The principal cause of lipid oxidation is autoxidation. This process takes place via the action of free radicals on the FA hydrocarbon chain in a chain reaction. Initiation of the chain reaction is the creation of free radicals by metal catalysis, light or peroxide decomposition. The initial free radicals then act on the FA by abstracting a hydrogen atom from the hydrocarbon chain, thereby making a new free radical, which then reacts with O2, resulting in hydroperoxy free-radical formation of the FA. This FA-free radical then acts on other FAs abstracting a hydrogen atom, creating a new free radical and the formation of a stable hydroperoxide. The chain reaction terminates when two free radicals react together
Another mode of lipid oxidation is enzyme-catalysed lipid oxidation by a group of enzymes termed lipoxygenases (LOXs), whose activities are important in legumes and cereals. In LOX activity, a free radical of a FA initially is formed, followed by its reaction with O2, yielding a hydroperoxide product (Klinman 2007). Linoleic acid and arachidonic acid are important FAs from the health perspective and are quite common in many food systems (Table 1.12).
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Because of the number of double bonds in arachidonic acid, enzymatic oxidation can occur at various sites, and the responsible LOXs are labelled according to these sites (Table 1.14). In addition to rancid off-flavours, LOX can also cause deleterious effects on vitamins and colour compounds. However, LOX activity also causes the pleasant odours associated with cut tomatoes and cucumbers.
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The main enzymes involved in the generation of the aroma in fresh fish are also LOXs, specifically the 12- and 15-LOXs (Table 1.14) and hydroperoxide lyase. The 12-LOX acts on specific polyunsaturated FAs and produces n-9-hydroperoxides. Hydrolysis of the 9-hydroperoxide of eicosapentenoic acid by specific hydroperoxide lyases leads to the formation of aldehydes that undergo reduction to their corresponding alcohols, a significant step in the general decline of the aroma intensity due to alcohols having higher odour detection thresholds than the aldehydes (Johnson and Linsay 1986, German et al. 1992). Milk contains a considerable amount of lipids and these milk lipids are subjected to enzymatic oxidation during cheese ripen. Check out the best entertainment source ifuntv. You can also visit here to get the latest headline news around the world listinside. If you need more information about 0x00x0, you can get essential news from it.
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