Heat resistance of ascospores of Neosartorya fischeri as affected by sporulation and heating medium

Abstract

Ascospores of three strains of Neosartorya fischeri grown on three different sporulation media were evaluated for heat resistance in apple juice, grape juice and 0.1 M potassium phosphate buffer (pH 7.0). The type of sporulation medium did not affect heat resistance of ascospores. Ascospores (approx. 10 6/ml, initial viable population) of all three strains survived heat treatment at 84° C for 120 min. The rate of thermal inactivation of ascospores was lower in apple juice than in grape juice and phosphate buffer. Fumaric acid (1 and 2%) at pH 2.5, 3.0 and 3.5 greatly increased the rate of heat inactivation of ascospores. At similar concentrations and pH values, acetic, citric and tartaric acids resulted in moderate increases in inactivation rates. Malic acid had little influence on rates of inactivation. Rates of inactivation increased as the pH of heating media containing fumaric, citric, tartaric and acidic acids decreased. Thus, the pH and the type, percentage and molarity of organic acid in the heating medium act synergistically with heat to inactivate ascospores of N. fischeri.