The different pathways of spore germination and inactivation in dependence of pressure and temperature

Abstract

Bacillus subtilis spores (PS832) and isogenic strains, which lack the germinant receptors (FB114 and FB115), were treated up to 24 h between 100 and 700 MPa and 30–80 °C to improve the understanding of spore inactivation during the high pressure thermal sterilization. Flow cytometry was used to analyze the physiological state of the spores after pressure-treatment. A two step kinetic model was capable to derive a global model with an acceptable fit for all strains. The kinetic data demonstrated that spore germination without germination receptors is possible even at 300 MPa and no inactivation occurred up to 700 MPa and 50 °C. Contrary for PS832, a spore inactivation of more than 3 log 10 was detected at 300 MPa, whereas no inactivation was observed at 550 MPa and moderate temperature. Possibly, spores with nutrient receptors could proceed through stage II of germination at low pressure levels, whereas this step is retarded at higher pressures. The high pressure thermal sterilization (HPTS) is an emerging technology to reduce the thermal load of a sterilized food product, which consequently increases its sensorial and nutritional quality. However, an industrial installation of this promising technology has not been implemented, yet. An improved understanding of the underlying inactivation mechanisms of bacterial spores may contribute to an optimization of this process. The kinetic data of the current manuscript proves that a significant spore inactivation is possible at moderate treatment pressures and temperatures for long pressure dwell times. However, for a full spore inactivation in an economical treatment time, high pressures and elevated temperatures are needed. For B. subtilis a threshold pressure of 600 MPa was identified, whereas above this pressure the process temperature dominates the inactivation rate. Furthermore, the used two step kinetic model, applied for different kinetic data sets showed an excellent fit in the shared pressure–temperature range, which confirms that inactivation data derived under isothermal isobaric conditions could be transferred to other HP units. ► Spore germination is possible without nutrient receptors even at 200 MPa. ► Spores treated at pressure ≤ 400 MPa possibly proceed through germination stage II. ► To germinate the entire spore population high pressures and temperatures are needed. ► A two step kinetic model was capable to derive a global model for all spore strains. ► Kinetics from isothermal isobaric conditions could be transferred to other HP units.