SummaryAn improved distillation method is described for the quantitative determination of malonaldehyde in foods containing oxidized fats. The procedure is compared with other methods in current use for the determination of malonaldehyde. A high correlation of TBA numbers with rancid odor in cooked meats was established.

Oxidation of tissue lipids contributes markedly to undesirable flavor changes that occur in stored cooked meat products. Thiobarbituric acid values determined directly on the meat tissue increase rapidly in uncured cooked meats within a few hours after cooking and this increase is accompanied by development of off odors and flavors (8). This oxidative reaction will be designated as "tissue rancidity" since the available evidence indicates that the site of oxidation is a protein bound phospholipid fraction not readily extractable with fat solvents.This oxidative reaction is greater in uncured than in cured meats. Observations on cured meats in this laboratory suggested that they may be held at refrigerator temperature for a much longer period of time before tissue rancidity is noted. I t was considered possible that differences in the heme pigments of cured versus uncured cooked meats might be responsible for this observed difference in their oxidative behaviour.The catalytic effect of hemoglobin and other iron porphyrins on the oxidation of lipids is a generally acepted phenomenon. Earlier work, reviewed by Watts (13), demonstrates that this reaction brings about destruction of the pigment as well as oxidation of the fat.The mechanism of the reaction is incompletely understood. Banks ( I ) suggested that the active catalyst results from a combination of fat peroxide and iron porphyrin. In the most recent of a series of contributions by Tappel and coworkers, Maier and Tappel (5) propose the theory that catalytically active hemes form unstable compounds with fat peroxides, which then decompose to give two free radicals, each of which is capable of initiating an oxidation chain.There is no clear evidence in the literature of the role of the cured meat pigment, nitric oxide hemochromogen, in the oxidation of unsaturated fats. Chang and Watts (3) showed that the addition of nitric oxide hemoglobin to model systems accelerated rancidity. Since their pigment preparation was very unstable, however, it is not clear whether nitric oxide hemoglobin or its ferric oxidation product was responsible for the observed catalysis. Tappel (7) reports similar catalytic effects of extracts from cured pork, but here again the pigment actually responsible for the catalysis is not known. The denatured cured meat pigment is not soluble in water.I t may be hypothesized that the cured meat pigment, in which the 5th and 6th coordination places of the iron are occupied by denatured globin and nitric oxide, respectively, would not be expected to react with a fat

Distribution of iron in six fractions (water-soluble, water-insoluble, diffusate, hematin, total heme, and ferritin) of beef and chicken muscles hcatcd to 55, 70, 85, and 100°C was determined. Iron content decreased in water-soluble fractions and increased in water-insoluble fractions as temperature increased from 27°C to 100°C. Heme iron decreased more from 55°C to 85°C than from 27°C to 55°C or 85°C to 100°C. The increase in diffusate iron appeared to be less than the decrease in heme iron at each heating temperature. As temperature increased from 27°C to IOtYC, hematin iron content increased and extractable ferritin iron content decreased. These findings may help explain rapid development of oxidative rancidity in cooked meat.

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Oxidative rancidity in stored cooked ground turkey and beef was determined by the thiobarbituric (TBA) test and sensory evaluations. There was an increase in rancidity with storage for all samples; however, beef treated with onion juice or textured vegetable protein showed a slower rate of deterioration. Rancidity in turkey was effectively controlled by hot-water extracts of eggplant tissue, peels of yellow onions, potatoes, and sweet potatoes, although initial values were high. There were high positive correlations between TBA values and panel scores for cooked ground beef. Panel members were less sensitive to the rancid odor in turkey and failed to give low sensory ratings even though TBA numbers were high.