Indicadores y aumento de la productividad.

In wine grapes, flavonoids are widely found in the skin, stem and seed. Most flavonoids are classified as secondary metabolites and are not active in the primary metabolism of the grapevine. They are water soluble and secreted into the vacuole of grape berries as glycosides. Flavonoids play an important role in protecting the grape berry against strong light as well as UV radiation as a protective pigment screen. In particular, light proved to enhance the synthesis of these compounds, while high temperature had an opposite effect (Azuma et al. 2012; Bergqvist, Dokoozlian & Ebisuda 2001; Spayd

et al. 2002). During the growth cycle of the grapevine, sunlight will increase the concentration of

flavonoids in the grape berries with the development of flavonoids being an important objective of canopy management (Azuma et al. 2012; Cordon 2008; Downey et al. 2004).

Anthocyanins, tannins and flavonols, the three main flavonoids accumulated in berry skin, contribute significantly to grape quality and are responsible for the colour, stability, mouthfeel and health properties of wine (Bucchetti 2009). Anthocyanins are responsible for the colour of red grapes and wines and confer organoleptic characteristics on the wine. These compounds start to accumulate from veraison when the skin of red wine grapes changes colour from green to shading to red or black. As the sugar in the grape increases during ripening, so does the concentration of anthocyanins. In most grapes, anthocyanins are found only in the outer cell layers of the skin leaving the grape juice inside to be virtually colourless. Another major category of flavonoids are tannins which can affect the colour, aging, ability and texture of wine (Robinson 2006). Tannins are found in the skin, stem and seed of 25

wine grapes. Natural tannins found in grapes are known as proanthocyanins due to their ability to release red anthocyanin pigments when they are heated in an acidic solution. The tannins are formed by enzymes during metabolic processes in the grapevine. The amount of tannins naturally found in grapes varies among different grape cultivars, with Cabernet sauvignon, Nebbiolo, Syrah and Tannat being four of the most tannic grapes. The reaction of tannins and anthocyanins with the catechin creates influences the colour of red wine (Robinson 2006).

Another class of polyphenolic phytochemicals studied extensively are the flavonols. Flavonols are a ubiquitous class of flavonoids with photo-protection function. Flavonols are represented mainly by kaempferol, quercetin and myricetin, while other simple methylated derivatives such as isorhamnetin (quercetin 30-methyl) are also common (Makris, Kallithraka & Kefalas 2006). In grape, flavonol profiles vary among different grape cultivars. Red grapes usually contain myricetin, laricitrin and syringetin derivatives, whereas in white grapes the lack of expression of the enzyme flavonoid 3', 5'-hydroxylase restricts the presence of these compounds to quercetin, kaempferol and isorhamnetin derivatives (Flamini et al. 2013). The main flavonols in red grape berries are 3-O-glucosides, 3-O-galactosides, and 3-O-glucuronides of six flavonoid structures (kaempferol, quercetin, isorhamnetin, myricetin, laricitrin, and syringetin). In contrast to red grape cultivars, white grapes do not contain any flavonol derived from myricetin, laricitrin and syringetin (Isidro et al. 2011).

Flavonol composition in grape changes throughout berry development. These compounds can also be affected by many environmental factors including light exposure, particularly UV-B radiation (Cordon 2008; Downey et al. 2004; Gregan et al. 2012; Liu et al. 2014). Recently, a detailed examination of sunlight exposure and temperature on the contents of quercetin, myricetin and kaempferol glycosides revealed that berries (Vitis vinifera cv. Merlot) from sun exposed clusters might contain as much as ten times the content found in samples obtained from shaded clusters (Spayd et al. 2002). The effects of UV radiation on flavonol contents in grapes post-harvest was also illustrated (Cantos et al. 2000). Flavonol contents in Napoleon table grapes showed no notable increase under UV-B and UV-C treatments after harvest, which means the effects of UV must take place while the grape is growing but not after harvest.

2.6.2

Flavonoids in wines

Flavonoids are important quality components in wine. These compounds contribute to the colour, taste, mouthfeel as well as the stability of wines. For red wine, anthocyanins and tannins are the most important flavonoid classes (Kennedy 2008). Anthocyanins are responsible for the colour and confer organoleptic characteristics on the red wines, whereas tannins contribute to the mouthfeel of wines. Anthocyanins and tannins also associate together and form pigmented polymers to provide the stable pigments required to give red wine its long term colour stability (Robinson 2006). The concentration 26

of flavonols in red wines is high when both aglycones and glycoconjugates are taken into consideration. A survey of the concentration of quercetin, myricetin and their glycosides on 65 red wines showed flavonols varied from 4.6 to 41.6 mg/L (McDonald et al. 1998). Compared with the red wines, the total content of flavonols in white wines was much lower varying from 0.5 to 1.5 mg/L. These values included both glycosides and aglycones of quercetin and myricetin determined after hydrolysis (Hertog

et al. 1993).

The concentration of flavonoids in wine varies according to grape cultivars, wine making treatments and vineyard practices. Flavonoid profiles of grapes and juice can be degraded upon exposure to heat, enzymes and oxidative chemical species such as free radicals (Makris et al. 2006; Makris & Rossiter 2000). The processing treatments of grapes and juice might affect the flavonol profiles in wine products. For example, when Thompson Seedless juice was treated by enzymic clarification, extensive hydrolysis of quercetin derivatives were detected in wine (Spanos & Wrolstad 1992). Similarly, different pressing methods (cold-press or hot-press) and storage at different temperatures cause very large variations on total flavonol concentration in Muscadine juice (Talcott & Lee 2002). Geographical location is another factor altering flavonol concentrations of wine. A survey on the free and conjugated myricetin and quercetin content in red wines with different geographical origins showed the flavonol concentrations of wines are affected by their origins (McDonald et al. 1998). In addition, it has been known that the final flavonol concentrations in the finished wines are greatly influenced by vineyard managements, such as leaf removal and sunlight exposure (English et al. 1990; Gregan et al. 2012; Tarara et al. 2008). Leaf canopy management and sunlight exposure alter the contents of quercetin, myricetin and kaempferol glycosides in berries, which eventually change the final flavonol concentration in wines.