Freezing

The rate of loss of color, flavor, structure, and nutrients in foods is a function of temperature; thus, lower storage temperatures prolong the useful life of foods. However, below 0°C, the free water in food forms ice crystals as a function of moisture content, solute composition, and storage temperature. Ice formation is both beneficial and detrimental. Benefits include strengthening of structures and removal of free moisture, which reduces water activity. Benefits, however, are often far outweighed by the deleterious effects of ice crystal formation, the partial dehydration of the tissue surrounding the ice crystal, and the freeze concentration of potential reactants. Ice crystals disrupt cell structures mechanically, and the increased concentration of cell electrolytes can result in the chemical denaturation of proteins.

The technology of food freezing emphasizes as short a passage of time through the temperature zone of maximum ice crystal formation as possible. The formation of as small an ice crystal as possible minimizes the mechanical disruption of cells and possibly reduces the effects of solute concentration damage. Rapid freezing can only be accom-

Table 6. Approved Uses of Food Irradiation in the United States

Food product

Purpose of irradiation

Dose (kGy)

Date approved®

Fresh fruits and vegetables

Dehydrated aromatic vegetable substances (herbs, seeds, spices, teas, vegetable seasonings)

Food

Enzyme preparations, dehydrated

Pork

Hawaiian papaya

Packaged poultry products, fresh or frozen, including ground, hand-boned and skinless products

Mechanically deboned poultry meat

Meat, meat by-products, and certain meat food products

Inhibition of growth and maturation Control of foodborne pathogens

Anthropod pest control Control of foodborne pathogens Control of Trichinella

Fruit flies

Control of foodborne pathogens

Control of foodborne pathogens Control of foodborne pathogens

4.5 (max) for refrigerated products 7.0 (max) for frozen products

April 1986 (FDA) April 1986 (FDA)

April 1986 (FDA) April 1986 (FDA)

January 1986 (USDA) July 1985 (FDA) February 1989 (USDA) September 1992 (USDA) May 1990 (FDA)

September 1992 (USDA) May 1990 (FDA)

Proposed by the USDA in February 1999

Approved by the FDA in December 1997

"Final rules published in the Federal Register by the Food and Drug Administration (FDA) or the United States Department of Agriculture (USDA) unless otherwise indicated.

plished by large temperature differences and high heat-transfer coefficients.

Many processed foods and certain animal products tolerate freezing and thawing because their structures can accommodate ice crystallization, movement of water, and the related changes in solute concentrations. Starches can be modified to form gels that accept several freezing and thawing cycles without breakdown. By contrast, most fruits and vegetables lose significant structural quality on freezing because their rigid cell structures fail to accommodate ice crystal formation. However, it is not possible to store foods at temperatures low enough to ensure complete conversion of all water to ice; as a result, commercial frozen food storage temperatures represent an economic balance between storage costs (time, energy, and capital investment) and projected shelf life.

The freezing process disrupts tissue structures and allows cell contents to become mixed so that undesirable enzyme reactions can take place at significant rates even at storage of — 18°C. These reactions can generate off-flavors, reduce nutrient concentrations, and cause major changes in the structure and appearance of foods. The amount of free liquid or drip found after a freeze-thaw cycle is a good indication of the structural damage.

Heat treatment (blanching) prior to freezing eliminates enzyme-mediated changes in color, flavor, and structure. Most deteriorative enzymes are inactivated by exposure to a temperature of 100°C for 1 to 5 min. The enzymatic oxidative deterioration of frozen fruits can be inhibited with sulfur dioxide, sucrose, and combinations of citric acid, sodium chloride, and ascorbic acid (preceded by vacuum removal of oxygen if heat is not used).

Most frozen foods have a useful storage life of one year at — 18°C; however, foods high in fat (eg, sausage products) may become rancid in two weeks. Frozen storage can result in moisture loss from the food through a freeze-drying process, because the heat-transfer surfaces used to maintain storage temperatures are at a lower temperature than the storage area. For this reason, frozen foods must be protected against drying by a moisture barrier. In addition, foods subject to oxidative deterioration must be protected from air.

Freezing-preservation equipment can be classified by method of heat transfer. Usually air is the heat-exchange medium for freezing foods. Foods are loaded on a belt or vibrating conveyor and passed through air flowing upward at up to 5 m/s at temperatures as low as — 40°C. The air is recycled through coils and fans located next to the conveyor and returned through the conveyor. Because the air has a partial water vapor pressure lower than the food, freeze drying can occur, and some of the water in the product is removed and deposited as ice on the heat-exchange coils. As a freezing medium, air has other drawbacks. The low gas-solid heat-transfer coefficient and the heat capacity of air require either low temperatures or high velocities to obtain needed high heat fluxes. Low air temperatures increase refrigeration costs and high air velocities generate additional fan heat loads. For these and other reasons, freezing by conduction or by liquid heat-transfer methods are less costly in terms of capital investment and energy.

Liquid heat-transfer media that are used for direct-immersion freezing include food-grade dichlorodifluoro-methane, nitrous oxide, and water solutions of various edible salts, sugars, alcohols, acid, and esters. Liquid heat-transfer agents offer a high-heat-transfer coefficient and reduced pumping costs, eliminate product desiccation, and allow operation at high low-side equipment temperatures. Drawbacks include possible changes in food flavor and costs of processing. Although dichlorodifluoromethane offers major operating advantages as compared with other heat-transfer media, cost and environmental concerns have reduced its potential usefulness as an ideal direct-immersion freezant. There is a need for a direct-immersion liquid freezant that is safe, low cost, thermodynamically efficient, and compatible with foods with respect to flavor, color, and odor.

Conduction freezing between chilled plates is a cost-effective method of heat removal provided the product can be assembled in a geometry compatible with the plate surfaces. Packages having semiinfinite slab geometry are loaded between stacks of platens through which refrigerant is circulated. Good heat transfer is maintained by maintaining a pressure on the stack of platens.

Other freezing methods use direct immersion in liquid nitrogen, exposure to solidified carbon dioxide, and immersions of packaged products in liquid freezants, for example, sodium or calcium chloride brines, methanol, or propylene glycol solutions.

The quality of frozen food is related to storage temperature; however, because constant storage temperatures are not always feasible, the shelf life is often determined by the highest temperature and total length of time that food is exposed to that temperature before use. Maintaining a —18°C storage environment from time of freezing until use continues to be a major technical problem facing the frozen-food industry.

REDUCTION OF WATER ACTIVITY (DEHYDRATION)

Microbes require a specific minimum level of water activity (aw), defined by the relative humidity (measured in equilibrium with the food) for growth and reproduction at a given temperature and substrate composition. Foods possess characteristic equilibrium relationships between water activity and moisture content at given temperatures, which are known as sorption isotherms. Preservation against microbial spoilage by dehydration requires a moisture content equal to a water activity below 0.65 (Fig. 1). Dehydration contrasts with food concentration where water is removed for reasons other than for effective reduction in water activity.

Living tissue, when dried to a water activity below 0.97, suffers irreversible disruption of metabolic processes. Deteriorative chemical reactions, enzyme-catalyzed or not, are generally a function of water activity and reactant concentration. Thus, nonenzymatic browning (Maillard reaction), oxidation, and internal rearrangements (eg, staling and protein cross-linking) can increase in rate as water activity is reduced because of the increase in concentration of reactants. Many reactions show a minimum rate in the range of aw = 0.4.

Prior to dehydration, foods are usually heat treated to inactivate enzyme systems. Those foods susceptible to rapid nonenzymatic browning resulting from high concentrations of reducing sugars are treated with sulfur dioxide, and products subject to oxidative rancidity can be treated with antioxidants and packaged to prevent exposure to oxygen. Low-temperature storage (5°C) reduces chemical deterioration. Blends of dehydrated products must be assembled from ingredients having the same water activity.

High-quality dried foods can be obtained only if the drying system is designed to match heat penetration with the rate of release of moisture from the food. Typically, continuous-belt dryers are staged to provide three or more zones into which the product is reapplied into progressively deeper beds. Each zone operates at a dry- and wet-bulb temperature, a through-flow (upflow and downflow through a bed of materials) air velocity, and a bed depth that optimize product quality, energy use, and production rate.

Liquids and pastes are commonly dried in spray, drum, or freeze dryers. In spray drying, product is atomized through a nozzle (rotary, pneumatic, or high pressure) into a drying chamber in a continuous operation with hot air entering at typically 200 to 300°C. Evaporative cooling provides rapid cooling of the inlet air and prevents heating of the product until in a dry, stable state. Drum drying is generally a much more severe heat treatment unless done under vacuum. Freeze drying preserves original product qualities (flavor, structure, nutrients, etc) to the greatest extent, but it is a slow and costly process and therefore finds application mostly with high-value products such as spices and coffee. Particulate foods can be dried in continuous conveyor systems, fluidized beds, freeze dryers, or batch tunnel systems. Fluidized beds use pneumatic flow to fluidize particulate product and have the advantage of optimizing drying surface area for heat and mass transfer per unit volume of dryer. Some fruits, salted animal products, nuts, berries, and many field crops are sun-dried. Other food processes that produce dehydration include osmotic dehydration (in which intact fruits and vegetables are exposed to hypertonic solutions), vacuum dehydration, microwave dehydration, extrusion cooking, pneumatic dehydration, foam-mat dehydration, and gun puffing.

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