All protein fractions were analysed by SDS poly- acrylamide gel electrophoresis according to the method of Laemmli (1970).
The extent of proteolysis was monitored by the modified method of Greene and Babbitt (1990). The oligopeptide content in the supernatant was measured by the method of Lowry, Rosebrough, Farr, and Randall (1951) and expressed as micromole of tyrosine released/g dry matter. Free a-amino acid was deter- mined using TNBS according the method of Benjakul and Morrissey (1997). Free a-amino acid was calculated
and expressed as micromoles of leucine/g (dry weight