The proline-rich -casein was digested in vitro with trypsin, and the oligopeptides produced were then isolated by RP-HPLC and subsequently identified by amino acid analysis and ion mass spectrometry. The peptide fractions from the complete digestion were then treated with purified x-prolyl dipeptidyl peptidase (X-PDP) extracted from Lactobacillus casei ssp. casei LLG. Two bitter peptides (f53-97 and f203-209) containing X-Pro-Y-Pro in their amino acid residues were completely hydrolyzed by X-PDP while several peptides with a high degree of hydrophobicity were also decreased in peak area. Debittering effect of X-PDP from Lactobacillus casei ssp. casei LLG on enzyme modified cheese (EMC) was also investigated both by subjective and objective methods. The bitterness of Cheddar cheese slurries supplemented with Neutrase® 0.5 L was completely eliminated after treatment with crude enzyme extract from Lactobacillus casei ssp. casei LLG. Two hydrophobic peptides in EMC with Ala-Pro-Phe-Pro-Glu-Val, and Phe-Leu-Leu residues were hydrolyzed by crude enzyme extract. The RP-HPLC and subsequently ion mass spectrometry analysis have shown that the debittering effect on EMC was due partially to the presence of X-PDP.

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