At near optimal temperatures ( i.e., 60 – 65 ☌) viable cell and spores counts initially decreased in the milk effluent but began to increase consistently after 3-6 h indicating attachment, germination and proliferation of cells and production of spores. Studies were conducted at temperatures from 45 to 70 ☌. The kinetics of cell and spore attachment to stainless steel were studied by inoculating the flow-through system with spores of G. stearothermophilus W14 in the milk flowing through the system and monitoring the change over time in vegetative cells and spores in the milk leaving the system (i.e., the ‘effluent’). G. stearothermophilus strain W14 showed fastest growth at ~60 ☌ in anaerobic conditions with a doubling time of 26 min for vegetative cells, which was similar to growth rates under analogous aerobic conditions. The bioreactor was employed mainly to enable a comparison of aerobic and anaerobic growth rates by reliably producing anaerobic conditions. Growth rate data of G. stearothermophilus strain W14 under either anaerobic (100% N2) or aerobic (20% O2) conditions were also generated using a bioreactor (fermenter apparatus) at 55 ☌, 60 ☌, 65 ☌, and 70 ☌. Growth rates observed for G. stearothermophilus W14 were representative of the average response of all strains and this strain was selected for use in subsequent studies. Nonetheless, no strain had growth rates that were systematically different to that of the pooled data. The results also showed that growth rate variability of all 16 strains is large compared to growth rate variability reported for non-spore forming cells. Studies at different water activities (aw) suggested that the minimum water activity for growth was ~0.975. Consistent with published reports the generation time at 60 ☌ was estimated to be ~ 22 – 25 min. The studies showed that growth occurred in the temperature range 45 to ~70 ☌ with fastest growth occurring at ~60 ☌. Over 300 growth curves were generated at temperatures in the range 45 to 75 ☌, using different incubation methods, enumeration methods and growth media, although not all data sets were used because many were deemed to be unreliable due to insufficient or erratic growth of the spore-forming thermophiles under apparently well-controlled growth conditions, a phenomenon reported anecdotally by others. under time-varying temperature conditions and to identify temperatures optimal for growth and biofilm formation. The model was developed to predict the growth of Geobacillus spp. were modelled as function of temperature using a four-parameter square-root (‘Ratkowsky’) model. Growth rates of the 16 strains of Geobacillus spp.
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