Racemization kinetics of aspartic acid in fish material under different conditions of moisture, pH, and oxygen pressure.

The racemization kinetics of aspartic acid in heat-treated whole herring have been studied under conditions of treatment comparable to those that may occur in processing of fish meal. D-Aspartic acid content in the samples has been measured by RP-HPLC with precolumn automatic derivatization. The major parameters affecting the rate of racemization of aspartic acid k(Asp) have been demonstrated to be temperature (elevation of temperature from 95 to 120 degrees C resulted in an increase of k(Asp) from 0.46 to 3.39x10(-3) min(-1)), moisture of the raw material (reduction of the moisture content of the raw material from 80 to 15% lowered k(Asp) measured at 95 degrees C from 0.46 to 0.06x10(-3) min(-1)), and to a lesser extent, pH (k(Asp) at 95 degrees C was lowered from 0.46 to 0.37x10(-3) min(-1) following a decrease of pH from 7.0 to 4.0). No significant effects on the racemization rate of aspartic acid was observed for reducing the oxygen pressure to 0.8%. The results from the present study show that the content of D-aspartic acid in fish material is a function of heat exposure and may be used to predict the thermal history of fish meal.