Proteolytic changes in lactose hydrolysed UHT milks during storage

摘要

Proteolytic changes in lactose unhydrolysed and hydrolysed direct UHT-treated milks were studied during a storage period of 12 weeks. Enzymatic hydrolysis was performed either before (=prehydrolysed) or after (=posthydrolysed) UHT-treatment. The enzymatic hydrolysis of lactose resulted in an increase in proteolysis compared to unhydrolysed milk, during the storage regardless whether hydrolysis was performed before or after the UHT-treatment. The highest degree of proteolysis was found at the higheststorage temperature tested (45°C), while proteolysis was almost non existent at the lowest storage temperature of 5°C as measured by a-amino nitrogen/total nitrogen or as changes in SDS-PAGE analyses. Proteolysis was also noticed in unhydrolysed milk where it was caused by the plasmin enzyme system and possibly by the microbial contaminants in milk. The pH in milks decreased during the storage at 22, 30 and 45°C respectively and there was high correlation (r~2=0.865) between pH drop and a-amino nitrogen/total nitrogen change.