Color of white wines

Assessing the color of white wines is much more complex, as the spectrum has no defined maximum in the visible range. Absorption is continuous from 500 to 280 mn, with a maximum in the UV range. It is difficult to use the spectrum to translate the visual impressions corresponding to dry white wines, sweet white wines and oxidized dry white wines. As the characteristic absorption wavelength of yellow substances is 420 mn, measurements of optical density at this value provide only an approximate assessment of color. 

Contribution of the Various Components to the Color of White Wines... 

The color of white wines therefore involves the oxidation of phenolic compounds. However, the consequences of enzymatic and chemical oxidation are not the same. Chemical oxidation of a catechin solution produces maximum absorption at 400 nm, with a more intensely yellow color than in solutions where oxidation is catalyzed by laccase. 

Absorbent charcoal has been used for many years. There are many different preparations (based on animal or plant charcoal), which have been subjected to various activation processes. These prod-nets are relatively suitable, either for eliminating nnpleasant smells or for removing stains or mader-ized color from white wines. The use of doses up to 100 g/hl is permitted by European legislation for the treatment of white wines. Doses of 10-50 g/hl are generally sufficient to treat the color of white wines, while effective deodorization may require... 

Finally, let us end this section with a few words on the origin of wine color, whether it is red or white. Call it the unifying theory. Everybody knows that red is given by the grape anthocyanins (Brouillard et al, 1997). In our opinion, the pale yellow color of white wines could arise from the presence of iron-chalcone complexes (see Fig. 1.4), and the balance between iron and anthocyanins would determine the final aspect of a white wine. Thus, anthocyanins may be stored in living tissues under almost colorless or yellow combinations. If it is the case, phytochemists looking for new anthocyanin natural structures may turn their attention toward yellow and even white flowering plant species. 

PVPP is commonly used to remove undesirable brown or pink pigments from wine (Jackson, 1994). However, because much of the color of red wine is due to polyphenolic compounds, treatment with PVPP or other polyamides can diminish the red color and so must be carefully controlled. Additions of gelatin or egg white (egg albumin has about... 

Chemical Analysis. It is suggested that a set procedure be established for running specific analyses according to a specific timetable. For example, a visual and taste inspection of all wines every two weeks (two months for wines in barrels), malic and lactic acid analyses at weekly intervals during the malo-lactic secondary fermentation, and S02 and color analyses of white wines at least every two weeks until the wine is bottled. Basic analyses to monitor are V.A., S02, alcohol, pH, and T.A. 

Among polyphenolic compounds, two types of flavonoids, the anthocyanins and flavanols (i.e., catechins, proanthocyanidins, condensed tannins), are particularly relevant to the quality of red wines, as they are key compounds for color definition and astringency. Other flavonoids such as flavonols may have some influence on color and bitterness, although they are present in red wines in much lower amounts. Phenolic acids and hydrolysable tannins, released from barrel wood, may also have an influence on wine taste and color, and hydroxycinnamoyl derivatives from grape must are involved in the oxidative browning of white wines together with flavanols. Besides, some of these perceptions may be modified by other sensory characteristics (e.g. sourness, sweetness) related to other wine components (Preys et al. 2006). 

The chemical interpretation of the yellow color in white wines has always been a little-known field. Phenolic compounds are certainly involved, but concentrations are low and their contribution has never really been established. Many studies have investigated the oxidative browning of wines, independently of enzyme mechanisms. Other molecules are involved besides tannins (Sapis and Ribereau-Gayon, 1968), especially compounds that have a high absorption in the visible—and especially ultraviolet—spectrum (Somers and Ziemelis, 1972). Cafeic and coutaric acids are responsible for browning in white wines (Cheynier, 2001). 

The flavonoids (Figure 2B) are yellow-colored pigments that give the pale yellow color to white wines. They are also present in red wines but their color is masked by the anthocyanins (Figure 2C), the coloring substances of red wines. Like the anthocyanins, the flavonoids exist primarily as heterosides where the 3-position on the heterocycle contains a sugar moiety. The latter may in turn be acylated with cinnamic acid, which confers greater stability on the molecule. 

Several important measures of wine quality ean be evaluated by mathematical combination of absorbance values at multiple wavelengths (Bain). Wine color intensity, a measure of how dark the wine is, is ealeulated from the sum of the absorbanees at 420, 520, and 620 nm. The wine hue is a measure of the appearance of the wine and is ealeulated from the ratio of absorbance at 420 nm to absorbance at 520 nm. The Thermo Fisher Seientifie software on the Evolution Array UV/VIS spee-trophotometer ean ealeulate the intensity, hue, and the CIE L a b values, as well as the eolor dif-ferenee values (the delta values) eompared to a standard. Red wines exhibit an absorbanee between 400 and 650 nm, eentered at about 500 nm, due to the presence of anthocyanin. No such peak appears in the spectra of white wines. The CIE eolor measurements are earried out in transmittance mode. Table 5.12 shows results for the color and color difference measurements. 

The color of red wines has been studied much more than that of whites. The importance of color to the commercial value of red wines was stressed by Vogt (1935), Winkler and Amerine (1938), and others. 

Nevertheless, the dissolvent effect of sulfiting, with respect to phenolic compounds, is obvious in the case of limited maceration. This operation is not recommended for crushed white grapes before must extraction by pressing. The sulfiting of grapes also has an impact on the color of rose wines. 

Muller-Spath (1977) was the first to contest the need to sulfite white juice before alcoholic fermentation. His research clearly showed that adding pure oxygen to non-sulfited juice before clarification improves the stability of white wine color without producing oxidation-type flaws. This process, called hyperoxidation or hyperoxygenation, consists of oxidizing juice polyphenols to precipitate them during clarification and eliminate them during alcoholic fermentation. 

The effects of juice hyperoxygenation on the stability of white wine color are variable, due to the existence of several reactional mechanisms. 

Table 13.19. Influence of wood origin (Allier, Limousin) and toasting intensity of the color and wood character of white wines (Chatonnet, 1995)...

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