Relative lipid oxidation

Standards and Controls. In all experiments, the 85 g standard patties were made from freshly ground top round steaks (excess fat trimmed) and immediately frozen in covered glass petri plates until the day of the assay. The fat content was routinely from 4-5%, determined by the method of Koniecko (57). The standards generally had relatively low values for hexanal, total volatiles (TV) and TEARS, and low intensity values for painty (PTY), cardboardy (CED), sour (SUR) and bitter (ETR). These results indicated the absence of lipid oxidation and no formation of off-flavors. As expected, the desirable flavor notes, cooked beef/brothy (CEE), beefy/meaty (EM), brothy (ERO), browned/caramel (ERC) and sweet (SWT) had high intensity values. 

Elemental mercury vapor is relatively lipid soluble and is readily absorbed from the lungs following inhalation and is oxidized in the red blood cells to Hg2+. Elemental mercury may also be transported from red blood cells to other tissues such as the CNS. Elemental mercury readily passes across the blood-brain barrier into the CNS and also into the fetus. The metallic compound is only poorly absorbed from the gastrointestinal tract, however. 

The contribution of lipid oxidative products to off-flavor development has been studied by many workers, and a review of these studies has been presented by Nagy (38). It is generally agreed that the contribution of the lipid oxidative products to the flavor deterioration of processed citrus products is relatively minor when compared to the contributions by the products formed by the acid-catalyzed hydrolysis of flavoring oils and the products of Maillard browning (39,40). 

The basic mechanism of autoxidation at elevated temperatures is similar to that of room-temperature oxidation, i.e., a free radical chain reaction involving the formation and decomposition of hydroperoxide intermediates. Although relative proportions of the isomeric hydroperoxides, specific for oleate, linoleate and linolenate, vary with oxidation temperatures in the range 25°C -80°C, their qualitative pattern is the same (. Likewise, the major decomposition products isolated from fats oxidized over wide temperature ranges are those reflecting autoxidation of their constituent fatty acids (2 -6). The mechanisms and products of lipid oxidation have been extensively studied. The reader is referred to the numerous monographs, reviews and research articles available in the literature (1,A,7,8,9,10,11). 

To gain insight into the effect of physical state and/or molecular organization on lipid oxidation, a variety of model systems have been used. These include dispersions, liposomes or vesicles (37,38), monolayers adsorbed on silica (39,40,41), and red blood cell ghosts (42). In most of these studies, oxidation was conducted at relatively low temperatures, i.e., 20 - 40°C. Very little information is available on the effects of physical state on high temperature oxidative reactions or interactions of lipids. 

Volatile compounds generated by model systems of zeln, corn amylopectin and corn oil extruded at barrel temepratures of 120°C and 165°C were analyzed by GC and GC/MS. The largest quantities of lipid oxidation products were detected in systems containing all three components. In each system, the quantity of 2,4-deca-dienal was low relative to the quantities of hexanal, heptanal and benzaldehyde. Identification of the Maillard reaction products, 2-methyl-3(or 6)-pentyl-pyrazine, 2-methyl-3(or 6)-hexylpyrazine and 2,5-di-methyl-3-pentylpyrazine, suggested that lipid-derived aldehydes might be involved in the formation of substituted pyrazines. 4-Methylthiazole was identified as a major decomposition product of thiamin when corn meal containing 0.5% thiamin was extruded at a final temperature of 180°C. 

Rancimat induction Lag in lipid oxidation in a stream of air at relatively high temperature (90-130 °C) Anese and Nicoli446... 

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). 

Citation of the classic chain reaction for lipid oxidation persists even though, as product analysis and studies of mechanisms have become more sophisticated, there is now considerable evidence that only Reactions 1, 2, and 5 (and perhaps also 6) of Figure 1 are always present. Research has shown that, although hydrogen abstraction ultimately occurs, it is not always the major fate of the initial peroxyl or alkoxyl radicals. Indeed, lipid alcohols from H abstraction are relatively minor products of lipid oxidation. There are many competing alternative reactions for LOO and LO that propagate the radical chain but lead to different kinetics and different products than expected from the classic reaction sequence (5, 6, 21). A more detailed consideration of each stage shows how this basic radical chain sequence portrays only a small part of the lipid oxidation process and products, and a new overall reaction scheme for lipid oxidation is needed. 

Radical oxidation chains are initiated when LOOH is decomposed to initiating LO and OH radicals by light and heat, to LO /LOO by metals, or to LO by the enzyme itself (120). Very low levels produced in plant or animal tissues may provide the invisible initiators that make lipid oxidation sometimes appear spontaneous. Perhaps just as important, LOOH produced by lipoxygenase can accumulate to relatively high levels under appropriate conditions (e.g., cold and dark, as in frozen unblanched materials), then lead to a cascade of rapid oxidation when LOOH decomposes. 

Analysis Measure relatively low levels of oxidation (below 1%) and include measurement of initial or primary products of lipid oxidation (e.g., hydroperoxides, conjugated dienes) as well as secondary decomposition products of lipid oxidation (e.g., carbonyls, volatiles, dialdehydes). 

Monohydroperoxides are the primary products of lipid oxidation. A variety of hydroperoxides with positional and geometrical isomers are formed depending on the position and number of double bonds of the unsaturated fatty acids and the oxidation mechanism. A number of reviews have been published on the composition of isomeric hydroperoxides formed from oxidation of oleate, linoleate, and linolenate (286, 287-291). The hydroperoxides formed are odorless, but they are relatively unstable and are the precursors of a variety of volatile and nonvolatile scission products that are important to the oxidized flavor. 

In native state, proteins exist as either fibrous or globular form. Protein should be denatured and unfolded to produce an extended chain structure to form film. Extended protein chains can interact through hydrogen, ionic, and hydrophobic bonds to form a three-dimensional stmcture (24). Protein films are excellent gas barriers but poor moisture barriers because of their hydrophilic nature. Mechanical properties and gas permeability depend on the relative humidity (1). Al-ameri (25) smdied the antioxidant and mechanical properties of soy, whey and wheat protein, and carrageenan and carboxymethyl cellulose films with incorporated tertiary-butylhy-droquinone (TBHQ), butylated hydroxytoluene (BHT), fenugreek, and rosemary extracts. Armitage et al. (26) studied egg albumin film as a carrier of natural antioxidants to reduce lipid oxidation in cooked and uncooked poultry. 

Lipid oxidation starts in raw beef and continues during cooking. Mottran et al. (21) demonstrated that the intramuscular lipids (not the adipose tissues) are responsible for the formation of most of the lipid-derived volatiles. Intramuscular lipids consist of marbling fat made primarily of triglycerols and structural or membrane lipids made of phospholipids. The phospholipids contain relatively high amount of unsaturated fatty acids more prone to oxidation. In beef, the intramuscular tissue phospholipids are sufficient in imparting a full meaty aroma (22). 

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