Protein hydroperoxides carbonyl compounds

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. 

Fig. 1. a-Oxidation of amino acids. Hydroxyl radical (or other reactive radical) abstracts hydrogen atom from the a-carbon. The C-centered free radical formed may react with other amino acid residues or dimerize in the absence of oxygen, which leads to protein aggregation. In die presence of oxygen the carbon-centered radical forms peroxyl radical. Reduction of peroxyl radical leads to protein hydroperoxide. Decomposition of hydroperoxide leads to formation of carbonyl compounds via either oxidative deamination or oxidative decarboxylation. Oxidation of the new carbonyl group forms a carboxyl group. 

The major cause of deterioration of food products is lipid oxidation, from which low-molecular-weight, off-flavor compounds are formed. This deterioration is often caused by the oxidation of the unsaturated lipids present in foods. Off-flavor compounds are created when the hydroperoxides, formed during the initial oxidation, are degraded into secondary reaction compounds. Free radicals are also formed which can participate in reactions with secondary products and with proteins. Interactions with the latter can result in carbonyl amino... 

Antioxidative activities of the hydrolysates from porcine myofibrillar protein were measured in a linolenic acid oxidation system The hydrolysates at the concentration of 0.02, 0.2 and 2% exhibited antioxidative activities. All hydrolysates exhibited stronger antioxidative activity, as the concentration was higher in the production of hydroperoxides. On the other hand, the addition of 0.2% hydrolysate suppressed the production of TEARS most strongly, and the antioxidative activity in the addition of 2% hydrolysate was lower than that of 0.2% in the method of TEARS. It is well known that 2-thiobarbituric acid (TEA) reacts with aldehyde compounds including malondialdehyde (MDA) formed by Upid peroxidation. Two percent hydrolysates contained a lot of amino confounds including free amino acids and peptides. Therefore, the production of aldehyde compounds seems to be accelerated by amino-carbonyl reaction between hydrolysate and lipid in lipid peroxidation system including 2% hydrolysate. This might be the reason why the antioxidative activity of 2% hydrolysate was lower than that of 0.2 % hydrolysate in the method of TEARS. 

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