Oxidative Rancidity in Meat

In view of the fact that Lea (1939) has published a comprehensive and critical monograph on the broad topic of rancidity in edible fats, no attempt will be made to review this earlier work in detail except as it may be necessary for purposes of orientation. Furthermore, while great strides have been made in clarifying the chemistry of the oxidation of unsaturated fatty acids in the past decade, this work as it applies to the extracted fats from animal tissues has been ably reviewed by Bailey (1951). Therefore only the salient facts without extensive documentation will be presented here in order to leave space for more comprehensive coverage of variations in the composition of the fat of meat animals which 

Rancidity, at least as the term is used in meat products, results from the oxidative decomposition of unsaturated fats. The first step in this decomposition is the addition of oxygen at a carbon atom adjacent to an unsaturated carbon to form a hydroperoxide  

Although this reaction can occur in fatty acids having a single double bond, such as oleic, the methylene group between two double-bonded carbons is very much more susceptible to oxidative attack than carbons adjacent to a single double bond. Thus, linoleic acid, with one active methylene group, oxidizes ten to twelve times as rapidly as oleic acid. Linolenic acid, with two such labile carbons, oxidizes twice as fast as linoleic (Gunstone and Hilditch, 1946). 

The hydroperoxides formed are intermediates in the oxidative process. They do not themselves contribute to the rancid odor, but they are unstable and break down to a great variety of decomposition products, some of which contribute to rancidity. Much recent work has been devoted to the isolation and identification of such decomposition products, but the literature is too voluminous to review here. 

The course of the oxidation of any animal fat follows a typical pattern. A period of very limited oxygen uptake (the induction period) is followed by a phase of rapid oxidation (see Fig. 4). The induction period is due to the presence in animal fats of varying amounts of a natural antioxidant, alpha tocopherol. The activity of this and other antioxidants is discussed in Section V. 

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Tappel (5) showed that ascorbic acid acts synergistically with food antioxidants and because of the great increase in eflFectiveness resulting from a small amount of ascorbic acid, he suggested mixtures would be eflFective in preventing oxidative rancidity in meats, poultry, and fish. 

Measures for the control of oxidative rancidity in meats can begin with the feeding of the meat animals. Rancidity of the fat in situ is influenced by the characteristics of the fat itself and by the aqueous medium with which it is intimately associated. 

To improve our understanding of the complex oxidation reactions in meat and fish products, evaluations should be carried out with each product stored under realistic conditions. Lipid oxidation should be followed by several and more reliable methods than the TEA test. These methods should measure specific lipid oxidation products (e.g. hydroperoxides or conjugated dienes) acting as precursors of rancid flavors, and specific volatile products (e.g. propanal, hexanal and other volatiles associated with fish flavors) that serve as reliable indicators of flavor deterioration. Instrumental analyses of volatile oxidation products should be supported and confirmed by sensory evaluations. 

Uses Texturizer, retention agent in meat and dessert prods. sequestrant inhibiting oxidative rancidity in foods moisture loss regulator during thawing/cooking emulsifier for fat and protein in petfoods, prod, of food starch, as egg wash boiler water additive... 

In tire processed meat field, citric acid, along with oxidants, is used to prevent rancidity in frankfurters and sausages. Sodium citrate is used in processing livestock blood, which is used to manufacture some sausages and pet loods,... 

Tims and Watts (2) showed that a combination of ascorbates and phosphates acted synergistically to retard development of rancidity. Sato and Hegarty (3) verified the antioxidant activity of the combination and suggested that ascorbic acid acts by keeping part of the iron in the Fe2+ state. It has been shown that ascorbic acid and phosphates act synergistically in preventing oxidation of cured meat (23), and probably help in explaining the virtual absence of WOF in cured meats. 

Aldehydes are by far the most numerous compounds identified as dry-cured ham odorants, with different odors (green, rancid, toasted) and thresholds in air ranging from 0.09 to 480 ng/L (Table 1). Most of them were identified in the first works focused on dry-cured ham volatile compounds (7,2). Aldehydes are essential for meat flavor (70), but large quantities in meat and meat products have been related to lipid oxidation and deterioration (77). The effect of several quality factors has been researched and it was found that the rearing system of pigs (S) and ripening conditions (7) influence on the contribution to odor and the content of some aldehydes. 

The largest values for the 14-days group matched with the largest scores for the rancid note (it ). In fact, pentanal and hexanal (two usual indicators of lipid oxidation in meat, 11) were much more abundant in the 14-days group. It is known that during ripening of hams an increase of lipid oxidation volatile compounds takes place, and it is also followed by a important decrease (2). The decrease in some compounds could be related to the decrease of lipid oxidation caused by the depletion of the oxygen available in the tubes and to the lost of odorants by reactions with other components. 

Studies related to oxidation of IM meats have been discussed by Okon-kwo (1984) and Okonkwo et al. (1992a,b). With smoked beef prepared by cook-soak equilibrium in a solution containing sodium chloride, sodium nitrite, and potassium sorbate and smoked for 4 or 18 hr at 50°C, oxidation was not a serious problem. TEA values were low and all samples possessed no detectable rancidity. 

There is increasing evidence that degradation of proteins is associated with the development of the so-called warmed-over flavor . Selective methods to measure protein oxidation may be useful as a complementary approach in evaluating oxidative deterioration of meat products. The determination of protein carbonyls (as dinitrophenyl hydrazine derivatives) is now used for this purpose. More reliable methods are needed, however, to determine specific products of lipid oxidation and their interaction products acting as precursors of flavor compounds to establish the relative contribution of heme and nonheme iron to the development of rancidity in various meat products. 

This review is concerned with two types of oxidative changes which occur in meat, namely, oxidation of the fat, resulting in rancidity, and... 

A number of painful experiences have served to impress upon the author the fallaciousness of this assumption. Time schedules for the analysis of experimental samples have had to be revised repeatedly when controls which were expected to be rancid in two months still showed no oxidative deterioration after as many years. Optimistic claims made for the stability of meat treated in a specified way have had to be abandoned when another lot of meat treated in the same way failed to measure up. It was not unusual to find that inherent variations in the meat itself were of far greater importance in determining the keeping time than any of the experimental treatments under investigation. 

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