Standardization of methods for detecting heat resistant fungi.

Heat resistant fungi can be defined as those capable of surviving temperatures at or above 75°C for 30 or more minutes. The fungal structures which can survive these temperatures are ascospores, and sometimes chlamydospores, thick walled hyphae or sclerotia (Scholte et al., 2000). During the last three years, spoilage incidents involving heat resistant fungi occurred increasingly in various products examined in our laboratory. Paecilomyces variotii, Fusarium oxysporum, Byssochlamys fulva, B. nivea, Talaromyces trachyspermus and Neosartorya species were often encountered in pasteurized fruit, dairy products and soft drinks. A questionnaire sent to many laboratories showed that inappropriate methods were used for the detection of heat resistant fungi, or that sometimes there was no special protocol at all. The use of inappropriate media, such as Sabouraud agar, wrong incubation conditions and the analysis of inadequately sized samples were often encountered. In addition, accurate identification of the isolated colonies to species level often was not performed. In the literature many methods are described for the detection of heat resistant fungi (Murdock and Hatcher, 1978; Beuchat and Rice, 1979; Beuchat and Pitt, 1992). Beuchat and Pitt (1992) described two methods: the Petri dish method or plating method and the direct incubation method. In the first method, test tubes are used for the heating of the sample. Subsequently, the sample is poured into large Petri