Fermentation rate, yeast protein and sensory profiles of wines from fungicide treated Chenin Blanc grapes

Abstract

Fungal diseases in vineyards are one of the main causes leading to economic losses within the viticultural sector and are continuously increasing over years. The most common of these fungal diseases are powdery mildew, downy mildew and grey mould. Commercial fungicides to treat the above-mentioned diseases are available and their usage is regulated under Act 36 of 1947 to comply with Good Agricultural Practises (GAP). However, the application of less-harmful, natural alternative fungicides to control vineyard diseases are currently an important research focus since the demand for organic products by consumers and retail companies are increasing. However, fungicide residues can alter the fermentation process and prevent some biochemical pathways of yeast metabolism involved in phenolic and/or aroma compound production that are critical for sensory quality. Therefore the aim of the study was to investigate the effect of fungicide treatments on the fermentation rate, yeast proteins expressed, aroma compounds released and sensory profile of wines produced. In the study, Chenin Blanc grapes treated with chemical and natural fungicides (1x treatment and 2x treatment) under Good Agricultural Practises (GAP) were used to produce small-scale wines and laboratory-scale fermentations. Laboratory-scale fermentations were conducted in duplicate using the commercial Saccharomyces cerevisiae (S. cerevisiae) Active Dry Wine Yeast (ADWY) strains VIN 13 and VIN 7. The fermentations were monitored by frequently weighing until they stabilised (CO2 weight loss). Small-scale wines were produced using the commercial S. cerevisiae ADWY strain VIN 13 only. Wines were made according to the standard Nietvoorbij experimental winemaking procedure. At the end of fermentation, lees samples were plated onto Yeast Extract Peptone Dextrose (YPD) agar and colonies grown were subjected to CHEF gel electrophoresis to confirm that the S. cerevisiae yeast strain inoculated at the beginning of the fermentation completed it. Moreover, fermenting wine samples, collected at the start (lag phase) and at end of fermentation (stationary phase), were subjected to protein extraction, quantification and characterisation in order to investigate fermenting wine yeast proteins. Furthermore, the final wines were subjected to chemical analyses as well as measurement of aroma enhancing metabolites (esters, higher alcohols, fatty acids and thiol compounds) using GC – FID and MS. Additionally, duplicate samples of the wines were evaluated sensorially by a trained panel of 12 winemakers and researchers, using an unstructured line scale. Wines were compared to the control wine according to visual (colour), flavour (tree fruit, tropical fruit, and wine foreign), taste (body mouthfeel, acidity) and overall quality. The data collected from the study was statistically analysed using a two-way analyses of variance (ANOVAs) and subject to a multiple factor analysis (MFA). From the results obtained in the above study, it was concluded that yeast strains used for winemaking completed the fermentations at a similar rate to their respective controls. In addition, small-scale cellar fermentations showed that fungicide treatments (1x treatment and 2x treatment) compared to the controls had no notable negative effects on wine aroma and sensory profiles although differences were observed in the proteins expressed after the fermentation. Overall, the fungicide treatments did not negatively affect the yeast performance, yeast protein expressed, aroma compounds released and sensory profiles of the wines produced. Further studies are recommended on other white as well as red wine grape cultivars to fully assess the effects of the fungicides.