Lipid Oxidation

When fresh meat is held in conditions free from bacterial spoilage, the meat will age and become more tender. In the presence of even small amounts of oxygen, oxidation will also take place at the double bond of the polyunsaturated fatty acids, leading to formation of peroxides that react chemically to produce secondary reaction products (120). These at first have a bland flavor but later result in a typical rancid flavor reminiscent of "old socks." The fats concerned are mainly the phospholipids associated with the cell membrane rather than the depot fatty tissues.

The lipids of raw meat are initially protected from oxidation by endogenous antioxidants, but eventually these antioxidants are inactivated and the rancidity process starts. Exposure to light and heavy metals promotes the onset of rancidity. Rancidity, being initiated by a free-radical reaction, continues even in the absence of additional oxygen, so conditions to minimize its initiation are most important. Vacuum packaging is one option; freezing is another. As all chemical reactions proceed more slowly at lower temperatures, meat should be stored at as low a temperature as practical.

One study on lamb (121) showed that at — 5°C, rancidity was initiated, and it continued to progress even when the meat temperature was reduced to — 35°C; rancidity did not appear to be initiated at the — 35°C temperature, however. This study illustrates that, when considering rancidity development, the entire history of the meat should be considered, rather than mere storage duration at various temperatures.

Although oxidative rancidity changes occur slowly in frozen meat, they occur extremely rapidly in meat after cooking and produce a characteristic "warmed-over flavor." Although the development of warmed-over flavor is reduced if cooked meat is chilled rapidly, this flavor development makes commercial storage of cooked meat for more than a few days a practical impossibility, unless oxygen is prevented from reaching the meat by storing and cooking meat in its own oxygen-impermeable package. The stability of meat lipids to oxidation is lowest in pork and poultry, intermediate in beef, and highest in lamb.

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