100A-11

Yeast response in laboratory media formulated at a_w 0.95, pH 4.5 or 3.5 when using sodium chloride, sucrose or glucose to adjust a_w, in combination with acetic, citric, phosphoric or tartaric acid to reduced pH was evaluated.

For each yeast, media formulated with each factor combination was inoculated with 10³-10⁴ CFU mL^-1, and incubated at 27ºC up to 200 h. Periodically during incubation, the number of viable cells was determined and compared with the initial population, determining if growth, inhibition (growth delay) or yeast death occurred.

An ANOVA demonstrates that individual variables as well as their interactions significantly (p<0.05) affected each yeast response. Acetic acid significantly (p<0.05) inhibit yeast growth in more cases than the other evaluated acids; including yeast type, growth was observed in only 12 of the 30 evaluated combinations. With tartaric acid growth was observed in 20 cases, in 22 cases when using citric acid and in 23 with phosphoric acid. At pH 3.5, acetic acid promoted cell death independently of solute type. The use of sodium chloride to adjust a_w causes yeast inhibition or death in more cases than the use of glucose or sucrose.

The studied yeasts not have the same behavior to equal combinations of a_w and pH, demonstrating that acid and solute type exhibited an important effect on their response. D. hansenii was more resistant to the evaluated effects, followed by C. krusei, and Z. bailii, being S. cerevisiae the most sensitive.