Factors influencing the recovery of psychrotrophic, mesophilic and thermophilic Bacillus spp from bulk raw milk

Abstract

Because of the wide range of physiological properties found in the genus Bacillus one of the main problems in the standard procedure for isolating these organisms from milk and dairy products is the difficulty in defining conditions which are suitable for the activation and outgrowth of all spores present. This study investigated the effects of various raw milk heat treatments, the addition of L‐alanine (a nutrient encouraging germination) to the milk after heat treatment, different incubation temperature‐time combinations and two confirmatory media on the recovery of psychrotrophic, mesophilic and thermophilic Bacillus spp occurring in raw bulked milk. There was no significant difference (p >0.05) between the confirmatory media used, ie, starch milk agar and milk plate count agar. However, it was found that heat treatment, addition of L‐alanine and the incubation period had significant effects on the recovery of the organisms from raw milk. The highest numbers of psychrotrophic and mesophilic spores were recovered after a heat treatment of 80d̀C for 10 min followed by the addition of L‐alanine to the milk at 0.1% wlv, and incubating at 6.5°C for 15 days for the former and 30d̀C for 15 days for the latter. The highest numbers of thermophilic spores were recovered from raw milk by a heat treatment of 80d̀C for 30 min, addition of L‐alanine and incubating at 55°C for 7 days.