Occurrence and ecological distribution of Heat Resistant Moulds Spores (HRMS) in raw materials used by food industry and thermal characterization of two Talaromyces isolates

In this study, screening of some raw materials used to produce pasteurized products was carried out to determine the occurrence and ecological distribution of heat-resistant fungi. The search for Heat Resistant Mould Spores (HRMS) resulted in the isolation of a limited number of fungal genera: Arthrinium, Aspergillus with either Eurotium-type or Neosartorya-type ascoma, Byssochlamys, Hyphodermella, Monascus, Penicillium, Rasamsonia, Talaromyces and Thermoascus. Sexual aspergilli constituted an overwhelming percentage of the mycobiota, totaling 93.5% of the heat-resistant fungi detected, and being the only fungi to be simultaneously detected in discrete concentrations on almost all matrices found positive for HRMS. Talaromyces spp., Penicillium spp. and Monascus sp. occurred at low percentages (up to 2.1%), though they were the most commonly occurring genera in lemon cells (Talaromyces, Monascus) and blueberries (Penicillium spp.). Among these isolates, two Talaromyces spp. (T. trachyspermus and T. bacillisporus) were tested for heat-resistance in both blueberry and grape juice or in buffered glucose solution, in order to assess their D- and z-values. Data obtained from thermal death curves and statistical elaboration of raw data showed that D-values of T. trachyspermus ranged between 50.0 and 90.9 min at 75 °C; 13.6 and 20.8 min at 78 °C; 5.1 and 12.4 min at 80 °C; 1.6 and 2.6 min at 82 °C. D values of T. bacillisporus ranged between 44.4 and 60.9 min at 82 °C; 11.9 and 15.5 min at 85 °C; 2.7 and 4.1 min at 88 °C and were equal to 1.2 min at 91 °C, depending on the medium. The heating times needed for inactivation were comparable to those applied to most heat-resistant species, but significantly lower than those applied to Talaromyces macrosporus or less common ascospore-forming fungal species such as Hamigera avellanea and Thermoascus crustaceus. Therefore, a traditional pasteurization process would be insufficient to avoid potential spoilage problems with T. trachyspermus or T. bacillisporus, even if HRMS contamination of the raw materials processed by food industries is generally low (< 100 CFU/kg), since the food industry generally tries to achieve five or more log-reduction in their products.