Proteolytic side-activity of lactase preparations

Storage-induced changes in hydrolysed-lactose (LH) ultra-high-temperature (UHT)-treated milk were studied using proteomics. Five commercial lactase preparations were used to hydrolyse lactose in semi-skimmed milk after UHT treatment. The proteolytic side-effects of lactase preparations on UHT milk during storage for up to 60 days were compared with their effects on conventional UHT milk. The level of free amino groups increased more during storage in all LH UHT milk samples than in conventional UHT milk. The increase was partly ascribed to the release of C-terminal amino acid residues from intact β- and αS1-casein, indicating carboxypeptidase activity. During storage, three of the enzyme preparations increased release of peptides derived from the hydrophobic regions of β- and αS1-casein, which could potentially contribute to development of bitterness in the milk samples. The authors propose that residual protease side-activity in the lactase preparations accelerate product deterioration and decrease shelf-life of LH UHT milk.