Inactivation of Spores of Bacillus cereus in Cheese by High Hydrostatic Pressure with the Addition of Nisin or Lysozyme

The objective of this work was to study high hydrostatic pressure (HHP) inactivation of spores of Bacillus cereus ATCC 9139 inoculated in model cheeses made of raw milk, together with the effects of the addition of nisin or lysozyme. The concentration of spores in model cheeses was approximately 6-log10cfu/g of cheese. Cheeses were vacuum packed and stored at 8°C. All samples except controls were submitted to a germination cycle of 60MPa at 30°C for 210min, to a vegetative cells destruction cycle of 300 or 400MPa at 30°C for 15min, or to both treatments. Bacillus cereus counts were measured 24h and 15 d after HHP treatment. The combination of both cycles improved the efficiency of the whole treatment. When the second pressure-cycle was of 400MPa, the highest inactivation (2.4±0.1-log10cfu/g) was obtained with the presence of nisin (1.56mg/L of milk), whereas lysozyme (22.4mg/L of milk) did not increase sensitivity of the spores to HHP. For nisin (0.05 and 1.56mg/L of milk), no significant differences were found between counts at 24h and 15 d after treatment. Considering that mesophilic spore counts usually range from 2.6 to 3.0 log10cfu/ml in raw milk, HHP at mild temperatures with the addition of nisin may be useful for improving safety and preservation of soft curd cheeses made from raw milk.