BITTER TASTE OF ENZYMATIC HYDROLYSATES OF CASEIN .1. ISOLATION, STRUCTURAL AND SENSORY ANALYSIS OF PEPTIDES FROM TRYPTIC HYDROLYSATES OF BETA-CASEIN

Beta-Casein A2 was isolated from milk of a homozygous cow and hydrolys ed with trypsin. The hydrolysate was separated by RP-HPLC into 18 pept ides, all but one of which could be attributed to the sequence of beta -casein on the basis of the amino acid composition. Some peptides over lapped. In total, they represented about 97% of the protein sequence. Only three peptides had a bitter taste, namely I49-N68 (recognition th reshold 1.0 mg/ml, 0.45 mmol/l), I49-K97 (1.5 mg/ml, 0.28 mmol/l) and G203-V209 (0.175 mg/ml, 0.23 mmol/l). The contribution of the three pe ptides to the overall bitterness of the beta-casein hydrolysate (2.67 mg/ml) was about 11, 21, and 60%, respectively. Peptide I49-K97 was pr esent in the hydrolysate together with its fragments I49-N68 and S69-K 97. Remarkably, the smaller and more hydrophobic fragment I49-N68 was less bitter than 149-K97 on a molar basis, whereas the larger and more hydrophilic fragment S69-K97 had a neutral taste. These results show that in the case of larger peptides neither hydrophobicity nor size ar e responsible alone for bitter potency, but that conformational parame ters must be of great importance. Furthermore, it can be concluded tha t only a part of the structure is responsible for the contact with the receptor. The bitterness of G203-V209 is discussed in connection with related synthetic peptides in the literature.