CONCURRENT EFFECTS OF HIGH HYDROSTATIC-PRESSURE, ACIDITY AND HEAT ON THE DESTRUCTION AND INJURY OF YEASTS

Saccharomyces cerevisiae ATCC 2373 and Zygosaccharomyces bailii ATCC 3 6947 were exposed to hydrostatic pressures ranging from 1,500 to 3,000 atmospheres for 10, 20 and 30 min in 0.1 M citrate buffer at pH 3.0, 4.0 and 5.0 at 25 and 45-degrees-C. Inactivation of inoculated yeast c ultures was achieved in spaghetti sauce with meat at 25-degrees-C with 3,000 atmospheres for 10 min and also at 45-degrees-C and 2,500 atmos pheres for 10 min. Viable counts were determined on potato dextrose ag ar (PDA) incubated at 30-degrees-C for 48 h. Pressure-induced injury w as demonstrated by plate count differential between PDA and PDA supple mented with glucose (PDAG). A reduction of 7-log10 cycles colony formi ng units (CFU)/ml was seen for both strains at 3,000 atmospheres for 1 0 min at 25-degrees-C at all pH levels and at 2,250 atmospheres, pH 5. 0 for 20 min at 45-degrees-C. At 2,000 atmospheres, pH 3.0 for 30 min, the increase in temperature from 25 to 45-degrees-C increased the ina ctivation of yeast by 6-log10 cycles. Lowering the pH from 5.0 to 3.0 enhanced lethality up to 2-log10 cycles at 2,250 atmospheres, 25-degre es-C for 30 min. Injury was most apparent at exposure parameters that produced 3- to 5-log10 cycle reductions on PDA. This was achieved (99% injury) at 2,250 atmospheres, 25-degrees-C for 30 min. These data ind icate that mild heat and acidity contribute to the effectiveness of th e inactivation and injury of yeast by high hydrostatic pressure (HHP).