Occurrence and Thermoresistance of Spores of Psychrophilic and Psychrotrophic Aerobic Sporeformers in Soil and Foods

Abstract

Spores of psychrotrophic (able to grow at 5°C) aerobic sporeformers occurred in soil in high numbers (2 × 10 3 ‐5 × 10 6 /g), whereas psychrophilic (able to grow at 0°C) spores were present at significantly lower levels (500–10 5 /g). Psychrotrophic spores were absent in herbs and spices: in pasteurized meals prepared industrially their numbers varied from <10 to 1000/g. For spores harvested from Trypticase Soy Agar (TSA), the heat resistance of the cold‐tolerant sporeformers was low with D 90°C ‐values from 1–11 min. The recovery of heated psychrophilic spores on this medium at 5°C was equal to their recovery at 20°C. However, the recovery of heated psychrotrophic spores was lower at 5°C than at 20°C, whereas unheated spores gave the same counts at both temperatures. The heat resistance of naturally occurring spores of cold‐tolerant sporeformers washed from soil was comparable with the resistance of spores formed on TSA.