Investigation into the effect of winemaking techniques on Sauvignon blanc wine quality and development of procedures that will produce optimum quality wines
The pinking phenomenon, a negative quality characteristic of Sauvignon blanc wines, was investigated. The effects of grape temperature (0°C vs 20°C, and 10°C vs 25°C), oxygen contact of juice prior to fermentation, skin contact for 15 hours at 0°C and 20°C respectively, the use of ascorbic acid and yeast strain (VL3C, VIN 13, VIN 7, NT 116 and NT 7) on Sauvignon blanc wine composition and quality were also investigated. Depending on the investigation, wines were sensorially evaluated for fruitiness intensity, green pepper/asparagus intensity and overall wine quality, as well as chemically analysed for 2-methoxy-3-isobutylpyrazine (ibMP), monoterpenes, total polyphenols, esters and higher alcohols. Furthermore, the sensory impact of ibMP and 4-mercapto-4-methylpentan-2-one (MMP) on Sauvignon blanc wine aroma was determined by the addition of these components to water and wine.
Various factors have an effect on the pinking susceptibility of Sauvignon blanc wines. They are: climate, sunlight exposure of grapes, grape ripeness, reductive wine production, juice oxidation prior to fermentation, skin contact, lees contact and ascorbic acid use. Pinking can be prevented by applying not too reductive conditions prior to fermentation and by treatment of wines with polyvinylpolipyrrolidone (PVPP). Grape temperature, skin contact and oxidative or reductive conditions prior to fermentation affect wine composition and quality significantly. Most component concentrations were enhanced by skin contact. Harvesting of grapes and skin contact at low temperatures (10°C and lower) yielded more typical, delicate and higher quality Sauvignon blanc wines. Polyphenol concentrations, which may affect wine quality negatively, were lower in wines produced oxidatively (prior to fermentation).
The use of ascorbic acid normally yields fresh/fruity wines, but should be used judiciously, because it may cause oxidation during wine ageing. Initially, problems were encountered with the commercial ascorbic acid/meta-bisulphite preparate, namely the development of sulphur-like off-odours. The components responsible for these aromas could not be identified. After conclusion of the study, the commercial preparate was improved and no longer yields the relevant off-odours. Sulphur-like aromas, however, still periodically occur in Sauvignon blanc wines and should be further investigated.
Regarding yeast strains, it was found that the French yeast strain (VL3C) formed sulphur-like off-odours under South African conditions. The local yeast strains, NT 116 and VIN 13, yielded relatively high ester concentrations and consequently fruity wines. They can be used with Sauvignon blanc wines with strong, typical cultivar characters. In the case of neutral Sauvignon blanc wines, however, the fruitiness may mask the sought-after green pepper, asparagus character, and it is recommended that yeast strains that produce low ester concentrations, like VIN 7, are used.
Addition of ibMP and MMP, important Sauvignon blanc impact components, to water and wine yielded the following nuances: Depending on the concentration levels and medium they varied from dusty, grassy to green pepper for ibMP and from guava, sulphur-like to cat urine for MMP. The possibility of a synergistic action between these two components was mentioned.
Cultivar-typical and high quality Sauvignon blanc wines can be produced when the above-mentioned results are applied, i.e. low temperatures during harvesting of grapes, crushing and skin contact, the judicious use of ascorbic acid, or the non-use thereof and the use of suitable yeast strains. Most of the practices are already applied in the industry.
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