D. A. Ledward

Relative role of catalysts and reductants in the formation of metmyoglobin in aerobically stored beef

The formation of metmyoglobin (metMb) in sterile minced and intact slices of beef has been determined during storage at 1°C. The oxidation of oxymyoglobin in minced beef is first order, the rate constants for different muscles, although being pH dependent, are similar for all the muscles studied. The rate is about four or five times faster than that reported for oxymyoglobin in solution, indicating that catalysis occurs. In intact slices the reaction profile is more complex as a pseudoequilibrium level of metmyoglobin forms after a few days at 1°C, the concentration of metMb remaining constant for several days and then being followed by further rapid oxidation to 100% metMb. Due to variations in an aerobic reducing system present in intact beef the duration of the pseudoequilibrium varies markedly from muscle to muscle; this reducing system is destroyed by mincing. The rate of metMb formation in intact muscles varies with sample shape, thin slices being more colour stable ones. Reasons for this dependence on sample shape are discussed.

Reactivity of glycerol in intermediate moisture meats.

Non-enzymic browning (NEB) reactions between glycerol and some of the amino acids present in meat have been observed at 38 and 65°C; l-lysine is the most reactive of the amino acids studied and l-cysteine and l-arginine the least. These NEB reactions also occur between glycerol and proteins (gelatin and casein). The reactions are not unique to glycerol as other polyhydric alcohols also form brown pigments with lysine. On prolonged air storage at 65°C glycerol itself undergoes mild oxidation, ultimately yielding a brown solution.

Scanning calorimetric studies of some protein-protein interactions involving myoglobin

To aid in understanding the formation of cooked meat haemoprotein, the thermal denaturation behaviour of metmyoglobin in 0·05(M) phosphate solutions (pH 6·0) of bovine serum albumin and ovalbumin was analysed by differential scanning calorimetry. Although, in general, there is minimal interaction between proteins prior to and during thermal denaturation, it was found that in the presence of an excess of protein of relatively low thermal stability (bovine serum albumin) myoglobin denatures and precipitates with this protein. It is suggested that this destabilisation of myoglobin is due to specific interaction between the denatured protein and the haematin of a conformationally altered, but undenatured, myoglobin.

Prediction of water activity, aw, in cook-soak equilibrated intermediate moisture meats

The relationship between the moisture content and water activity, a(w), of several meat samples adjusted to intermediate moisture water activities by cook-soak equilibration in glycerol solutions containing 9·5% sodium chloride was determined. Within experimental error the relationship was found to be unique for all the lean meats studied. It was also found that a modification of the Grover method (Grover, 1947) in which a(w) is determined from the well established relationship between a(w) and sucrose equivalent, E(s), was an excellent predictor of the final a(w) of the ssystem. E(s) was calculated from E(s) = E(1)X(1) + E(1)X(2) + … etc. where E(1) are specific constants for each humectant and X(1) and X(2) are given as weight of humectant in the system/weight of water in the system. E values of 9·0 and 2·2 for sodium chloride and glycerol were applicable for initial salt concentrations of 0 to 20 (w/w)_and glycerol concentrations of 0 to 90% (w/w) The method was equally applicable to propylene glycol (E value 3·2)/salt systems and sorbitol (E value 1·0)/salt systems.