In Vitro Enzymatic Determination of the Protein Nutritional Value and the Amount of Available Lysine in Extruded Cereal-based Products

Abstract

An in vitro method, using proteolytic enzymes for protein hydrolysis, was used to evaluate the effect of extrusion cooking on protein quality. The products studied were protein-enriched biscuits processed in a twin-screw extruder under different conditions. In the product processed at low temperature (170°C), the digestibility was comparable with that of the raw material. With increasing temperature (170 to 210°C), the in vitro availability of amino acids decreased. This decrease was most pronounced in the case of lysine, but the recovery of methionine, arginine, glycine, aspartic acid and aspargine was also affected. A loss of lysine, methionine and arginine could also be detected after acid hydrolysis. The total amount of lysine present in the raw material was not completely available, as judged from the in vitro assay as well as from balance experiments in rats. With increasing severity of heat treatment, the loss of biologically available lysine was more pronounced than the loss of total lysine. However, the loss of enzymatically available lysine was in good agreement with the results obtained in animal experiments. An increase in moisture content of the feed during processing, from 13 to 18%, improved the availability of lysine in particular.