Intermediate Thermoresistance in Black Yeast Asexual Cells Variably Increases with Culture Age, Promoting Survival and Spoilage in Thermally Processed Shelf-Stable Foods

Abstract

Black yeasts are a functional group that has caused spoilage in cold-filled and hot-filled beverages, as well as other water activity-controlled food products. We established quantitative thermoresistance parameters for the inactivation of 12 Aureobasidium and Exophiala isolates through isothermal experiments and a challenge study. Culture age (2 versus 28 days) variably affected the thermoresisitance among the black yeast strains. Variation in thermoresistance exists within each genus, but the two most resistant strains were the Exophiala isolates. The two most heat-resistant isolates were Exophiala phaeomuriformis FSL-E2-0572, with a D60-value of 7.69 ± 0.63 min in 28-day culture and Exophiala dermatitidis YB-734, with a D60-value of 16.32 ± 2.13 min in 28-day culture. Although these thermoresistance levels were, in some cases, greater than those for conidia and vegetative cells from other common food spoilage fungi, they were much more sensitive than the ascospores of heat-resistant molds most associated with spoilage of hot-filled products. However, given that black yeasts have caused spoilage in hot-filled products, we hypothesized that this intermediate degree of thermoresistance may support survival following introduction during active cooling before package seals have formed. A challenge study was performed in an acidic (apple cider) and water activity-controlled (maple syrup) product to evaluate survival. When apple cider was hot filled at 82°C, black yeast counts were reduced by 4.1 log CFU/mL 24 h after the heat treatment, but the survivors increased up to 6.7 log CFU/mL after 2 weeks. In comparison, the counts were below the detection limit after both 24 h and 14 days of shelf life in both products when filled at the boiling points. This suggests that ensuring water microbial quality in cooling tunnels and nozzle sanitation may be essential in mitigating the introduction of these fungi.