Impact of sublethal treatments on dormant and non-dormant populations of Bacillus cereus spores during spray drying and on their recovery during whole milk powder storage

Abstract

This study evaluated the resistance induced by sublethal thermal stress to mimic heat processes employed during dried milk production chain on the survival of Bacillus cereus spores during spray drying. Three B. cereus strains (strains 436, B63 and 540) were exposed to four sublethal stresses before spray drying of whole milk. In treatments 1, 2 and 3, heat shocks were applied at two different temperatures (80 °C and 90 °C) and exposure time ranged between 10 and 30 min. Treatment 4 was a combination of heat shock at 72 °C/15 s, cold shock at 4 °C/12 h and another heat shock at 72 °C/15s. The counts of B. cereus spores were conducted with no heat shock and also applying heat shock (75 °C/20 min). The powdered milk obtained was stored at room temperature for 180 days and the counts of B. cereus were also determined employing no heat shock and heat shock (75 °C/20 min). The results indicated that sublethal treatments increased spray drying resistance of strains B63 and 540 and their survival during the storage period. The counts of B. cereus spores after 180 days of storage varied between <1.00 and 4.4 log₁₀ spores/g of dry weight. These results demonstrate the importance of constant monitoring of the milk quality; once the thermal stresses occur during the dried milk production chain, it may increase the resistance of B. cereus spores, making it hard to effectively control this bacterium.