Thermophilic spore-forming bacteria (TSFB) of the genera Geobacillus and Anoxybacillus can develop to high numbers on hot surfaces in milk powder processing plants where conditions are favorable for attachment, colonization and biofilm development. High spores loads in end-product develop after 16-24 hours of continuous process operation and, while not harmful to consumers, render milk powder products commercially unacceptable. Thus, the manufacturing process must be stopped periodically stopped for cleaning, an expensive undertaking. Being able to limit thermophile growth in milk powder processing plants promises considerable cost savings.

To enable prediction of growth of TSFB under time-varying temperature and water activity conditions relevant to powder processing plants, a mathematical model was developed from >200 growth rate determinations of 16 strains of Geobacillus spp. (The intent of the model was to identify probable ‘hot spots’ of colonization in powder plants.

 A representative strain, G. stearothermophilus W14, was selected for further studies. W14 biofilm development on stainless steel surfaces exposed to flowing milk, at temperatures from 45-75^°C, was assessed using a flow through device mimicking powder plant evaporator sections. Vegetative cells (VC) and spores in the milk effluent were enumerated frequently over 24h periods.

At optimal temperatures, VC initially decreased in the milk effluent, but then increased after approximately 6 hours. Spores increased after 8 hours but had counts always 2 to 3 orders of magnitude lower than VC. These time dependent changes reflect attachment, biofilm development, sporulation, and detachment processes.

The flow through system is a useful to study the growth and sporulation of thermophilic spore formers, and to explore potential interventions against spore contamination. Experiments in which temperature was cycled during the 24h processing time revealed substantially lower G. stearothermophilus spore counts for up to 24h at near optimum temperature, compared to numbers at equivalent constant temperature treatments and suggest that process run times may be able to be extended.