Volatile organic compounds from Starmerella bacillaris to control gray mold on apples and modulate cider aroma profile

Gray mold caused by Botrytis cinerea is a fungal disease that can determine significant economic losses of apple during the storage phase. An alternative to reduce the use of traditional synthetic fungicides is to employ the yeast Starmerella bacillaris as biological control agent (BCA), also with positive effect on apple juice fermentation for the production of cider. Thus, we aimed to evaluate the safety of 16 S. bacillaris strains and their ability to control B. cinerea. In addition, the fermentation performances in apple juice and the volatile organic compounds (VOCs) profile were assessed, both in single-strain and in sequential fermentations with Saccharomyces cerevisiae. The in vitro assays showed that all S. bacillaris strains can be considered safe from the analyzed virulence factors, and were able to significantly constrain the growth of B. cinerea, reducing mycelial growth of 50% in dual-culture and of 90% through VOCs. Moreover, in vivo antagonistic assays revealed a visible decrease of gray mold rot symptoms on apples confirming the potential of S. bacillaris as BCA. GC-MS analysis of the ciders obtained showed increased concentrations in the sequential fermentation of some higher alcohols and terpenes, positively correlated with the cider aromatic quality, and suggested the involvement of benzyl alcohol, known for its antimicrobial action, in the biocontrol efficacy.