Effect of Ripe Rot of Grapes (Colletotrichum spp.) on the Chemical Composition and Off-Flavour Compounds in Grapes and Wine

Ripe rot of grapes (Vitis vinifera) is a bunch rot disease caused by species of the fungus Colletotrichum. The disease occurs predominantly in those grape growing regions which have warm and wet conditions close to harvest time. Grapes affected with ripe rot have a bitter taste and taint. Other unwanted flavours and aromas are also found in wine made from ripe rot affected grapes. Despite the widespread occurrence of this disease in sub-tropical and tropical viticulture, the chemical nature of the off-flavours has not been defined. This body of research sought to better characterise the impact of ripe rot on both grape and wine quality. As the fungal agents responsible for causing ripe rot of grapes are also pathogens of other fruit crops, the research was extended to include infections involving blueberries (Vaccinium corymbosum). Prior to this investigation, there was little information, if any, in the literature on the chemical nature of compounds responsible for off-flavours in grapes and other fruit crops affected with Colletotrichum. An optimised method was developed for the quantitation of compounds potentially responsible for off-flavours in grapes and wine. These compounds included 2-iopropyl-3-methoxypyrazine, 2-isobutyl-3-methoxypyrazine, 3-octanone, fenchone, 1-octen-3-one, trans-2-octen-1-ol, Fenchol, 1-octen-3-ol, 2-methylisoborneol, 2,4,6-trichloroanisole, geosmin, 2,4,6-tribromoanisole and pentachloroanisole. These compounds were selected as they had been previously reported in grapes affected with an unrelated fungus, Botrytis cinerea (grey mould) and in wine made from grapes affected with this fungus. The method developed involved a simple headspace-solid-phase-micro-extraction gas-chromatography mass-spectrometry (HSSPME'GC'MS) for the simultaneous determination of thirteen off-flavours compounds in a single run. Sample temperature and fibre extraction time were optimised using response surface methodology, in both wine base and grape base, to maximise signal intensity for compounds at low concentrations. Low limits of detection (0.05-5 ng/L), quantitation (0.2-17% CV), good recoveries (83- 131%) and repeatability (4.3-11.1% CV) and reproducibility (1.9-10.9% CV) indicated that the method has excellent sensitivity and is suitable for the analysis of off-flavour compounds in wine and grape samples. This method has allowed the investigation of compounds that contribute to the off-flavours found in bunch rot affected grapes and wines made from them to be done in a fast single run. Cabernet Sauvignon wines previously vinified from both healthy and ripe rot affected grapes from the Hastings Valley, (NSW) were analysed for the presence of fungal derived volatile compounds using the optimised method. Four off-flavours compounds, geosmin, 1-octen-3-ol, 1-octen-3-one and 2-methylisoborneol were found to be significantly higher in wines (P<0.05) made from ripe rot affected grapes than wines made from apparently disease-free grapes. The results indicate that the metabolic activities of Colletotrichum spp., responsible for bunch rot diseases in the vineyard, contribute to unwanted volatile flavour compounds in wine. Three fining agents, polyvinylpolypyrrolidone, carbon and bentonite, were investigated at two concentrations for remediation of wines affected by ripe rot. At higher concentrations of the four off-flavour compounds, these fining agents significantly (P<0.05) reduced geosmin, 1-octen-3-ol, 1-octen-3-one concentrations in affected wine, but had no impact on the concentration of 2-Methylisoborneol. In order to have a better understanding of how Colletotrichum spp. causes off-flavour in grapes and wine, the formation of off-flavour compounds in fruits was also investigated. Two host fruits were chosen, V. vinifera (cultivars Chardonnay and Cabernet Sauvignon) and V. corymbosum (blueberry), and inoculated with different isolates of C. acutatum (DAR 77282, 76888, 32068, 76925) and of C. gloeosporioide