Effect of Bacillus cereus enzymes on the milk quality following ultra high temperature processing

Abstract

Using a model case of contamination of long-life semi-skimmed milk with the spores of six B. cereus strains, isolated from the farm environment and raw milk, proteolysis was monitored by measuring changes in protein content by infra-red spectroscopy; free tyrosine was measured by the Lowry method according to Juffs, and the reduction in casein fractions by SDS-PAGE. Lipolysis was monitored by the dilution extractive method. At a storage temperature of 4°C for 4 months no enzyme processes were observed, whereas at a storage temperature of 24°C a marked enzyme activity was found during maximum 3 weeks as well as sensory changes of UHT milk. After three weeks of storage, a reduction in protein content from 34.55 g·1-1 milk to 29.46 ± 2.00 g·1-1 milk, and a reduction in the free tyrosine from 0.65 to 2.13 ± 0.28 mg·ml-1 was found, as well as increased molar contents of free fatty acids (FFA) from 41.97 to 1617.22 ± 68.17 mmol·kg-1 milk fat. After six days of storage, α-casein, β-casein and κ-casein dropped to 69 ± 10%, 56 ± 16% and 43 ± 10%, respectively. Majority of changes in UHT milk depended on the B. cereus strain used, initial microbial counts and the method of heat inactivation of spores.