Ergosterol as an objective indicator for grape rot and fungal biomass in grapes

Abstract Sound grapes are an essential prerequisite for the production of high-quality wines. However, pricing of grapes is so far mainly based on the must weight of grape deliveries, although e.g. highly botrytised grapes become raisined bringing about higher contents of soluble solids than sound ones. Besides the desired ‘noble rot’, in particular infection of unripe fruits by grape rot decreases the perceptual quality by destroying fruit flavours typical of the grape variety, furthermore leading to off-flavours, off-odour, bitterness and colour-loss. Moreover, the formation of mycotoxins, in particular ochratoxin A, associated with fungal infestation highly affects food safety of the products. Consequently, there is a strong need of an objective assessment of the phytosanitary status of grapes to be used for vinification and other grape derived products. To date, the commonly applied visual examination is subjective, and particularly challenging when grapes were mechanically harvested. Objective methods suggested in the literature, such as the analysis of major and secondary metabolites of moulds and the determination of related enzyme activities, are requiring tedious sample preparation being generally time-consuming. Furthermore, these methods are prone to interferences, and mostly lacking selectivity and sensitivity. Ergosterol is a characteristic component of the fungal cell membrane, while it is completely or nearly absent in animal, plant and bacterial cells. Thus, our group proposes ergosterol as a specific and quantitative marker for fungal infection of grapes. Mould strains relevant for grape rot were demonstrated to contain considerable amounts of ergosterol. In grapes infected with several pure mould strains and in naturally contaminated grapes, ergosterol was unambiguously detected and quantitated. In contrast, only traces were found in sound grapes, presumably originating from the presence of ubiquitous yeasts and moulds. Ergosterol levels highly correlated with the degree of fungal decay in blended samples of mashes made from sound and rotten berries. An analytical HPLC-method for the quantitation of ergosterol was developed allowing a simple and objective evaluation of the phytosanitary status of grapes, thus enabling fair and incentive payments between grape growers and wineries.