to between 131 and 158°F (55 and 70°C), to destroy harmful bacteria without substantially altering the liquid's composition, flavor, or nutritive value. In addition to destroying potential disease-causing bacteria, pasteurization can greatly increase the storage life of milk by inactivating enzymes that reduce quality. There are two basic methods of pasteurization, batch or continuous. In the batch method a vat of liquid is heated to 145°F (63°C) for thirty minutes, rapidly cooled, and stored below 50°F (10°C). This method is used for milk and its by-products (e.g. creams, chocolate). Beer and wine are pasteurized at about 140°F (60°C) for twenty minutes. Water can also be pasteurized at 149°F (65°C) for six minutes, or to a higher temperature for a shorter time, to kill germs, viruses, and parasites. In contrast, the most common continuous processing method uses high temperature, short time (HTST) pasteurization. Milk is heated to 161°F (72°C) for fifteen seconds, while beer and wine are heated to 158°F (70°C) for about thirty seconds, and bottled under sterile conditions. The continuous process has several advantages over the vat method, the most important being time and energy saving. Radiation pasteurization of foods uses low doses of gamma rays, X-rays, and electrons to control foodborne pathogens on beef, pork, lamb, and fish.

Refrigerated storage. Cold storage. Most food stores best at temperatures near 32°F (0°C) because chemical and biological processes are slowed down. Maximum storage life for meat, eggs, dairy products, and all processed food is at 32°F (0°C). Most fresh fruits and vegetables are also best stored at 32°F (0°C). They include apples, berries, broccoli, cabbage, carrot, corn, grapes, lettuce, and pears. However, some fruits and vegetables are sensitive to low temperatures and are damaged if stored below 50°F (10°C). These chilling-sensitive commodities include asparagus, avocado, banana, beans, cucumber, eggplant, grapefruit, melons, peppers, potatoes, squash, sweet potato, tomato, and watermelon.

Best product quality is maintained under constant temperatures. Typically the storage temperature should vary no more than ±1.8°F (1°C) from the desired temperature, but even this variability may be excessive if it allows the temperature to fall below the freezing point of the commodity. Large swings in temperature can result in unwanted freezing or chilling injury, condensation of water on the product that promotes microbial growth, accelerated water loss, and reduced storage life. All fresh fruits and vegetables lose water in storage and must be properly packaged and/or stored under high relative humidity to prevent excessive water and weight loss.

Freezing. Living cells are mainly dilute aqueous solutions of salts, sugars, organic acids, proteins, and lipids. These solutes lower the freezing point of pure water from 32°F (0°C) to 31.6°F (<0.2°C) for lettuce, 30.6°F (<0.8°C) for bananas, 29.5°F (<1.4°C) for carrots and sweet potatoes, 26.6°F (<3°C) for beef, fish and poultry, and 17.6°F (<8°C) for peanuts. Most frozen foods can be kept at 0°F (<18°C) for a year with little loss of quality. Both free and bound water exist in foods. Bound water exists in combination within an insoluble matrix such as cellulosic cell walls and muscle protein. As the temperature falls, ice crystals first form where the water is the purest and therefore has the highest freezing point. Growing ice crystals remove pure water from the solution, and it becomes more concentrated. Slow freezing produces large ice crystals and highly concentrated solutions, both of which are detrimental to quality retention. Rapid freezing retains quality by either producing small ice crystals or a vitrified solid. The differential effects of freezing on bound and free water give rise to the complex freezing pattern of foods.

Water activity and humidity. Drying is perhaps the oldest method used to store food. Drying fish, meat, fruits, and vegetables in the sun or over a fire or by ventilation with heated air is still used to prolong storage life. Both the harvested food and the spoilage organism (microbe or insect) need water to live. Removal of water by drying, brining, salting, cooling, or freezing prevents the growth of microorganisms and insects, deactivates enzymatic pathways, and reduces the many chemical reactions that accompany a reduction in quality during storage.

Different foods can have different equilibrium moisture contents under the same relative humidity and different microbial stability under the same moisture content. The idea of water activity (a_w) was introduced to better understand how water availability in foods affects microbes. The water activity (a_w) of pure water is 1.00. The a_w of an aqueous solution is calculated by dividing the vapor pressure of solution by the vapor pressure of water at a given temperature. Under steady state conditions, water activity in food can be approximated by dividing the relative humidity of the ambient air by 100.

Few spoilage and pathogenic bacteria can grow below a_w's of 0.90, while levels below 0.70 prevent the growth of most yeasts and molds. An a_w that influences microbial growth is usually lethal to grain, fruits, and vegetables. Food quality can still deteriorate at a_w's below 0.60 from the enzymatic and nonenzymatic oxidation of lipids, vitamins, and pigments.

For most fresh fruits and vegetables, a relative humidity of 90 to 95 percent is recommended during storage. A relative humidity close to 100 percent or the condensation of water on a commodity that frequently accompanies such high humidity may cause cracking of the skin. Surface condensation may also accelerate the growth and spread of microorganisms. Large evaporator surfaces will improve the relative humidity in direct expansion refrigeration systems. A 41 to 50°F (5 to 10°C) temperature split, as commonly designed, will maintain 70 percent to 80 percent relative humidity; a 32.9°F (0.5°C) split would be required to maintain 95 percent relative humidity. In practice, supplementary humidification with fogging nozzles, spinning disc humidifiers, or steam humidifiers is often used to maintain high humidity in the storage of fruits and vegetables.

Dry cereal grains have an extremely low respiration rate and show only slight reduction in total sugars and quality even after years at a cool temperature and low oxygen concentration. In general, grain in equilibrium with 30 percent and 50 percent relative humidity air has a moisture content of about 8 percent and 12 percent, respectively. Grain respiration is particularly affected by moisture, increasing as the water content of the seed rises above 14 percent. Respiration produces heat and water, both of which contribute to increased respiration and an accelerated loss of quality. Warm temperatures and higher humidity also facilitate the growth of insects and molds. The moisture requirement for the growth and reproduction of different insect species in grains varies from 8 percent to 12 percent.

Controlling the moisture content is extremely important in preventing spoilage. Grains (e.g., wheat, rye, oats, barley, soybeans, corn, and sorghum) must be dried to around 12 percent moisture for safe storage during warm weather. Most grains are dry enough to store when harvested. Artificial drying of grains that are harvested before they are dry enough can be accomplished in a few hours with heated forced air. Longer times are required using dry (<70 percent relative humidity) ambient air. Rice storages must have driers since rice is customarily harvested before it is dry enough for storage. Temperature differences and air movement within grain stored at the proper moisture level can redistribute moisture and cause local increases conductive to spoilage. Respiration of the grain and associated pests (e.g., mold and insects) produces water that promotes their growth. Periodic ventilation and turning can remove or redistribute moisture to maintain proper storage conditions.

Low oxygen and/or high carbon dioxide. Researchers in the 1920s showed that altered concentrations of oxygen and carbon dioxide in the storage atmosphere retarded the germination and growth of microorganisms on fruits, vegetables, and meat. Low oxygen and moderately elevated carbon dioxide atmospheres retard respiration and the synthesis and action of the plant hormone ethylene and extend the storage life of fruits and vegetables. High carbon dioxide levels are more effective in prolonging the storage of meat.

Modified atmospheres have many advantages for the storage of both living and processed food, but the long treatment period may be incompatible with the marketing system. If crops sensitive to ethylene are to be stored together with crops that produce large amounts of ethylene, then either periodic or continuous venting may be necessary to prevent deleterious concentrations of ethylene from accumulating.

Fermentation. Oxygen is necessary for respiratory metabolism. Exclusion of oxygen causes a shift from aerobic to anaerobic metabolism and the production of many fermentative products such as ethanol and lactic acid. Fermentation is an ancient, low technology method of preserving food. When Ch'in Shih Huang Ti was constructing the Great Wall of China in the third century b.c.e., the laborers were given mixed fermented vegetables as part of their rations. Fermentation is a complicated anaerobic process in which naturally present bacteria hydrolyze sugars to organic acids (e.g., acetic acid) or ethanol. Salt may be added to the initial mix to draw water from the vegetables and encourage brine formation. Brine promotes the growth of desirable bacteria; acid (vinegar or acetic acid) may be added to rapidly lower the pH and discourage the growth of undesirable bacteria.

Ionizing radiation. Ionizing radiation from gamma rays or electron beams is a cost-effective method of preparing food for storage. Low-dose irradiation controls insect infestation of grain and flour (0.2 to 0.7 kGy), foodborne parasites (1 kGy), and inactivates non-spore forming pathogenic bacteria (3 to 5 kGy). Higher dosages (20 to 50 kGy) are required to inactivate enzymes, kill spores and sterilize products. Levels of 0.2 to 1.0 kGy are not immediately lethal, and irradiated insects can survive for several weeks, but they feed less and are usually infertile. Higher dosages cause immediate mortality, but they can also reduce vitamin content and alter textural properties of grains. There is no residual effect of irradiation, so reinfestation must be controlled by appropriate handling and packaging. Low doses inhibit sprouting of potatoes and onions, but higher doses stimulate wound responses in fresh fruits and vegetables that lead to reduced quality. Spices and vegetable seasoning are the products most commonly irradiated. Aseptically packaged irradiated meat can be stored without refrigeration for many months. The U.S. military has used irradiated food for a number of years. Consumers view irradiated food either as being adulterated or as being additionally protected against spoilage.

Sanitation, fumigation, and quarantine treatments. Since the leading cause of food spoilage is pests such as insects, fungi, and bacteria, storage life can be extended by removing these organisms. Using acceptable cultural practices during production of the raw material and sanitation practices during harvest and preparation for storage significantly decreases the initial number of pests present in stored food. Since many pests increase rapidly under favorable storage conditions, a significant reduction in their initial number can greatly extend the storage life of a commodity before expensive control measures are needed. Once stored, there are only a few techniques available to eliminate pests already present in raw and processed food.

Some of the most important control measures are fumigation, heat treatments, and controlled atmospheres. The anaerobic storage of grains in airtight containers can reduce the need for chemical fumigation to control insects and mold and does not require the technical sophistication required for the safe application of toxic fumigants. The hermetic storage of living tissue such as grains, fruits or vegetables is a form of modified atmosphere storage that has been used for centuries. The respiration of the commodity creates an atmosphere rich in carbon dioxide and deficient in oxygen. Some of these fast-acting treatments can be used as quarantine treatments. Many production areas throughout the world contain pests that importing countries do not want introduced into their production areas. Increasing international trade in staples and exotic foods and the elimination of effective, commonly used fumigants (e.g., methyl bromide) are promoting the development of new quarantine treatments that are not only environmentally safe, but also fast-acting and effective on a wide range of commodities.

Methods of Storage

Food can be stored alive (apples, cows, tomatoes, wheat), or dead (applesauce, beefsteak, spaghetti sauce, flour). Storage technologies must produce drastically different environments to effectively store these many kinds of food. Cereal grains store best at a relative humidity below 50 percent to reduce respiration, while fresh fruit and vegetables need humidity above 90 percent to prevent excessive weight loss. Some ripening fruits (bananas and tomatoes) are injured if stored below 10°C (50°F), while others (apples and pears) store best at 0°C (32°F). Carbon dioxide levels above 50 percent reduce the spoilage of meat, and exclusion of oxygen prevents rancidity of potato chips and nuts, but most fruits and vegetables will not tolerate oxygen levels below 2 percent or carbon dioxide above 3 percent. Although the word "storage" may conjure up images of large grain elevators, warehouses, and sophisticated refrigeration technology, there still exists a range of less sophisticated techniques that are widely used today.

Storage before slaughter or harvest. Animals can be slaughtered as necessary where the climate is mild or protective structures exist, and where animal feed is available or can be stored. However, animal products (eggs and milk) may need to be collected periodically to maintain production.

For most perishable food, maturity occurs over a short period of time, and harvest must take place during that period. However, a range of crops can be left on the plant (or in the ground) for up to a few months. The storage of potatoes and other root crops in the ground and the storage of citrus and avocados on the tree are good examples of this. Storage on the plant overcomes the need for capital investment in storage facilities and delays exposure of the commodity to water and nutritional loss encountered during storage. It also reduces the damage and disease inevitably associated with harvesting, handling, and storage. Problems include exposure of the commodities to environmental disasters (hail, frost, diseases, and sunburn), the cost of keeping the land idle (for field crops) or of reducing the following seasons' yield (for tree crops).

Storage after slaughter or harvest. Food can be stored after slaughter or harvest as fresh, still-living commodities (fresh fruits and vegetables), or as a processed, dead product (cured meat, canned tomatoes, etc.). Low humidity and temperature are two traditional means of facilitating the storage of naturally dormant crops such as grains (wheat, corn, and rice), bulbs (onions and garlic) and, roots and tubers (carrots, potatoes, and yams). The rates of the many chemical reactions that reduce quality are naturally suppressed in these crops. Using chemicals or treatments that interfere with those physiological processes that naturally terminate dormancy can lengthen the duration of storage.

Common (unrefrigerated) storage. One of the oldest methods of storage of perishables is the use of "clamps," which are basically piles of a commodity laid in the field and then covered with straw and soil to insulate and waterproof the clamp. This method is still in use for the storage of potatoes, turnips, parsnips, celeriac, rutabagas ("Swedes") and other "hard" vegetables. In large clamps, ventilation with chimneys and perforated channels through the stack are needed to remove the heat of respiration, and reduce the risk of anaerobiosis or CO₂ toxicity.

A step in sophistication above field clamps is storage in rooms dug into the ground, such as root cellars, and natural or man-made caves, and buildings that are often heavily insulated. Temperatures in caves and underground (1 m) are fairly stable at 10 to 15°C (50 to 59°F) throughout the year. They have the advantage of low maintenance costs, stable temperatures, and high relative humidity. These unrefrigerated stores are best suited for long-lived commodities like potatoes, onions, and winter squash. But these structures are only effective in climates where the ambient temperatures during the storage period are low enough to maintain product quality for a reasonable length of time. Old-fashioned storage of apples, cabbage, and pumpkins in root cellars is a good example of this type of storage. This technique is still used for potatoes, particularly those intended as propagation material.

Grain storage facilities come in a number of sizes and shapes: rectangular wood-framed bins; cylindrical prefabricated metal structures; flat, ground-level piles on concrete floors in warehouse-type buildings; and overhead bins above driveways. One of the most common storage and processing facilities is the granary or grain elevator; a tall (c. 30 meters tall) cylindrical building equipped with machinery for unloading, weighing, cleaning, mixing, storing, and loading grain. Livestock farms may have a number of small granaries to store feed grain at locations adjacent to feedlots. Every town in a grain producing area has an elevator to accumulate grain from farmers and store it for a short period of time while it is cleaned, conditioned, dried, and graded. Larger elevators are located at terminal grain markets and shipping centers where the grain is stored for use by millers or to await shipment. Grain arriving by truck, railroad car, or ship is unloaded and moved by auger, or belt, bucket, or pneumatic conveyer to an area above the cylindrical storage bins where it is cleaned and weighed. A number of bins can be built side-by-side to form facilities that can store millions of bushels of grain. Governments in some countries subsidize the long-term storage of grain to stabilize prices and protect against famine. In contrast to a granary or elevator that stores grain, a silo is any structure or container used for the storage of large masses of high-moisture forage or silage for animal feed.

Once processed, food is more physically stable than the original living material, but it is also a better source of nutrients to microbes. Processed food can be preserved for extended periods by a combination of aseptic packaging such as canning or bottling to exclude microbes and oxygen and moderate to low temperatures.

Night air storage. In warmer climates, a modification of common storage can be used if there is a substantial difference between day and night temperatures during the storage season. The technique used is termed "night air ventilation." The produce is placed in a common storage room, which is well-insulated and supplied with a ventilation system to enable air to be drawn into it and distributed through the produce during the coolest part of the night. This technique can be used to remove field heat and cool produce before refrigerated storage. It can also be used to maintain produce at the proper storage temperature when the nights are cold. In modern stores of this type, microprocessor technology can be used to regulate the ventilation system to optimize the storage environment.

Refrigerated storage. By far the largest proportions of fresh commodities are stored in insulated rooms provided with mechanical refrigeration. At low temperatures, the biological activity (and deterioration) of the product is dramatically slowed, and the growth rate (and sometimes the viability) of microorganisms is reduced, as is the rate of water loss.

Several methods are employed to provide cold storage in the absence of mechanical refrigeration. In some areas, cold well or lake water may be used. The air of the store may be cooled to near the water temperature with a good heat-exchange system. In cold climates, ice harvested from lakes and ponds during winter can be stored and used for summer refrigeration.

In areas where the relative humidity is low, an evaporative cooler may reduce the temperature of the air sufficiently to be an economical source of refrigeration. In the western United States, for example, the mean wet bulb temperature in growing areas ranges from 8 to 21°C (46 to 79°F) during the harvest season. A well-designed evaporative cooling system will provide cooling air at one or two degrees above the wet bulb temperature. Chilling-sensitive commodities can be cooled very satisfactorily by this technique. An additional advantage is that the high relative humidity produced during cooling significantly reduces water loss from the commodity.

Mechanical refrigeration. Without a doubt, the invention of mechanical refrigeration in the 1850s and its commercial application starting around 1875 was crucial to the modern storage of perishable food. Mechanical refrigeration relies on the basic principle that substantial amounts of energy (i.e., heat) are absorbed during the vaporization of a liquid and released during the condensation of a gas. Using a mechanical pump, a gaseous refrigerant is compressed, cooled, liquefied, and stored in the receiver. When cooling is needed, the liquid refrigerant is allowed to enter the evaporator where it evaporates into a gas as it absorbs heat from the cold storage enclosure. Fans blow air warmed by the commodity over the cold evaporator coils where heat is transferred from the air to the refrigerant. Repeated cycles of condensation and evaporation of the refrigerant "pumps" heat from the cold storage enclosure. Ammonia was the first and most commonly used refrigerant for many years. The development of Freons has produced refrigerants that are less toxic to plants and humans but that are being reassessed for their environmental impact.

There are a number of different techniques for using mechanical refrigeration in the cold storage of food. In the direct expansion method, the evaporator coils are in the cooled space. This system is simple, but it results in low relative humidity. Special and expensive modifications are necessary to provide high humidity for the storage of fresh fruits and vegetables. However, low relative humidity is beneficial for the storage of some commodities such as grains and onionsand for canned or packaged processed food.

See also Cereal Grains and Pseudo-Cereals ; Frozen Food ; Preparation of Food ; Seeds, Storage of .

BIBLIOGRAPHY

Abeles, Frederick B., Page W. Morgan, and Mikal E. Saltveit. Ethylene in Plant Biology. 2nd ed. San Diego: Academic Press, 1992.

Calderon, Moshe, and Rivka Barkai-Golan, eds. Food Preservation by Modified Atmospheres. Boca Raton, Fla.: CRC Press,1990.

Council for Agricultural Science and Technology. Foodborne Pathogens: Risks and Consequences. Report No. 122. Ames, Iowa: Council for Agricultural Science and Technology, 1995.

Diehl, Johannes F. Safety of Irradiated Foods. 2nd ed. New York: M. Dekker, 1995.

Fields, P. G., and Muir, W. E. "Physical Control." In Integrated Management of Insects in Stored Products, edited by Bhadrivaju Subramanyam and David W. Hagstrum, pp. 195–221. New York: M. Dekker, 1996.

Gould, G. W., ed. New Methods for Food Preservation. New York: Chapman and Hall, 1995.

Hardenburg, Robert E., Alley E. Watada, and ChienYi Wang. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. Agriculture Handbook No. 66 (revised). Washington, D.C.: U.S. Department of Agriculture, Agricultural Research Service, 1986.

Kilcast, D., and Persis Subramaniam, eds. The Stability and Shelflife of Food. Cambridge, U.K.: Woodhead, 2000.

Ooraikul, B., and M. E. Stiles, eds. Modified Atmosphere Packaging of Food. New York: Ellis Horwood, 1991.

Saltveit, Mikal E. "Discovery of Chilling Injury." In Discoveries in Plant Biology, edited by Shain-Dow Kung and Shan-Fa Yang, vol. 3, pp. 423–448. Singapore: World Scientific Publishing, 2000.

Saltveit, Mikal E., ed. Physiological Basis of Postharvest Technologies. International Society for Horticultural Science Acta Horticulturae 343 (1993).

Taub, Irwin A., and Singh, R. Paul Food Storage Stability. Boca Raton, Fla.: CRC Press, 1998.

VanGarde, Shirley J., and Margy Woodburn. Food Preservation and Safety: Principles and Practice. Ames: Iowa State University Press, 1994.

Mikal E. Saltveit

Methods Used to Store Food

Natural methods

Harvest as needed (field storage)
Slaughter animals as needed
Keep plants in the ground (carrots, cassava)
Keep fruit on the tree (avocados, citrus)

Harvest and keep alive
Selection of naturally dormant plants
(grains, nuts, bulbs, tubers)
Store in field or natural
(clamps, curing, root cellars)

Technological methods