Phenols are natural substances which occur in plants and, of course in the grapes used to make wine. Collectively known as polyphenols, they are a group of several thousand compounds which affect the colour, flavour and texture of wine.
There are two broad groups of phenols: flavonoids and non-flavonoids. In the first group there are Anthocyanin (from Greek = blue flower), a phenol occurring in the grape-skin responsible for its dark red to blue colour, and anthoxanthins with white to yellow colour. Then there is Tannin (from German “Tannenbaum” or fir tree), a phenol which has no aroma or flavour but which can give a bitter character to the wine as it reacts with proteins on the palate. It is found in the skins, stems and pips of the grape, and also the wood in barrels. It helps with wine stability and the bitterness fades over time as the tannins polymerise and fall out of solution. This is why red wines are laid down, but phenol levels can be reduced in the winery by encouraging this process. Catechins are phenols produced by the vine as a defence and have colour stability and antioxidant properties.
The non-flavonoid group includes Resveratrol, another phenol produced by the vine as a defence against microbes. It is found in red and white grapes but more in red wine because of the maceration of the skins during production. White wines are usually made without maceration of the grape-skin and thus mainly contain the phenols found in the pulp. Resveratrol has beneficial effects on the human heart and extracts are already being produced. In winemaking everything is interdependent. There are so many variables such as grape variety, ripeness, pressing, extraction, barrel ageing and many, many more.
Some very old Sherries – those aged oxidatively - can contain significant phenol levels which have been extracted from the old oak butts over decades, up to a century or more. It is extremely rare for any other white wine to have such extensive wood ageing. Since Phenols are alcohol-soluble, the longer the wine ages, and the stronger it is, the more phenols it will absorb, especially as evaporation concentrates the wine. But it is the oxidation over time which gives them their dark colour, whereas with Finos and Manzanillas the ageing period is shorter so they have a lower phenol content and the flor protects them from oxidation making them paler.
Nowadays we have sophisticated means of analysis and measurement and so polyphenol content can be measured, usually expressed in mg/l of gallic acid. The Total Polyphenol Index (TPI) is used and a typical sturdy red Cabernet Sauvignon might register around 90 on this scale while a more everyday red would register more like 50-60, and an oxidatively aged Sherry a little less – depending on age. It is a useful guide to the extraction and structure of the wine without the need to taste it, but virtually never appears on wine labels.