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.

SUMMARY The literature on reflectance spectrophotometry, as it applies to meat pigments, is critically reviewed, and improved methods are suggested for determination of total pigments and of the percent metmyoglobin from reflectance data on raw meat. The suggested method for total pigment was based on reflectivity of the meat samples at 525 mμ, the isobestic point for myoglobin, oxymyoglobin, and metmyoglobin. The reflectivity data, when calculated as the corresponding ratios of the absorption coefficient K to the scattering coefficient S were linearly related to total pigment extract from the meat with acidified acetone. K/S values of pigment‐free (peroxide‐treated) samples were obtained as a base line. Lowering the pH of the meat decreased the K/S value. This was attributed to changes in texture which increased S. Metmyoglobin was determined from the ratio K/S 572 mp/K/S 525 mμ. Limiting values for the ratio were established for meat containing 100% and 0% metmyoglobin, and a linear relation was assumed between the ratios and intermediate amounts of metmyoglobin.

SUMMARY Methods for measuring reducing capacity of meats are described. These include changes in oxidation‐reduction potentials of ground meat and changes in oxygen tension of meat slurries, as well as reduction of metmyoglobin. Except for a small residual utilization of oxygen in meat slurries (ascribed to nonenzymatic oxidation), all reductive activity in meat can be stopped by inhibitors of DPNH oxidation via the electron transport chain. Added DPN accelerates all reductive activity. Metmyoglobin reduction does not occur until oxygen has snbstantially disappeared from the meat. Meat contains little or no snccinate. Added succinate greatly accelerates oxygen utilization, but affects metmyoglobin reduction only indirectly by establishing anaerobic conditions more rapidly. It is concluded that both oxygen utilization and metmyoglobin reduction in meat are normally mediated through DPN.

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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