Aspartic acid racemization in fish meal as induced by thermal treatment

The effect of heat treatment during fish meal processing on amino acid racemization was studied. The hydrolysis-induced racemization rate (R) and the D-isomer content in the sample before hydrolysis (I) were differentiated by means of deuterium labelling and gas chromatography-mass spectrometry (GC-MS) analysis in selected ion monitoring mode. A preliminary experiment on laboratory-made herring meals cooked at 125°C for different time intervals showed aspartic acid (Asp) as the only amino acid with significant racemization before hydrolysis. Aspartic acid racemization rate appeared to be a nearly linear function of the duration of thermal treatment (R2 = 0.93; P <0.01). Analyses were carried out on several samples of commercial fish meals from different origin. Low-temperature-dried fish meals had a D-Asp content, expressed as I = 100 [D-isomer concentration before hydrolysis I (D- + L-isomer concentration before hydrolysis)], at less than 1%, while the D-isomer content of high-temperature dried fish meals exceeded 2%. Differences between the two commercial categories were statistically significant (P < 0.001). Further studies are required in order to evaluate the effects of D-Asp in protein of fish feeds and the role of the raw material and processing parameters in inducing amino acid racemization in protein of fish meals.