Mathematical models for the biofilm formation of Geobacillus and Anoxybacillus on stainless steel surface in whole milk

Biofilm formation of Geobacillus thermodenitrificans, Geobacillus thermoglucosidans and Anoxybacillus flavithermus in milk on stainless steel were monitored at 55, 60, and 65 °C for various incubation times. Although species of Geobacillus showed a rapid response and produced biofilm within 4 h on stainless steel, a delay (lag time) was observed for Anoxybacillus. A hyperbolic equation and a hyperbolic equation with lag could be used to describe the biofilm formation of Geobacillus and Anoxybacillus, respectively. The highest biofilm formation amount was obtained at 60 °C for both Geobacillus and Anoxybacillus. However, the biofilm formation rates indicated that the lowest rates of formation were obtained at 60 °C for Geobacillus. Moreover, biofilm formation rates of G. thermodenitrificans (1.2-1.6 log₁₀CFU/mL∙h) were higher than G.thermoglucosidans (0.4-0.7 log₁₀CFU/mL∙h). Although A. flavithermus had the highest formation rate values (2.7-3.6 log₁₀CFU/mL∙h), this was attained after the lag period (4 or 5 h). This study revealed that modeling could be used to describe the biofilm formation of thermophilic bacilli in milk.