Influence of sporulation aw on heat resistance and germination of Penicillium roqueforti spores

Spores of Penicillium roqueforti were produced using potato dextrose agar which was solute adjusted using either NaCl, sucrose, or glycerol to water activity (a(w)) levels of: 0.99 (control), 0.97, 0.90 and 0.88. All spore crops, regardless of the sporulation a(w) on which they were produced, exhibited near total germination within 24 h at 22 degrees C in sterile water. However, when spores were initially heated either in water (42 degrees C, 2 h or 55 degrees C, 10 min) or in air (60 degrees C, 20 min) and then germinated, levels were observed to decrease particularly for spores produced on 0.88 a(w), adjusted media. Germination levels were also observed to decrease with time when harvested spores were initially suspended in sterile distilled water (0 to 24 h, 2 degrees C) and then heated. Control spores (0.99 a(w)) showed no effects. Heat studies of spores suspended in water were used to calculate thermal decimal reduction times (D). Control spores and spores produced at 0.88 a(w) with glycerol, exhibited D-55 degrees C values of 10.3 and 2.3 min, respectively. Specific effects attributable to the nature of the solute appeared to influence D values; however, no clear trend was observed. The results of this study indicated that spores that were produced at reduced a(w) became heat-sensitized perhaps owing to injury and/or damage incurred during sporogenesis. (C) 1998 Academic Press Limited.