Food sterilisation under high pressure- Fundamentals, new insights and challenges

Consumers all over the world demand high quality and convenient products with natural flavour and taste, and greatly appreciate the fresh appearance of minimally processed food. Sterilization under pressure has been shown to be valuable for global production of high quality products. However, the mechanism of inactivation of bacterial spores by heat and pressure is still a topic of discussion. Obviously, the change of the dissociation equilibrium under pressure and temperature plays a dominant role in inactivation of microorganisms. Heat and pressure inactivation of Geobacillus. stearothermophilus spores at different initial pH-values in ACES and phosphate buffer confirmed this view. Thermal inactivation in ACES buffer at 122°C resulted in higher logarithmic reductions. Contrary, after pressure treatment at 900 MPa with 80°C phosphate buffer showed higher inactivation. These results indicated the different dissociation equilibrium shifts in buffer systems by heat and pressure. Due to preparation, storage and handling of highly concentrated spore suspensions, the clumping and the formation of aggregates can hardly be avoided. After particle analysis it could be shown that the lag phase often found in thermal spore inactivation can sufficiently be described by first-order inactivation kinetics when the agglomeration size is considered. The physiological response of B. licheniformis spores to pressure and thermal inactivation was investigated using multiparameter flow cytometry. For pressure treated spores, four distinct subpopulations were detected. For these sub-populations, we suggest a three step model of inactivation involving a germination step following hydrolysis of the spore cortex, an unknown step, and finally an inactivation step with physical compromise of the spore’s inner membrane. With an improved understanding of the spore inactivation mechanism it will be possible to assess the benefits and sustainability of emerging technologies e.g. pressure assisted thermal sterilization with regards microbiological safety and stability.