The influence of different winemaking techniques on the extraction of grape tannins

[摘要] ENGLISH ABSTRACT: Grape and wine phenols consist of flavanols which is the building blocks for tannins.These building blocks are called monomers which consist of catechins, epicatechins,epigallocatechins and epicatechin-gallate. Tannin is important in wine as it contributesto bitterness, mouth feel (astringency) and maturation potential of the wine. Futhermoreit has a health benefit as an antioxidant. Anthocyanins are responsible for the colour ofred wine. The anthocyanins combine with tannins to form stable polymeric pigments.Due to the importance of tannins and anthocyanins in wine, it is imperitative thatdifferent winemaking techniques are used to extract as much of these components aspossible and that the analysis is done quickly and accurately.The aim of this study was to evaluate different winemaking techniques and theirextraction of tannins and anthocyanins into the wine. Too much tannin extraction canhave a negative effect on the sensory quality of the wine. Therefore a second aim wasto evaluate the mouth feel properties of a Shiraz wine. A third aim was to compare thetwo tannin precipitation methods in terms of time efficiency, repeatability and the easeof practice.To investigate the amount of tannin concentration extracted by different winemakingtechniques, two cultivars (Cabernet Sauvignon and Shiraz) were used. Thesetreatments included the addition of an enzyme during fermentation [E], cold maceration[CM], post maceration [PM] and the combination of cold and post maceration [CM+PM].The grapes were harvested in two different climatic areas during the 2008 and 2009vintages. The two climatic areas were classified according to the Winkler scale as a III(Morgenster) and a IV (Plaisir de Merle). The grapes were harvested at two differentripeness levels in order to evaluate the effect of the different winemaking processes onthe extraction of tannins and anthocyanins. One harvest was before (LB) and the otherafter (HB) the commercial harvest.The results of this study showed significant differences in the phenolic composition ofthe wines. It was found that the warmer area showed higher tannin concentrations thanthe cooler area for both cultivars. In the 2008 Cabernet Sauvignon the CM extractedhigher concentrations of tannin from the cooler area at both ripeness levels. In thewarmer area, CM extracted the highest tannin concentration HB, but the CM+PMextracted the highest tannin concentration from Cabernet Sauvignon at the LB and CMat the HB of the warmer area. In 2009 the PM extracted the highest concentration oftannin at the lower ripeness level, while the E treatment extracted the highestconcentration from the warmer area. In the cooler area the CM+PM extracted thehighest concentration of tannin at a lower ripeness level, while there were no siginicantdifferences between the different treatments at the higher ripeness level. The highestanthocyanin concentration was found in the cooler area. The CM treatment was foundto have no effect on anthocyanin extraction.Different methods are available to quantify the tannin concentration in wine. Two of themost popular tannin analytical methods are the bovine serum albumin (BSA) and themethyl cellulose precipitable tannin (MCP) methods. The BSA method is a very complexmethod which uses at least 3 times more reagents than the MCP method. The MCPmethod only analyzes tannins, while the BSA method analyzes tannins, monomericpigments (MP), small polymeric pigments (SPP) and large polymeric pigments (LPP).In this study a good correlation was found between the two tannin precipitation methods(R2 – 0.88). There is controversy regarding the variability of these methods. Somescientists found that the two methods show a good correlation with HPLC, while othersfound that there was no such correlation between the precipitation methods and theHPLC. The MCP method had a practical advantage as it could be performed in half thetime required for the BSA method. This has a significant impact in scenarios where ahigh sample throughput is required although it only measures total tannin.The phenolic composition and mouth feel of the wine was strongly influenced by theclimatic area. In the warmer area the effect of tannin concentration on mouth feel wasmuch less than in the cooler area. The wine made of riper grapes, was more grippy,bitter and numbing than the wines made from greener grapes. The E treatment wasespecially associated with a dry, grippy sensation.

[发布机构] Stellenbosch University