High hydrostatic pressure come-up time and yeast viability

The effects of the come-up time at selected pressures (50 to 689 MPa) on Sa ccharomyces cerevisiae and Zygosaccharomyces bailii viability were evaluate d at 21 degrees C. For Z. bailii the effects of the water activity (a,) of the suspension media and the stage of the growth cycle were also investigat ed. Pressure come-up times exerted an important effect on the yeast surviva l fraction, decreasing counts as pressure increased. An increased sensitivi ty to pressure treatments was observed with yeast cells from the exponentia l growth phase. Lethality increased as a(w) of the suspension media increas ed. For an a, of 0.98 and cells from the stationary growth phase, pressure treatments at less than 200 MPa had no effect on Z. bailii viability; howev er, no survivors (<10 CFU/ml) were observed in treatments applied only for the time needed to reach pressures greater than 517 MPa. Yeast survivor cur ves showed an excellent fit (r > 0.996) when described by a phenomenologica l model based on the Fermi equation, S(P) = 1/{1 + exp[(P - P-c)/k]}, where S(P) is the survival fraction, P is the pressure, P-c is a critical pressu re corresponding to 50% survival, and k is a constant representing the stee pness of the curve.