Effects of weak acid preservatives on the growth and thermal death of the yeast Pichia membranifaciens in a commercial apple juice

Pichia membranifaciens exhibited a dissociative temperature profile (the temperature range of thermal death was distinct from the temperature range of growth) when incubation took place either in a commercial apple juice (AJ) or in a synthetic mineral medium with glucose and vitamins (MGV). In AJ the maximum temperature for growth (Tmax) was 38.6°C, which decreased to 36°C in the presence of either 1 mM sorbic or 1 mM benzoic acid. The minimum temperatures of thermal death (Tmind) were, respectively, 40 and 38°C with either of the acids. The yeast could grow with up to 2 mM sorbic or 3 mM benzoic acid, at 25°C, which is close to the optimum temperature for growth (Top). At temperatures slightly above Tmind, sorbic acid was an actual enhancer of death rather than benzoic, the latter conferring some protection. However, these effects were reversed at higher temperatures (above 43°C), at which benzoic acid was the most operative, in contrast to sorbic which was highly protective of the yeast against thermal death. The addition of acetaldehyde to sulphur-dioxide-containing juice reduced the lag phase and increased the overall specific growth rates. Sporulated or stationary vegetative cultures were more heat-resistant than exponential cultures, particularly at temperatures above 45°C.