Chemical Effects during Storage of Frozen Foods W. D. Powrie Department of Food Science, University of British Columbia, Vancouver, B.C., Canada Foods containing more than 26%water may deteriorate a t ambient temperatures hecause of the microbial breakdown of organic constituents. For microorganisms to grow, available water in food is essential to hydrate cell walls and membranes, t o ensure normal cell metabolism, and for transport of nutrients and metabolic by-products. The removal of available water from food can contribute to the reduction of microbial growth. High concentrations of hydrophilic compounds such as inorganic ions and organic polyols can restrict growth of organisms through the water-withdrawing effect, restricted nutrient transfer to cells, and intracellular solute effects ( I , 2). The term "water activitv" has been introduced to exnress the water-binding capacity i f foods (I, 2). Water activit;(a,) is defined as the ratio of the water vaoor nressure for a food to that of pure water a t the same temperaiure. Differences in the degree of microbial resistance to environmental aqueous solutions with high soluble solids has led to a classification of organisms as osmosensitive and osmotolerant. Microbial growth ceases below a water activity of 0.6 (2,3). When f w d is frozen. the available water is reduced through ice crystal formation, and the water activity of the unfrozen aaueous phase drops because of an increase in concentration ofhydrophilic sol"tes. The solute concentration in the unfrozen water ohase in food frozen at about -10°C is sufficiently high to impede growth of all typesof microorganisms (3.4). At -lO°C. the ice contents of skim milk, beef, strawberries, and peas are about 93,83,83, and 81%,respectively, of the total moisture content. The early settlers took advantage of the subzero winter temperatures in the northern part of North America to presew; meat. Around 1870, podtry and fish were frozen commercialhf by using a mixrun? of ire and salt for freezing and frozen storage. when ammonia refrigeration equipment appeared about 1880, freezing of food became more attractive as a commercial preservation technique. Rapid growth of the U.S. frozen food industry occurred after World War 11. Now, American consumers purchase about 90 lb of frozen food per capita each year. For the most oart. . . fruits. veeetables. meat. milk. and eees in the fresh state are consibered to haw high consumer acceotabilitv. The ohiertive in commerrial freezine and frozen &rage of?& is to-retain as much as possible of ;he desirable aualities of the fresh commodities (3-5). T o achieve this goal. conditions must be exercised to minimize undesirable phGica1 and chemical chanees in frozen stored ~roducts. With frozen storage temperatures of ahout -20% (used in the food distribution sector). chemical chanees oroceed a t reaction rate levels governed by the concentrati& okreactants, pH, oxygen diffusion into the unfrozen water phase, and product temperature (4,6). The types of chemical changes that occur in frozen foods during storage include protein insolubilization, lipid hydrolysis and oxidation, natural pigment dearadation, vitamin deterioration, and brown pigment for.. mition (4,5,7). With some fluid food systems, extensive rheolo~calchanges may occur during frozen storage. Under certain conditions, frozen-stored thawed yolk attains a pasty character, whereas frozen-stored concentrated milk can have a gel-like consistency upon thawing (3,5,8). Proteins in these systems aggregate to form three-dimensional networks. Chemical changes in muscle foods during frozen storage lead to a reduction in water-holding capacity, decrease in 340
Journal of Chemical Education
tenderness, and rancid flavor development (4,9,10). The total soluble protein content decreases through aggregation of myosin and actomyosin in frozen-stored muscle. Free fatty acids, released from lipids during frozen storage, can interact with muscle proteins to decrease their water-holding capacity. Oxidation of unsaturated fatty acids in frozen muscle can bring about a rancid off-odor (11j. Textural chanees in fruits and veeetahles during freezing and frozen storage can he attributed% structural a k e r a t i o i of orotein membranes and cellulosic cell walls bv ice crvstal g ~ i w t hand an increase of solute concentration (4;12,13j. Off odors in frozen vegetables, either unhlanched or underblanched, become apparent as enzymic oxidation of lipids proceeds (7). Chlorophyll can he oxidized in the presence of oxidizing lipids. Unfrozen Food Systems
The specific arrangement of constituents in a food system is responsible for well-defined sensory, physical, and chemical properties. Any alteration in the arrangement of the components will lead to changes in the characteristics of the entire system. T o understand how the freezing process brings about changes in food systems, a thorough knowledge of the characteristics, interrelationships, and distribution of food constituents, including water, is needed. Food systems can he divided into two major groups: gross intact tissue systems and small-particle dispersion systems (14). Intact tissue systems have much more complex structures than dispersion systems. In the first place, the cell wall encircles a suspension of numerous compounds such as proteins, carbohydrates, pigments, enzymes, and salts as well as These particles such as lipid droplets and protein aggregates. . . . constituents are not distributed uniformly throughout the protodasm, hut are concentrated in specific cell areas. For kxamp~e,sugars and salts are present in the aqueous solution of the vacuole of some plant cells, and chlorophyll pigments reside in plastids near the cell wall (15). In muscle cells, protein fibrils are aligned parallel to the long axis of the cell and are surrounded by a protein solution called sarcoplasm (16). Simple food dispersions are classed as emulsions (oil droplets in water or water droplets in oil), sols (particles dispersed in water), and foams (gas bubbles distributed in water). The uniform distribution of particles, droplets, and gas buhbles in a stable dispersion is dependent on such factors as the size of particles, the viscosity of the continuous phase, and narticle hvdration. Water is the major component in unfrozen vegetables, milk, and e m oroducts (4). . . From a textural standnoint. . . water contributes to juiciness, tenderness, viscosity, and gel character of foods. For enzymic reactions to occur, the presence of water in food is essential. In plant and animal tissues as well as some food disnersions such as eels and foams. water is hound and immokized in such a f&hion as to prevent or retard exudation (17). Water that is bound strondv to polar groups of solutes isnot freezable. Water that is i&nobilized bv beine held in microcavities (membranes. muscle channels) or by Ging trapped in macro&vities (vacuoles in plant parenchyma cells) is freezahle. Upon freezing food, some of the immobilized water will he converted to ice, which may bring about cell wall damage resulting in drip upon thawing,
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membrane breakdown with the release of enzvmes. . and an increase in reactant concentration in the unfrozen phase. The water molecule is a polar compound with an HOH angle of about 104.5' (4,131. The diameter of liquid water molecules is about 3.3 A. A model of the water molecule is presented in Fig. 1. Such a small molecule can fit into the small spaces on the surfaces of polymers to produce hydrated compounds. The charge distribution of the water molecule is in the form of a tetrahedron with two positive and two negative corners. Thus. the water molecule can he regarded as a-dipole. Each of the four c h a r ~ e dareas of a water molecule can attract another water molrrule with the formation of a hydrogen hond (Fig. 2). One water mulrculr then can attract a maximum of four water molecules. Hvdroeen-bondine between water molecules " is responsible for the outstanding physical features of liquid water and ice. The hvdroeen . .. hond enerw .,. between two water molecules is uhout 4.5 kcill~molc..Thc second hydrogvn bond to a water molecule is sliehtlsereater than the first. the third stronger than the seconi, andUsoon. So far. no structural model of liauid water has been put forth to account for all of the knownbroperties of water (i8). Many researchers have attempted to construct a model on the basis of X-ray diffraction, neutron-scattering data, infrared and Raman spectra, as well as statistical thermodvnamic treatment. M& researchers agree that, during the melting of ice, free non-hydrogen-honded OH groups appear. At O0 and 60°C, about 10%and 20% respectively, of the total OH groups are in the free form (18). Frank and Wen (19) proposed the flickering-cluster model based on the concept of cooperative hydrogen bonding. When one hydrogen hond is formed, there is a tendency for several more to form and when one hydrogen hond breaks, a number of other honds are broken. The cooperativeness leads to the formation of extended regions of H bonds and formation of water clusters. The water clusters have a varietv of sizes and shapes and a lifetime of about lo-" s. The flicdering-cluster model is in aereement with the viscositv of liauid water and the activation energy of 5 kcal/mole, the hind energy for surface water molecules of clusters. Numerous theories on the structure of liquid water have been nuhlished since the mid 1940's. Manv of the theories can he grouped into two main classes:
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Hvdroeen hondine of water molecules with wlar erouos . on pro~ins,carhohydrkes,and lipids is important in food systems from the standpoint of structural stability, chemical reactivity, and water-holding capacity (4). The most important polar groups of food constituents are -OH, >NH, and -C=O (20). The binding of water molecules by hydrophilic, polar com~oundsthrough - hvdroaen - - hondina is known as hydration i 4 , 1 3 , l X ) .The hound waterofprvt& in solution ~ H d M W e. vrokin. has heen estimated to he lw~ween20 t * 50r A peptide linkage -
