Reactions in frozen foods; the reactions of myosin and single amino acids with some aldehydes

Abstract

Malonaldehyde and related substances occur in foods as secondary decomposition products of oxidized polyunsaturated fatty acids. The reaction of malonaldehyde (and glycoaldehyde) with rabbit myosin or single amino acids (glycine, lysine methionine or asparagine) was studied in vitro for its potential to cause protein denaturation in muscle. The reactions were followed at nine temperatures between 45°C and −40°C. At each temperature the influence of pH (5.2, 7.0, 9.2) and ionic strength (0.3, 0.6) was examined, and the influence of the freezing rate on the reaction rates was also investigated. The decrease of malonaldehyde concentration in the reaction mixture was followed by the TBA-test. The temperature-dependence of reaction rates was very similar for all the reactions. As expected, the rate of reaction decreases when the temperature is reduced from 45°C to freezing point. However, when the mixture is frozen the reaction rate increases with decreasing temperature down to −24°C. Further lowering of the temperature to −40°C showed a reduction again in the reaction rate. There was no difference in the reaction rate for different ionic strengths. Lowering of pH gave an increase in reaction rate. A low freezing rate gave a higher reaction rate than was obtained with a fast freezing rate. The observed increase in the rate of reaction between malonaldehyde and protein components when lowering the temperature from the freezing point to the eutectic point is explained on the basis of the so-called ‘freeze concentration effect’ and a consequent pH-shift to values more favourable to the reactions.