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Mouth-feel of different white wine treatments
Anita Oberholster, Department of Viticulture and Oenology, Stellenbosch University
The article below is a shortened extract from an article published in the Australian Journal of Grape and Wine Research 15, 59 - 69, 2009.
INTRODUCTION
White grapes fermented like a typical red wine in the presence of skins and seeds result in wines with quite different astringent sensory profiles to those of red wines: they tend to be coarser and lower in astringency. The only apparent difference between the phenolic composition of pomace-fermented white wines and red wine is the anthocyanin content. Anthocyanins have been said to have only a mild indistinct taste , while it has been reported to possibly increase the astringency of seed and skin extracts . However, this does not explain the complex mouth-feel differences between red and white wines. The present study was conducted to further investigate the contribution of anthocyanins or tannin-anthocyanin reaction products to the mouth-feel properties of red wine. Anthocyanins were isolated and added to different white wine ferments to investigate the contribution of anthocyanins and their fermentation and aging products to sensory characteristics of wine.
MATERIALS AND METHODS
Chardonnay grapes (23.5 °Brix, pH 3.81, titratable acidity (TA) 5.2 g/L as tartaric acid) from the Adelaide Hills (Lenswood, South Australia) and Shiraz grapes (24.5 °Brix, pH 3.94, TA 4.3 g/L) from Nuriootpa Research Station (Barossa Valley, South Australia) were used. The anthocyanin extract was prepared from a portion of the Shiraz grapes (37.3kg), while the rest of the grapes were stored at 0°C. Pressed grape skins were extracted with isoamyl alcohol and purified by five re-crystallisations . Grapes were crushed and pressed and the isolated red and white skins and seeds were respectively back added where needed in ratio of the free run juice and skins and seeds produced during pressing of the respective grapes (see Table 1).
Phenolic composition of the wines was determined by reverse phase high performance liquid chromatography (RP-HPLC) analyses . Descriptive analysis was carried out using ten judges. Eleven mouth-feel terms ('fine grain', 'medium grain', 'coarse grain', 'silk', 'velvet', 'fine emery', 'dry', 'grippy', 'overall astringency', 'viscosity' and 'bitter') were generated with reference to the developed mouth-feel wheel . See Table 2 for description of attributes and reference standards used.
RESULTS AND DISCUSSION
Phenolic composition
The different wine treatments were analysed for phenolic composition after six months of bottle aging by RP-HPLC (Table 2). After six months of bottle aging the main differences between the treatment white free run juice (W) and the treatment with added white skins and seeds (WS) were expected to be due to skin and seed phenolic compounds extracted into the juice during fermentation and their subsequent reaction products during aging. Larger differences between the control W and treatment WS were anticipated according to HPLC analysis, but it were apparent that the control contained a higher concentration of phenolics then expected for a free run lightly pressed white wine . This is most likely due to the extended storage at 0ºC of the grapes prior to winemaking, allowing partial breakdown and extraction of skin phenolics into the berry. Wines from treatments RS and WRS had significantly higher concentrations of epicatechin, proanthocyanidins and anthocyanins than all the other treatments. The higher phenolic concentration of treatment RS compared to WRS may be related to a higher concentration of phenols released from the red skins into the free run juice compared to the white skins during pressing. Treatments WSA and RS were expected to be comparable, but as a result of some of the anthocyanins of the added anthocyanin extract being absorbed by grape skin components, precipitated or otherwise lost during fermentation, this was not true. In a future study it may be better to calculate the addition of anthocyanins according to the concentration of anthocyanins present in the red skins and not the extractable amount. When the treatments with added anthocyanins (WA and WSA) are compared with their non-pigmented counterparts (W and WS), the main differences were the presence of monomeric and polymeric pigments, as well as the fact that their concentration of polymeric phenols is at least double the amount (although WSA was not significantly different to WS).
(click table to enlarge)
Sensory analysis
An analysis of variance (ANOVA) of the descriptive data assessing the treatment effects was performed. All the attributes except 'silk' was significantly different across treatments. There were no significant differences between treatment replicates except for 'velvet'. Data for these two attributes will not be discussed further. To facilitate the interpretation of the differences and similarities of the samples, principal component analysis (PCA) was applied to the data. The first two components accounted for 95% of the variance. The first principal component (PC1) differentiated the samples according to the relative intensity of 'bitter', 'dry', 'viscosity', 'astringency', 'grippy', 'medium grain', 'coarse grain' and 'fine emery' (Figure 1). All of these attributes were correlated. The second principal component (PC2) separated the samples on the basis of the 'fine grain' attribute.
The presence of white skins and seeds (WA versus WSA and W versus WS) mainly increased the rating of 'fine grain', and only slightly increased astringency attributes such as 'drying' (Figure 1). It must be noted that the free run juice in the current study contained larger quantities of phenols than usual and the small effect of skin contact during fermentation may be due to an already astringent white wine control. However, the amount of flavan-3-ols extracted into WS were up to four times more than what was extracted in 24 hr in studies investigating the effect of prefermentative maceration .
It was clear that the presence of anthocyanins increased astringency, specifically the attributes 'dry' and 'grippy' and to a lesser extent resulted in an increase in 'viscosity' and 'fine emery'. This increase in astringency may have been related to low level phenolic impurities or by the formation of new polymeric pigments. Pigments other than monomeric anthocyanins made a 15 to 26% contribution to wine colour after six months of bottle aging in treatments WSA and WA, respectively. The treatments with added anthocyanin (WA and WSA) were rated as being more fine grained compared to the treatments with no added anthocyanin (W and WS). Vidal and co-workers determined that polymeric pigments isolated from grape marc contributed to the fine grain attribute . The treatments with added red skins and seeds (WRS and RS) contained the highest concentration of pigments and monomeric and polymeric phenols and were scored the highest in attributes 'dry', 'astringency', 'viscosity', 'grippy', 'medium grain' and 'fine emery'.
In this study, a white wine made like a red wine did not exhibit the same mouth-feel sensory attributes of a red wine: it was lower in viscosity, less particulate in nature and lower in astringency. It was found that differences in ratings of mouth-feel attributes could not be related closely to phenolic composition or structure. The reason for this was probably the relatively low concentration of phenolics in the wines investigated. It may have been possible to observe correlations with more phenolic red grapes and wines. Longer bottle-aging of the wine, might also have allowed larger differences in phenolic composition to be observed. The presence of anthocyanins during fermentation increases the intensity of astringency related terms and can partly explain the differences perceived between the mouth-feel properties of a white and a red wine. The addition of anthocyanins mainly contributed to the 'fine grain' sub-attribute of astringency. Further investigation of young versus aged wine treatments to determine the effect of anthocyanin polymerisation products, should be undertaken.
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