Color: In young red wines, the bright red (with purple tint) color is due to monomeric anthocyan pigments which are extracted from the skins during fermentation. During maturation, these pigments are progressively replaced by the polymeric form, which results from the combination of anthocyanin pigments with tannin.
During maturation, the wine is exposed to air. Oxygen plays an important role in the condensation reaction between anthocyanins and tannins, which results in the gradual loss of free anthocyanins and the formation of stable polymeric (anthocyanin tannin) pigments. It has been observed that the polymeric pigments account for 50% of the color density in one-year-old wine. As the wine matures and more polymeric pigments are formed, the color shifts from red to orange and brick red.
Taste and mouth feel: With proper maturation and aging, the wine becomes mellower and smoother, and acquires a richer mouth feel. Many compositional changes contribute to the improved taste. The important changes include polymerization of phenolic compounds and reduction in acidity. Phenolic compounds play an important role in the taste and flavor of wine. Bitterness and astringency are primarily attributed to flavonoid phenols. The monomeric flavonoids are more bitter than astringent. As flavonoid phenols polymerize, they become less bitter and more astringent. With further polymerization, the molecules become too large, and finally precipitate. This leads to a reduction in phenolic compounds and also in astringency. During maturation, oxidative and non-oxidative polymerization and precipitation of phenolic compounds (of larger molecules) occurs. This results in a wine with reduced astringency and a smoother, softer taste. Maturation, therefore, plays a key role in improving wine’s sensory appeal.
Another factor contributing to improved taste is loss of acidity. This occurs due to acid precipitation and ester formation. Acidity enhances the astringency and loss of acidity makes wine taste less astringent and more mellow.
Aroma: Significant changes in wine aroma occur during maturation and aging. These include the loss of certain grape or yeasty aromas, retention of the varietal aroma, formation of new aromas, and above all, integration of all flavors to produce a harmonious and pleasing fragrance. Many esters and higher alcohols formed by the yeast’s metabolic activity contribute to the fermentation aroma. During wine storage, the esters are hydrolyzed and the fresh and fruity aroma is lost.
Even more details on aging affects on wine can be found at: http://www.extension.iastate.edu/wine/w-aging