Challenge Test on Heat-resistant Fungus, Byssochlamys striata Inoculated in Blueberry Juice and Subjected to Heat Treatments

Introduction: Some strains of heat-resistant fungi have been reported to tolerate food processing steps such as heating. Spoilage caused by these organisms has frequently been reported in fruit and fruit juices. Byssochlamys striata, initially isolated from blueberry, produces eight-spored asci and has been shown to be resistant to heat treatments.    

Purpose: The objectives of this study were to investigate the response of heat-resistant fungal spores of B. striata ATCC 10501 at different growth age to various heat treatments and establish heat treatment time and temperature conditions to inactivate this fungus.

Methods: The 15 and 30 days old heat- resistant fungal spores were collected and inoculated into the blueberry juice at 10⁴ spores/ml.  Blueberry juice was then subjected to various heat treatments at 80, 85, 90, 93, 95, 96 and 99°C for different times. Mold counts were conducted and expressed as CFU/ml. Survivor curves were plotted, D-values and Z-values were determined and compared using ANOVA.

Results: The 30 days old spores of B. striata had significantly higher thermal resistance than that of the 15 days old spores (P < 0.05). Practical heat inactivation data were generated for different heat, time and temperature combination. The D-values of B. striata at 80, 85, 90, 93, 95, 96 and 99°C were found, and the Z-values calculated from the thermal death time curves were also determined.

Significance: The results established by this study may be used by blueberry processors to prevent losses due to spoilage caused by the heat-resistant microorganisms and to establish appropriate thermal process schedules for blueberry products.