Phenolics in Wine

• the greatest concentration of phenolics occurs in the seeds and stems
• destemming and even removing seeds, minimizes the concentration of complex green tannins and astringency, which results from excessive extraction of these plant part
• destemming is generally practiced when one desires a fruitier wine with less complex astringency
• macerating before fermentation, leads to additional color and tannin extraction from the pomace
• fermentation conditions, in particular the temperature, and the choice of the yeast strain will confer different chemical and flavor characteristics to wine
• the presence of yeast and bacteria or other fungal organisms in wine directly impacts the wine quality and flavor and influences the metabolism of phenolics in wine

The Influence of Oak Barrels on Flavor:
Among the 250 species of the genus Quercus, two European species, Q. petraea (sessile oak) and Q. robur (pedunculate oak) and one American species, Q. alba (white oak) have been used in the past and continue to be widely employed in barrel making.
Research into the composition of the wood has shown that:

• Q. petraea sessile oaks are richer in aromatic substances such as vanillin and methyl octalactone (characterized by a distinct odor of coconut, celery, and fresh wood
• Q. robur pedunculate oaks primarily contain phenolic compounds such as ellagitannins or catechol tannins
• Q. alba American white oak, contains fewer tannins than French oak, but more aromatic compounds, particularly methyl octalactone
• differences in the composition of American white oak and European sessile oak indicate that wines are likely to develop differently according to the origin of the barrels used for aging
• European sessile oak and American white oak are perfectly suitable for aging fine wine
• the low aromatic potential and the high ellagitannins content of pedunculate oak indicate that it should only be used for aging brandies
• wine character is also influenced by the age and preparation of the oak
• smaller barrels result in more phenolic and tannin content, due to the increased surface exposure area and extraction
• anthocyanin-tannin complexes can be formed stabilizing the color of red wines and resulting in wines that taste less fruity but also less astringent after aging

The effects of bottle aging:

• there is also some degree of continuous oxygen exposure during wine aging in the bottle
• the interaction of tannins and anthocyanins with oxygen, which diffuses during barrel storage, further polymerizes these compounds making them taste less astringent
• proanthocyanidins and other polyphenolics eventually aggregate into larger molecules accumulating as sediment at the base of the bottle
• red wines become lighter in color
• white wines often deepen in color, turning darker honey colors as they oxidize and age
• widespread existence of cork taint is now universally acknowledged
• cork taint is mainly due to the presence of 2,4,6-trichloroanisole (TCA) in the wine
• TCA in wine leads to an unpleasant, musty off-flavor and can render an otherwise excellent wine completely useless and may cause economic losses
• range of alternative closure techniques, which might vary in such aspects as degree of market acceptability, oxygen permeation rate, ability to carry out “flavor scalping” and their propensity to contribute “taints” of varying character are now available and used by the industry
• alternative closures such as roll-on tamper evident screw cap closures (ROTE) have become a popular alternative for Australian and New Zealand white wines
• high temperature during wine aging can negatively affect its sensory properties and visual stability
• exposure of wine to heat increases the loss of individual and total anthocyanins, reducing color density
• high temperature can accelerate the reaction between urea and ethanol causing the formation of ethyl carbamate

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.