Degree of hydrolysis of the extracts was determined by the mod-ified OPA method (Nielsen et al., 2001). Samples were dissolvedin OPA reagent (o-phthaldialdehyde in ethanol and SDS), and the spectrophotometric absorbance was measured at 340 nm and cor-rected with absorbance of the unhydrolysed sample. Serine was used as the standard.