Geobacillus spp. are thermophilic Gram-positive endospore-forming bacteria capable of forming biofilms, and are encountered in a wide range of environments. In the food industry, biofilm formation and subsequent release of spores can limit production run times (between cleaning). Alternative means of controlling spore development in biofilms may extend production times, enhance productivity, improve product quality and increase revenue. Sixteen strains of Geobacillus spp, isolated from milk powders plants, were characterised. The growth rates in response to temperature were assessed by culture on agar media, and by optical density using a Bioscreen instrument. Based on its consistent growth responses compared to the other 15 strains, strain W14 was selected for further research. A temperature-controlled continuous flow-through device was designed, built and used in experiments intended to mimic the evaporator section of milk powder plants. Sterile milk, inoculated with various spore loads, was fed continuously into the flow-through device and viable counts and spores enumerated in the effluent over time. Six temperatures (45 - 74°C), two flow rates and different modes of introduction of inocula were used to infer the development of biofilms from cell and spore counts in the milk leaving the chamber over time. The results showed growth rates were maximal over a broad temperature range (54 - 64°C), and that generation times in this range were of the order 20 - 25 minutes. Viable cell numbers increased after approximately 3 - 6 hours at different temperatures, while spore counts began to increase after 6 - 8 hours, irrespective of the flow rate or method of inoculation.
Keywords : Thermophilic bacilli, milk powder, non-continuous flow through, spore cells.