Inhibition of Bacillus cereus growth in carbonated fermented bifidus milk

Abstract

A Bacillus cereus strain isolated from commercial pasteurized milk was used to evaluate the carbonation of heat-treated skim milk as a method for inhibiting B. cereus growth during incubation at 37°C and cold storage at 4°C of fermented bifidus milk made with the strain Bifidobacterium infantis ATCC 15702. Carbonated (pH 6.0) both uncontaminated and contaminated (10 cfu ml −1) milks were compared with their respective non-carbonated controls. The population of Bif. infantis in milk was affected neither by CO 2 nor by the presence of B. cereus. The contaminating micro-organism multiplied actively during incubation at 37°C although growth was reduced by approximately 2 log units at 12 h of incubation in carbonated samples (5.80±0.57 log cfu ml −1) with respect to the non-carbonated ones (7.37±0.01 log cfu ml −1). Through the refrigerated storage at 4°C, CO 2 also prevented the proteolytic degradation of contaminated samples, inhibited the formation of organic acids by B. cereus and reduced acetic acid/lactic acid ratios in fermented milk. Finally, the use of milk acidified with carbon dioxide had no detrimental effects on the sensory properties of the product. Therefore, the carbonation of heat-treated milk prior to the bifidobacteria addition may contribute to reduce the contamination risk of bifidus milk by B. cereus.