Growth dynamics of Bacillus cereus and shelf-life of pasteurised milk

Growth dynamics of Bacillus cereus in pasteurised milk was examined in storage tests performed at 5, 7, 9, 11, and 13°C. The contents of B. cereus in pasteurised milk ranged from the absence in 4 ml of milk to 2.3 cfu/ml. The initial total plate counts varied from 1.1 × 104 to 3.0 × 104 cfu/ml (n = 15). Growth curves of Bacillus cereus showed that strains naturally present in pasteurised milk grew well at 5 and 7°C. A square root model was used for the growth rate analysis in relation to the storage temperature of milk (√μ = 0.026 (T - Tmin); R2 = 0.93). Lag-time of B. cereus was described by a modified Arrhenius-type equation (λ = -45.667 + 1035.3/T; R2 = 0.94). The comparison of the time prediction calculated for B. cereus to reach the density of 104 cfu/ml in pasteurised milk and that for the total plate counts to reach levels of 5.0 × 104 cfu/ml proved that these plate counts were reached in all tests earlier than the level of 104 cfu/ml could be reached by B. cereus. It is thus concluded that pasteurised milk is spoiled by the growth of saprophytic psychrotrophic bacteria before B. cereus can produce hazardous levels of its enterotoxin.