Effects of fat, casein and lactose on high-pressure destruction of Escherichia coli K12 (ATCC-29055) in milk

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High-pressure (HP) destruction of Escherichia coli K12 suspended in commercial pasteurized milk (0, 3.25 and 5% fat), peptone water (0.1%), and phosphate buffer (0.2 mol/l Na 2HPO 4 and 0.2 mol/l NaH 2PO 4) supplemented with different amounts of casein or lactose (1–4%) was investigated. HP treatment resulted in the highest level of E. coli destruction in the buffer (possibly because it contained no nutrients), followed by those in peptone water and least in milk. Therefore, milk and peptone provided some baro-protection to the destruction of E. coli during the HP treatment. Fat content in milk between 0, 3.25 and 5% had no significant ( P > 0.05) influence on the HP destruction of E. coli. The supplement of either casein or lactose to milk did not influence the level of HP destruction of E. coli. However, the baro-protection became significant during HP treatment when casein or lactose was added the buffer. The buffer solution supplemented with 1–4% casein showed a baro-protective effect equivalent to that in milk. Lactose supplement to buffer also caused baro-protection, though the effect was relatively less pronounced. However, no significant difference in the baro-protection effect was observed among the buffer samples supplemented with different levels (1, 2 and 4%) of lactose or casein. Therefore the major contributors for baro-protection of E. coli in milk during HP treatment appear to be casein and lactose, rather than the fat content.