Lipids and Rancidity

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

Potatoes contain a small amount of lipid, comprising approximately 0.1% of the fresh weight of the tuber (Galliard, 1973). Despite the minute quantity, lipids play an important role in the stability of processed potato products, since lipid degradation can lead to off-flavors and rancidity. 

Schwartz, D. P. 1974. The lipids of milk Deterioration, Part I. Lipolysis and rancidity. In Fundamentals of Dairy Chemistry. B.H. Webb, A. H. Johnson and John A. Alford (Editors). AVI Publishing Co., Westport, Conn., pp. 220-239. 

Malonaldehyde (MA) is a major end product of oxidizing or rancid lipids and it accumulates in moist foodstuffs (6). Several MA-protein systems have been studied. Chio and Tappel combined RNAase and MA to demonstrate fluorescence attributed to a conjugated imine formed by crosslinking two e-amino groups with the dialdehyde (7). Shin studied the same reaction and found it to be dependent on pH and reactant concentrations (8). Crawford reported the reaction between MA and bovine plasma albumin (BPA) also to be pH dependent, and of first order kinetics with a maximum rate near pH 4.30. At room temperature 50-60% of the e-amino groups were modified—40% in the first eight hours, the remainder over a period of days (9). 

Food lipids possess an inherent stability to oxidation, which is influenced by the presence of antioxidants and pro-oxidants. After a period of relative stability (induction period), lipid oxidation becomes autocatalytic and rancidity develops. Thus, the typical time-course of autoxidation, as measured by the concentration of hydroperoxides, consists of a lag phase (induction) followed by the rapid accumulation of hydroperoxides, which reaches a maximum and then decreases as hydroperoxide decomposition reactions become more important. The longer the induction period, the more stable the food to oxidation (Lundberg, 1962). 

In a biological system, an antioxidant can be defined as any substance that when present at low concentrations compared to that of an oxidizable substrate would significantly delay or prevent oxidation of that substrate (1). The oxidizable substrate may be any molecule that is found in foods or biological materials, including carbohydrates, DNA, lipids, and proteins. Food is a multicomponent system composed of a variety of biomolecules, and therefore, this definition describes well an antioxidant. However, regulatory bodies that overlook the food-supply categorize antioxidants under food additives and define them as substances used to preserve food by retarding deterioration, rancidity, or discoloration due to oxidation (Code of... 

Homogenization markedly reduces the propensity to oxidative rancidity, perhaps due to redistribution of the susceptible lipids and pro-oxidants of the MFGM (however, the propensity to hydrolytic rancidity and sunlight oxidized flavour (due to the production of methional from methionine in protein) is increased). 

The rancid lipid odor profile is made up of a mixture of several volatile compounds. Among them, the trans, cw-alkadienals, and vinyl ketones have the lowest flavor threshold in oils, while the threshold of hydrocarbons (alkanes and alkenes) is the highest (Min, 1998). The sensory effects depend on the composition of the participating compounds and on the composition of the food matrix, while the rancid off-odors and off-flavors of foods emanate from the interactions between lipids and other components, especially proteins. 

In actual lipid oxidation, one cannot overlook the critical role of trace metals, which complicate the kinetic sequences of initiation and decomposition of lipid hydroperoxides. These metals catalyse both initiation of free radicals and decomposition of hydroperoxides, which become particularly significant with polyunsaturated lipids containing more than two double bonds. With these polyunsaturated lipids, although the yields of hydroperoxides are reduced in the presence of metals, they produce volatile decomposition products that have a serious impact on flavor deterioration. In foods and biological systems, the mixture of trace metals and hydroperoxides is the most important initiator that plays a key part in the development of free radical oxidation and rancidity. The use of artificial azo compounds as initiators to study free radical oxidation is therefore not relevant. 

Our liquid CO2 extracts typically contain saturated lipids up to a carbon chain length of C12 though unsaturated lipids, oxygenated lipids and lipids of chain lengths in excess of C12 appear to be virtually absent. The mono, di and triglyceride lipids extracted do not readily oxidise and turn rancid but are of... 

The volatile components of raw chicken breast muscle include mainly carbonyls, thiols, sulfides and alcohols. The major volatile components of fried chicken are similar to volatiles of the meat of farm animals (e.g. common aldehydes, ketones, hydrocarbons and other compounds). Sulfur-containing compounds generated during thermal processing are also important for the flavour of chicken meat. Saturated and unsaturated aldehydes are very important - these are produced easily by oxidation of lipids and manifest negatively in stored poultry meat, which quickly becomes rancid. Feed plays an important role in imparting certain flavour characteristics to poultry meat. 

Chitosan and Chitosan derivatives (N-carboxymethylchitosan) are also used in preservation of cooked meat and meat products due to their anti-oxidative properties, since, these products are more susceptible to off-flavor and rancidity due to the presence of trrrsaturated lipids. 

This is considered by many as the ultimate test of quality control or assurance. The only drawback with this approach is the cost of using such a panel to get reliable shelf life stability data points at every testing time interval in order to obtain a representative sensory shelf life stabihty profile which reflects changes in the rancidity of the product over time. Lipid oxidation and rancidity represent two sides of the same coin oxidation is the process, and rancidity is the perceived result (Warner and Eskin, 1995). 

Gas chromatography Hexanal is a volatile closely related to extent of lipid oxidation and rancidity. It is determined in the headspace of the package and correlates with peroxide values (Pike 1994). 

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