Phenolics in Wine

“It’s Greek to me!”

You might hear winemakers talk about phenolics and to the general wine drinking population, they might as well be speaking Greek. But a little knowledge can a long way towards your general understanding of why wine looks and taste the way it does, as well as make you stand apart from the oenophile pack.

Phenols and phenolic acids are found in the pulp, skin, seeds and stems of grapes.  They are chemical compounds that effect aroma, flavor and texture. While the chemistry is complex, they are benzene rings. Many other compounds can hang off of them and that is what makes wines taste different from each other.

Phenolics are leached out when grapes are macerated and pressed. Longer maceration periods and harder pressings will create higher concentrations of phenolics. As such, red wines have greater phenolic intensity but skin fermented white wines also have a lot.

There are hundreds of phenols in wine. Flavonoids are the most important and within this group there are three principle types: anthocyanins, tannins and catechins.

Anthocyanins are largely responsible for the pigmentation in red wine. They do not add to the aroma or flavor. A wine’s color is in part determined by the ionization (the basic high school chemistry definition of ionization is the loss or gain of electrons to form a positive or negative charge) of anthocyanins of its pigments.

The basic wine colors

Here’s the tricky part. Wine has three pigments, red, blue and clear. Wines with higher acidity have a higher occurrence of ionization that creates a brighter red hue. On the other hand, wines with lower acidity are not as ionized and will have more of a purple color. Over time, anthocyanins react with other compounds to cause wine to turn more of a brick color and eventually, brown.

Tannins are what cause wine to seem astringent and they effect both the flavor and mouth feel. They are present in skins, stems and pits. Naturally occurring tannins are considered proanthocyanindins because anthocyanins are released as they heat up in an acidic solution, which is what happens to wine as it ferments. As wine ages, tannins form long polymer chains. You might hear people say that tannins break down over time but the opposite actually occurs and they become too large for people to perceive so they seem to soften. Also, anthocyanins and tannins create polymers that continue to grow until the whole molecule becomes unstable and falls out of the solution. This is how sediment is formed.

Oak has hydrolysable tannins that are created by ellagic and gallic acid found in wood and this is one of the reasons why winemakers ferment and age wine in oak barrels. New barrels or barrels that have only been used once have the highest level of tannin.

Vineyard fog

Catechins are the third component of the trio.  They are mostly in seeds and account for their bitter taste. In combination with anthocyanins and tannins, catechins stabilize color. Importantly, they play a role in fighting microbial activity in diseases such as downy mildew. Grapes from cool damp climates have evolved to produce more have more catachins than those from hot, arid areas.

Two important non-flavonoid phenols are stilbenoids and hydroxycinnammic acids. Of the hydroxycinnammic acids, tartaric acid is the most discussed as it plays a crucial role in the stabilization of wine. You will sometimes notice tartaric crystals (tartrates) in your glass or on the cork. These are formed during fermentation when tartaric acid attaches to lees and other particles. They are harmless.

Resveratrol is the most common stilbenoid. It is found in grape skins so there is a higher density in wines with increased skin contact. Like catechins, it provides defense against hostile microbes, especially those found in – you guessed it – cool, damp regions.

As mentioned, there are many other phenolics found in wine but these are the ones that are the most valuable for your own knowledge and will score you points if you want to impress a winemaker!

Frank Melis
Golden Gate Wine Cellars

P.S. As always, I am here to assist you and answer any questions that you may have about wine! Please don’t hesitate to give me a call at 415-337-4083 or email me at