Kinetic analysis and modelling of combined high-pressure-temperature inactivation of the yeast Zygosaccharomyces bailii

Eight foodborne yeasts were screened for sensitivity to high-pressure (HP) inactivation under a limited number of pressure-temperature combinations. T he most resistant strains were Zygoascus hellenicus and Zygosaccharomyces b ailii. The latter was taken for a detailed study of inactivation kinetics o ver a wide range of pressures (120-320 MPa) and temperatures (-5 to 45 degr ees C). Isobaric and isothermal inactivation experiments were conducted in Tris-HCl buffer pH 6.5 for 48 different combinations of pressure and temper ature. Inactivation was biphasic, with a first phase encompassing four to s ix decades and being described by first-order kinetics, followed by a taili ng phase. Decimal reduction times (D) were calculated for the first-order i nactivation phase and their temperature and pressure dependence was describ ed. At constant temperature, D decreased with increasing pressure as expect ed. At constant pressure, D showed a maximum at around 20 degrees C, and de creased both at lower and at higher temperatures. A mathematical expression was developed to describe accurately the inactivation of Z. bailii as a fu nction of pressure and temperature under the experimental conditions employ ed. A limited number of experiments in buffer at low pH (3-6) suggest that the model is, in principle, applicable at low pH. In apple and orange juice however, higher inactivation than predicted by the model was achieved. (C) 2000 Elsevier Science B.V. All rights reserved.