Food Factors Influencing Consistency

monoacylglycerol-water systems by Larson [94, 95], who observed the existence of closed water regions in these systems and proposed a cubic-phase model based on space-filling polyhedra packed in a body-centered cubic lattice (Fig. 16d). Cubic phases are usually very viscous and completely transparent. Mesomorphic states are of great importance in many physiological processes; for example, they influence the permeability of cell membranes [148]. Liquid crystals also play a significant role with regard to the stability of emulsions. 5.4.3.7 Factors Influencing Consistency [8] Several factors have important influences on the consistency of commercial fats: Proportion of solids in the fat. In general, the greater the solids content, the firmer the fat. It has been estimated that plastic commercial fats, at workable temperatures, increase in either firmness or viscosity by about 10% with each increment of crystals [8]. Number, size, and kind of crystals. At a given solids content, a large number of small crystals produces a harder fat than does a small number of large crystals. Larger, soft crystals are typically produced by slow cooling. Crystals composed of highmelting acylglycerols provide greater stiffening power than do those of lower melting acylglycerols. Viscosity of the liquid. Oils differ in viscosity at a given temperature and this will influence viscosity of the melt, as well as consistency of a solid-liquid lipid mixture. Temperature treatment. If a fat tends to supercool excessively, this can be overcome by melting the crystalline fat at the lowest possible temperature, holding it for an extended period of time at a temperature just above its melting point, and then cooling it. This facilitates formation of numerous crystal nuclei, numerous small crystals, and a firm consistency. Mechanical working. Crystallized fats are generally thixotropic; that is, they become reversibly softer after vigorous agitation and only gradually regain their original firmness. If a melted fat is mechanically agitated during solidification, it will be much softer than if allowed to solidify in a quiescent condition. In the quiescent state the growing crystals form structures of relatively great strength. These structures can be deformed by mechanical working. 5.5 Chemical Aspects 5.5.1 Lipolysis Hydrolysis of ester bonds in lipids (lipolysis) may occur by enzyme action [149] or by heat and moisture, resulting in the liberation of free fatty acids. Free fatty acids are virtually absent in fat of living animal tissue. They can form, however, by enzyme action after the animal is killed. Since edible animal fats are not usually refined, prompt rendering is of particular importance. The temperatures commonly used in the rendering process are capable of inactivating the enzymes responsible for hydrolysis. The release of short-chain fatty acids by hydrolysis is responsible for the development of an undersirable rancid flavor (hydrolytic rancidity) in raw milk. On the other hand, certain typical cheese flavors are produced by deliberate addition of microbial and milk lipases. Controlled and selective lipolysis is also used in the manufacture of other food items, such as yogurt and bread. In contrast to animal fats, oils in mature oil seeds may have undergone substantial Pag e 255 hydrolysis by the time they are harvested, giving rise to significant amounts of free fatty acids. Neutralization with alkali is thus required for most vegetable oils after they are extracted. Lipolysis is a major reaction occurring during deep-fat frying due to the large amounts of water introduced from the food, and the relatively high temperatures used. Development of high levels of free fatty acids during frying is usually associated with decreases in smoke point and surface tension of the oil and a reduction in quality of the fried food. Furthermore, free fatty acids are more susceptible to oxidation than are fatty acids esterified to glycerol. Phospholipase A and lipases from muscle tissues have been described. Significant phospholipid hydrolysis occurs in most species of fish during frozen storage and is usually associated with deterioration in quality. A number of studies appear to indicate that while the hydrolysis of triacylglycerols leads to increased lipid oxidation, phospholipid hydrolysis inhibits the oxidation [123]. Enzymic lipolysis is used extensively as an analytical tool in lipid research. As discussed earlier in this chapter, pancreatic lipase and snake venom phospholipase are used to determine the positional distribution of fatty acids in acylglycerol molecules. The specificities of these and numerous other enzymes make them particularly useful in the preparation of intermediates in the chemical synthesis of certain lipids, and in the manufacture of “structured fats” useful for specific nutritional, pharmaceutical, and cosmetic applications [2, 16, 141]. 5.5.2 Autoxidation Lipid oxidation is one of the major causes of food spoilage. It is of great economic concern to the food industry because it leads to the development, in edible oils and fat-containing foods, of various off flavors and off odors generally called rancid (oxidative rancidity), which render these foods less acceptable. In addition, oxidative reactions can decrease the nutritional quality of food, and certain oxidation products are potentially toxic. On the other hand, under certain conditions, a limited degree of lipid oxidation is sometimes desirable, as in aged cheeses and some fried foods. For these reasons, extensive research has been done not only to identify the products of lipid oxidation and the conditions that influence their production, but also to study the mechanisms involved. Since oxidative reactions in food lipids are exceedingly complex, simpler model systems, such as oleate, linoleate, and linolenate, have been used to ascertain mechanistic pathways. Although this approach has provided useful information, extrapolation to the more complex food lipid systems must be done with caution. It is generally agreed that “autoxidation,” that is, the reaction with molecular oxygen via a self-catalytic mechanism, is the main reaction involved in oxidative deterioration of lipids. Although photochemical reactions have been known for a long time, only recently has the role of photosensitized oxidation and its interaction with autoxidation emerged. In foods, the lipids can be oxidized by both enzymic and nonenzymic mechanisms. 5.5.2.1 General Characteristics of the Autoxidation Reaction Our present knowledge regarding the fundamental mechanisms of lipid oxidation resulted largely from the pioneering work of Farmer and his coworkers [36], Bolland and Gee [12], and Bateman et al. [10]. Autoxidation of fats proceeds via typical free radical mechanisms as characterized by (a) marked inhibition in rate by chemicals known to interfere with other well-established free radical reactions, (b) catalytic effects of light and free radical-producing substances, (c) high yields of the hydroperoxide, ROOH, (d) quantum yields exceeding unity when the oxidation Pag e 256 reaction is initiated by light, and (e) a relatively long induction period observed when starting with a pure substrate. Based on experimental results, mostly with ethyl linoleate, the rate of oxygen absorption can be expressed as shown here, where RH is the substrate fatty acid, H is an a-methylenic hydrogen atom easily detachable due to the influence of a neighboring double bond or bonds, ROOH is the hydroperoxide formed, p is oxygen pressure, and l and Ka are empirical constants. To explain the experimenta l results, a three-step simplified, free-radical scheme has been postulated: At high oxygen pressure (l[RH]/p<1), reactions (4) and (5) can be neglected to give this form: In this situation, the rate of oxidation is independent of oxygen pressure. At low oxygen pressure (l[RH]/p >1), steps (5) and (6) can be neglected to give this form: Since the reaction RH + O2 free radicals is thermodynamically difficult (activation energy of about 35 kcal, or 146 kJ/mol), production of the first few radicals (initiation) necessary to start the propagation reaction must be catalyzed. It has been proposed that initiation of oxidation may take place by hydroperoxide decomposition, by metal catalysis, or by exposure to light. More recently, it has been postulated that singlet oxygen is the active species involved, with tissue pigments such as chlorophyll and myoglobin acting as sensitizers. After initiation, oxidation is propagated by abstraction of hydrogen atoms at positions a to fatty acid double bonds, producing free radical species . Oxygen addition then occurs at Pag e 257 these locations, resulting in the production of peroxy radicals , and these in turn abstract hydrogen from a-methylenic groups of other molecules (RH) to yield hydroperoxides (ROOH) and new free radicals . The new groups react with oxygen, and the sequence of reactions just described is repeated. Due to resonance stabilization of these species, the reaction sequence is usually accompanied by a shift in position of the double bonds, resulting in the formation of isomeric hydroperoxides that often contain conjugated diene groups (atypical of unoxidized, natural acylglycerols). Hydroperoxides, the primary initial products of lipid autoxidation, are relatively unstable. They enter into numerous complex reactions involving substrate degradation and interaction, resulting in a myriad of compounds of various molecular weights, flavor thresholds, and biological significance. A general scheme summarizing the overall picture of lipid autoxidation is given in Figure 17, and some aspects of the reaction sequence are discussed in more detail later. FIGURE 17 Generalized scheme for autoxidation of lipids. Pag e 258 5.5.2.2 Formation of Hydroperoxides Qualitative and quantitative analyses of the isomeric hydroperoxides from oleate, linoleate, and linolenate have been conducted [22, 23, 39]. Oleate Hydrogen abstraction at carbons 8 and 11 of oleate results in formation of two allylic radical intermediates. Oxygen attack at the end carbons of each radical produces an isomeric mixture of 8-, 9-, 10-, and 11-allylic hydroperoxides. The amounts of 8- and 11-hydroperoxides formed are slightly greater than those of the 9- and 10-isomers. At 25°C the amounts of cis and trans 8- and 11-hydroperoxides are similar, but the 9- and 10-isomers are mainly trans. Linoleate The 1,4-pentadiene structure in linoleates makes them much more susceptible (by a factor of about 20) to oxidation than the propene system of oleate. The methylene group at position 11 is doubly activated by the two adjacent double bonds. Pag e 259 Hydrogen abstraction at this position produces a pentadienyl radical intermediate, which upon reaction with molecular oxygen produces an equal mixture of conjugated 9- and 13-diene hydroperoxides. Evidence reported in the literature indicates that the 9- and 13-cis, trans- hydroperoxides undergo interconversion, along with some geometric isomerization, forming trans,transisomers. Thus, each of the two hydroperoxides (9- and 13-) is found in both the cis,trans and the trans,trans forms. Linolenate In linolenate, two 1,4-pentadiene structures are present. Hydrogen abstraction at the two active methylene groups of carbons 11 and 14 produces two pentadienyl radicals. Oxygen attack at the end carbon of each radical results in the formation of a mixture of 9-, 12-, 13-, and 16-hydroperoxides. For each of the hydroperoxides, geometric isomers exist, each having a conjugated diene system in either the cis,trans or the trans,trans configuration, and an isolated double bond that is always cis. The 9- and 16-hydroperoxides are formed in significantly greater amounts than the 12- and 13-isomers. This has been attributed to (a) a preference of oxygen to react with carbons 9 and 16; (b) faster decomposition of the 12- and 13-hydroperoxides, or (c) a tendency of the 12- and 13-hydroperoxides to form six-membered peroxide hydroperoxides via 1,4-cyclization, as shown here, or prostaglandin-like endoperoxides via 1,3-cyclization. The cyclization mechanisms are thought to be the most likely explanations. 5.5.2.3 Oxidation with Singlet Oxygen As just discussed, the major pathway for oxidation of unsaturated fatty acids involves a self-catalytic free-radical mechanism (autoxidation) that accounts for the chain reaction of hydroperoxide (ROOH) formation and decomposition. However, the origin of the initial free radicals necessary to begin the process has been difficult to explain. It is unlikely that initiation occurs by direct attack of oxygen in its most stable form (triplet state) on double bonds of fatty acids (RH). This is because the C=C bonds in RH and ROOH are in singlet states and thus such reaction does not obey the rule of spin conservation. A more satisfactory explanation is that singlet oxygen (1O2), believed to be the active species in photooxidative deterioration, is responsible for initiation. Since electrons are charged , they act like magnets that can exist in two different orientations, but with equal magnitude of spin, +1 and -1. The total angular momentum of the electrons in an atom is described by the expression 2S + 1 where S is total spin. If an atom such as oxygen has two unpaired electrons in its outer orbitals, they can align their spins parallel or antiparallel with each other, giving rise to two different multiplicities of state, that is, 2(½ + ½) + 1 = 3 and 2(½ – ½) + 1 = 1. These are called the triplet (3O2) and singlet state (1O2), respectively. In the triplet state, the two electrons in the antibonding 2p orbitals have the same spin but are in Pag e 260 different orbitals. These electrons are kept apart via “pauli exclusion” and therefore have only a small repulsive electrostatic energy. In the singlet state, the two electrons have opposite spins, and therefore electrostatic repulsion will be great, resulting in an excited state. Singlet oxygen can be found in two energy states, 1d, with an energy of 22 kcal (94 kJ) above ground state, and 1e, with an energy of 37 kcal (157 kJ) above ground state. Singlet-state oxygen is more electrophilic than triplet state oxygen. It can thus react rapidly (1500 times faster than 3O2 with moieties of high electron density, such as C=C bonds. The resulting hydroperoxides can then cleave to initiate a conventional, free-radical chain reaction. Singlet oxygen can be generated in a variety of ways; probably the most important is via photosensitization by natural pigments in foods. Two pathways have been proposed for photosensitized oxidation [21]. In the type 1 pathway, the sensitizer presumably reacts, after light absorption, with substrate (A) to form intermediates, which then react with ground-state (triplet) oxygen to yield the oxidation products. In the type 2 pathway, molecular oxygen rather than the substrate is presumably the species that reacts with the sensitizer upon light absorption. Several substances are commonly found in fat-containing foods that can act as photosensitizers to produce 1O2. These include natural pigments, such as chlorophyll-a, pheophytin-a, and hematoporphyrin, the pigment portion of hemoglobin, and myoglobin. The synthetic colorant erythrosine also acts as an active photosensitizer. b-Carotene is a very effective 1O2 quencher, and tocopherols are also somewhat effective. Synthetic quenchers such as butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are also effective and permissible in foods. The formation of hydroperoxides by singlet oxygen proceeds via mechanisms different from those of free-radical autoxidation. The most important of these is the “ene” reaction, which involves the formation of a six-membered ring transition state. Oxygen is thus inserted at the ends of the double bond, which then shifts to yield an allylic hydroperoxide in the trans configuration. Accordingly, oleate produces the 9- and 10-hydroperoxides (instead of the 8-, 9-, 10-, and 11-hydroperoxides occurring by free radical autoxidation), and linoleate produces a mixture of 9-, 10-, 12-, 13-, 15-, and 16-hydroperoxides (instead of 9-, 12- , 13-, and 16-). In addition to the role of singlet oxygen as an active initiator of free radicals, it has Pag e 261 been suggested that certain products of lipid oxidation can be explained only on the basis of decomposition of hydroperoxides typical of those produced by singlet oxygen. However, there is general agreement that, once the initial hydroperoxides are formed, the free-radical chain reaction prevails as the main mechanism. Further research is needed to clarify the extent to which singlet-oxygen oxidation may be involved in the production of oxidative decomposition products. 5.5.2.4 Decomposition of Hydroperoxides Hydroperoxides break down in several steps, yielding a wide variety of decomposition products. Each hydroperoxide produces a set of initial breakdown products that are typical of the specific hydroperoxide and depend on the position of the peroxide group in the parent molecule [40]. The decomposition products can themselves undergo further oxidation and decomposition, thus contributing to a large and varied free-radical pool. The multiplicity of the many possible reaction pathways results in a pattern of autoxidation products so complex that in most cases their hydroperoxide origin is completely obliterated. It should also be mentioned that hydroperoxides begin to decompose as soon as they are formed. In the first stages of autoxidation, their rate of formation exceeds their rate of decomposition. The reverse takes place at later stages. The first step in hydroperoxide decomposition is scission at the oxygen-oxygen bond of the hydroperoxide group, giving rise to an alkoxy radical and a hydroxy radical, as shown. Carbon-carbon bond cleavage on either side of the alkoxy group (homolytic cleavage) is the second step in decomposition of the hydroperoxides: In general, cleavage on the acid side (i.e., the carboxyl or ester side) results in formation of an aldehyde and an acid (or ester), while scission on the hydrocarbon (or methyl) side produces a hydrocarbon and an oxoacid (or oxoester). If, however, a vinylic radical results from such cleavage, an aldehydic functional group is formed: Pag e 262 For example, with the 8-hydroperoxide isomer from methyl oleate, cleavage on the hydrocarbon side (at a) yields decanal and methyl-8-oxooctanoate, and scission on the ester side (at b) forms 2-undecenal and methyl heptanoate: In the same manner, each of the remaining three oleate hydroperoxides would be expected to produce four typical products; that is, the 9-hydroperoxide, shown here, would produce nonanal, methyl-9-oxononaoate, 2-decenal, and methyl octanoate; the 10- hydroperoxide, shown here, would produce octane, methyl-10-oxo-8-decenoate, nonanal, and methyl-9-oxononanoate; and the 11-hydroperoxide, shown here, would produce heptane, methyl-II-oxo-9-undecenoate, octanal, and methyl-10-oxo-decanoate. As mentioned earlier, autoxidation of linoleate produces two conjugated hydroperoxides, the 9- and the 13-hydroperoxides. Figure 18 shows the typical cleavage pattern of the 9-alkoxy radical. In addition to the compounds resulting from cleavage on either side of the alkoxy group, several other compounds have been observed. For example, in studies on heated ethyl linoleate, several esters and oxoesters with chain lengths g>b>a. However, the relative activity of these compounds is significantly influenced by temperature and light. Pag e 285 FIGURE 31 Resonance structures of the a-tocopherol radical. (From Ref. 9.) In biological systems, ascorbate is capable of “recycling” a-tocopherol by reducing the a-tocopherol radical [9]. Tocopherol as a Pro-Oxidant: It has been observed that under certain conditions tocopherol may act as a pro-oxidant. Under normal circumstances, where lipid concentration greatly exceeds the concentration of tocopherol, progressive oxidation depletes tocopherol, leaves the pool of lipid relatively unchanged and causes accumulation of ROOH. Accumulation of ROOH reverses the equilibrium reaction, thus stimulating the propagation reaction When a-tocopherol is present at a relatively high concentration, a pro-oxidant effect can occur via radical formation according to the reaction Gum guaiac is a resinous exudate from a tropical tree. Its antioxidant effectiveness, due mainly to its appreciable content of phenolic acids, is more pronounced in animal fats than in vegetable oils. Gum guaic has a reddish brown color, is very slightly soluble in oil, and gives rise to some off flavor. Both butylated hydroxyanisole (BHA) which is commercially available as a mixture of two isomers (Fig. 27), and butylated hydroxytoluene (BHT) have found wide commercial use in the food industry. Both are highly soluble in oil and exhibit weak antioxidant activity in vegetable oils, particularly those rich in natural antioxidants. BHT and BHA are relatively effective when used in combination with other primary antioxidants. BHA has a typical phenolic odor that may become noticeable if the oil is subjected to high heat. Pag e 286 Nordihydroquaiaretic acid (NDGA) is extracted from a desert plant, Larrea divaricata. Its solubility in oils is limited (0.5–1%), but greater amounts can be dissolved if the oil is heated. NDGA has poor carry-through properties and tends to darken slightly on storage, in the presence of iron, or when subjected to high temperatures. The antioxidant activity of NDGA is markedly influenced by pH, and it is readily destroyed in highly alkaline conditions. NDGA is reported to effectively retard hematincatalyzed oxidation in fat-aqueous systems and in certain meats. In the United States NDGA is not permitted for use as a food additive. However, it can be used in food packaging material. As would be expected from their phenolic structure and three hydroxyl groups, alkyl gallates and gallic acid exhibit considerable antioxidant activity. Gallic acid is soluble in water but nearly insoluble in oil. Esterification of the carboxyl group with alcohols of varying chain length produces alkyl gallates with increased oil solubility. Of these, propyl gallate is widely used in the United States. This compound reportedly is effective for retarding lipoxygenase-cat-alyzed oxidation of linoleate. In the presence of traces of iron and alkaline conditions, gallates will cause a blue-black discoloration. They are unstable during baking or frying. Tertiary Butylhydroquinone (TBHQ): TBHQ is moderately soluble in oil and slightly soluble in water. In many cases, TBHQ is more effective than other common antioxidants in providing oxidative stability to crude and refined polyunsaturated oils, without problems of color or flavor stability. TBHQ is also reported to exhibit good carry-through characteristics in the frying of potato chips. As would be expected from their similarity in structure, 2,4,5-trihydroxybutyrophenone (THBP) and the gallates exhibit similar antioxidant properties. THBP is not widely used in the United States. 4-Hydroxymethyl-2,6-di-tertiary-butylphenol is produced by substituting a hydroxyl for one hydrogen in the methyl group of BHT. It is therefore less volatile than BHT but otherwise behaves similarly as an antioxidant. Decomposition of Antioxidants Antioxidants may exhibit significant decomposition, particularly at elevated temperatures, and several degradation products can arise. The amounts of these products are very small because the concentrations of antioxidants allowed in food are small. However, some of these degradation products have antioxidative properties. The stabilities of four phenolic antioxidants have been studied during heating for 1 hr at 185°C [54]. Apparent stability increased in the order TBHQ

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