The effect of the extrusion temperature and the residence time on the D-amino acid content of corn extrudates

Abstract

Racemization of peptide-bonded amino acids results in a decrease of protein digestibility, and long-term negative health consequences due to the intake of dietary Damino acids cannot be excluded. Temperature and time dependence of racemization has been investigated in strong alkaline solutions mostly in cases of clear proteins. The aim of the research was to determine the amount of D-enantiomers and the level of racemization of amino acids with the highest rate of racemization and occurring in the largest quantities in corn grain extrudates treated with different heat effects (temperature and residence time combinations). Extrusion trials below 144°C with residence times of 28-72 s did not induce significant (P<0.05) racemization. Treatment at 175ºC and at 200ºC induced significant racemization of aspartic acid (2.4% and 6.1%, respectively). In case of serine and glutamic acid the ratio of D-enantiomers increased significantly due to the treatments at 200ºC (0.75% and 0.69%, respectively). The L-aspartic acid and L-lyisine content of the products extruded at 200ºC were significantly lower than in control and in products produced at lower temperatures. The primary cause of the loss of L-aspartic acid was D-aspartic acid formation. In contrast, racemization of lysine played a minor role in the decrease of L-lysine content.