Survival and growth of Bacillus cereus during Gouda cheese manufacturing

Survival and growth of Bacillus cereus was investigated during manufacturing of Gouda type cheese. The cheese was prepared in the pilot plant from pasteurised milk artificially contaminated with spores to give a final concentration of approximately 102 B. cereus spores per millilitre of cheese milk. B. cereus was enumerated by surface plating on B. cereus selective media and lactic acid bacteria were enumerated on lactic agar and MRS agar (de Man-Rogosa-Sharpe). Samples were taken for microbiological analysis of the milk before renneting, curd at cutting, at half whey removal, at final whey removal, at hooping of the curd, the cheese after pressing, after brining, after 1 week, after 2 weeks, after 4 weeks and after 6 weeks. The spores germinated into vegetative cells, which grew and reached a maximum of approximately 104 CFU per gram of cheese at hooping (about 4 h after renneting). After pressing (approximately 16 h after renneting ) the viable cells were reduced to less than 102 CFU per gram. After brining (about 40 h after renneting) B. cereus was not detected in the cheese curd. At this stage the conditions of the cheese, particularly lower moisture content and aw, lower Eh, high salt content, depleted lactose content combined with high acidity may have inhibited the growth of B. cereus. B. cereus did not affect the growth of lactic acid bacteria during cheese manufacturing. Lactic acid bacteria grew from 107 to 109 CFU per gram of curd during cheese manufacturing and stayed fairly constant at about 109 for 6 weeks.