Red and white wines possess obvious compositional differences, the most visible difference being the colour. The colour differences between red and white wines are a reflection of the chemical compositional differences that exist between the wines. These compositional differences are in turn a reflection of the differences in the manufacturing processes between red and white wines, the former including more of the solid parts of the grapes.
The polyphenolic flavonoids and stilbenes in grapes are localised almost exclusively in the skin, stems and seeds. It is the inclusion of polyphenols that determines the colour and chemical composition of the wine, with red wines undergoing maceration of the solid parts of the grape to allow transfer of the polyphenols to the wine. A fermentation step is then required for red wines to ensure transfer of the polyphenols to the wine to produce the red colour.
In contrast to red wines, white wines do not undergo a maceration step and instead is made by simple mechanical pressing of the red or white grapes. The lack of maceration and inclusion of solid material in the fermentation step prevents the transfer of large quantities of the polyphenols to the wine and this limits the final polyphenolic content of the wine. Because of its lower polyphenolic content, white wine is not considered as beneficial to the health as red wine.
Alcohol and Sulphur Dioxide
The concentration of polyphenol compounds in the final red wine is also dependent on other factors. In particular, the alcohol and sulphur dioxide content of the wine can modify the final amount of polyphenols in the wine. Red wine is often bottled into dark glass, and this is to hide the quite high content of solid residue in the wine that would be visible if the wine was in a clear bottle. Because of its lower polyphenol content, white wine can be bottled in clear bottles.
Seeds or Skins
The point at which the grapes are macerated can also affect the final composition of the red wine. Maceration at low temperatures before fermentation can increase the extraction of anthocyanins and proanthocyanidins from the skins. However, post fermentation maceration increases the proanthocyanidin content from the seeds. The final anthocyanins and proanthocyanidins content varies between 20 to 40 % of the original content of the grape, respectively.
During the aging of wine chemical and enzymatic processes change the chemical composition considerably. In particular, anthocyanins from the original grape, and tannins produced from the oxidation of flavonoids, are degraded to other phenolic compounds. This produces and highly complex mixture of chemicals. The exact composition of the final wine therefore depends on the original flavonoid content of the wine as well as the length of the ageing process.