Purpose: We illustrate the utilization of a novel, recently identified antifungal protein derived from Bacillus thuringiensis as a potential protectant against fungal spoilage.
Methods: Juice and tea matrices were inoculated with approximately 5-log Byssochlamys fulva H25 spores and incubated with serial two-fold dilutions of semi-purified antifungal protein. Minimum inhibitory concentrations (MICs) were evaluated in triplicate using a modification of the XTT cell proliferation assay while ambient temperature shelf life studies were inspected daily for visual signs of mold growth.
Results: The antifungal was active over a wide range of fruit juices and acidified tea beverages with MICs ranging from 1.56 ppm in apple juice and select acidified teas to approximately 3-12 ppm for lemonade, orange, white cranberry, blueberry, prune, cherry and grape juices. Apple cider and non-acidified teas generally exhibit a reduced efficacy with MICs exceeding 100 ppm. The antifungal extended the shelf life of each product in a concentration-dependent manner. Shelf lives ranged from a few days to several weeks depending on the concentration of protein utilized. However, initial efficacy did not seem to be a predictive indicator of shelf life. Lemonade, white cranberry and apple juice showed a high level of protection at just two and four fold concentrations above the MIC while apple cider, acidified black tea and blueberry juice only exhibited long-term stability at antifungal concentrations exceeding 20 times the MIC.
Significance: This protein is attractive as a potential alternative to currently available compounds and will provide a greater diversity of natural food protectants to combat fungal spoilage in beverages and fruit juices.