Journal of Food Composition and Analysis
Original ArticlePhenolic compounds in skins and seeds of ten grape Vitis vinifera varieties grown in a warm climate
Introduction
Polyphenolic compounds play an important role in the quality of grapes and wines. These constituents can be divided into two groups: non-flavonoid (hydroxybenzoic and hydroxycinnamic acids and stilbenes) and flavonoid compounds (anthocyanins, flavan-3-ols and flavonols). Anthocyanins are a family of polyphenols that are directly responsible for colour in grapes and young wines. Flavan-3-ols (monomeric cathechins and proanthocyanidins) are another large family of polyphenolic compounds, which are mainly responsible for the astringency, bitterness and structure of wines (Singleton and Essau, 1969; Gawel, 1998). The last group of flavonoids are flavonols (quercetin, myricetin, kaempferol, isorhamnetin and their glycosides), which seem to contribute to bitterness.
In grape berries, phenolic compounds are present mainly in skins and seeds. Flavonols are the most abundant phenolic compounds in grape skins, while grape seeds are rich in flavan-3-ol (Cheynier and Rigaud, 1986; Souquet et al., 2000). The concentration of phenolic compounds in grapes depends on the variety of grapevine and is influenced by viticultural and environmental factors (Broussaud et al., 1999; Cheynier et al., 1998; Ojeda et al., 2002).
Phenolic compounds of wine have attracted much interest due to their antioxidant properties and their potentially beneficial effects for human health (Shirkande, 2000). For this reason, grape seed extract has become popular in recent years as a nutritional supplement (Waterhouse et al., 2000).
These compounds are also responsible for browning reactions in grapes and wine (Macheix et al., 1991) and different reactions with anthocyanins that lead to the stabilization of colour in red wines. Anthocyanins may react with flavonols to produce more stable pigments, either directly (Francia-Aricha et al., 1997) or by means of different aldehydes (e.g. acetaldehyde, propionaldehyde) (Pisarra et al., 2003).
Lastly, polyphenols, particularly certain phenolic acids and flavonols, participate in the phenomenon of copigmentation. For this reason, anthocyanins display far greater colour than would be expected from their concentration (Boulton, 2001).
Therefore, the colour of red wines and its evolution depend not only on the concentration of anthocyanins; the concentration of the other non-coloured polyphenols is equally important. Consequently, the co-fermentation of red grapes of different cultivars has been proposed when any of the grapes do not present a good balance between the concentrations of anthocyanins and other polyphenols (Boulton, 2001). This may also apply to the co-fermentation of red grape varieties with white grapes that contribute these polyphenolic compounds (Gigliotti et al., 1985).
White grape must is not usually fermented with the skins of the grapes, hence until now the phenolic composition of grapes from white cultivars has been the object of fewer studies than that of red grape cultivars in which grape skins have a great impact on wine quality. This paper studies the phenolic composition of the skins and seeds of six white grape varieties, and compares them with those of four varieties of red grapes, all widely grown and of recognized prestige.
Section snippets
Materials and methods
We took 2.5 kg of grapes (approx. 0.5 kg at random×5 grapevines) of 26 samples of six white grape varieties (Chardonnay, Sauvignon blanc, Moscatel de Grano Menudo—a muscat cultivar, Gewürztraminer, Riesling and Viogner) and 44 of four red grape varieties (Cencibel, Cabernet Sauvignon, Merlot and Shiraz) grown in different parts of the region of Castilla-La Mancha (Spain). All the samples were collected when the grapes were at technological ripeness, i.e. between 12° and 13° Baumé for white
Results and discussion
Table 1, Table 2, Table 3, Table 4 show the results (mean value and standard deviation) obtained for the skins and seeds of the white and red grape varieties, expressed in mg/kg of fresh grape. The statistically significant differences found between the varieties for each compound analysed were highlighted with different superindexes. Many of the differences observed between the varieties studied could, according to González-Paramás et al. (2004), be an additional factor of variability together
Conclusions
The phenolic composition of grapes depends on multiple factors, including climate, degree of ripeness, berry size and grapevine variety. However, it may be concluded that the skins and seeds of white grape varieties present a very similar qualitative and quantitative composition to that of red grape varieties in terms of non-anthocyanic polyphenols. The only qualitative difference was observed in the skins of white grape varieties that lacked the myricetin glycosides present in the skins of red
Acknowledgements
The authors would like to express their gratitude to the Consejeria de Ciencia y Tecnología of the Junta de Comunidades de Castilla-La Mancha (Project PCC-02-003) for its financial support.
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