Welcome to part 3 of our series Terroir & the chemistry of wine. Having looked at soil and climate and dived into wine's acids and pH, we now turn our attention to the group of compounds that, more than any other, defines a red wine's colour, structure and ability to age: the phenolics.
For the experienced wine lover, phenolics are the point where the grape's anatomy, the choices in the cellar and the slow chemistry of time all meet. Here we go through where the compounds come from, how extraction is controlled, and what happens in the bottle over time. We stick to the mechanisms and let the chemistry explain what you already taste in the glass.
What you will learn
- The most important phenolic groups in wine and how they differ
- Where tannin and colour compounds come from in the grape
- How extraction and polymerisation shape a wine's structure
- How phenolics develop during ageing
What phenolics are
Phenolics make up a small but decisive share of the grape's overall composition, on the order of a fraction of a percent. Despite the modest amount, they are responsible for a disproportionately large part of what we experience in a glass: colour, flavour, mouthfeel and resistance to oxidation. They also contribute to the health-related aspects often attributed to wine.
Broadly, we divide the polyphenols into two families. The flavonoids are the large polymer molecules that account for the wine's colour and a large part of its structure. Chemically they are built around fifteen carbon atoms with two phenyl groups, formed via phenylpropanoid-malonylCoA reactions. The non-flavonoids are smaller molecules, and they are typically associated with flavour from oak. The non-flavonoids also include the stilbenes, which have two benzene rings connected by an ethane bond. The best-known stilbene in wine is resveratrol.
It is worth remembering that the flavonoids contribute more to a wine's browning than the non-flavonoids do. This becomes significant when we later look at how phenolics react with oxygen.
Tannins: sources and types
Tannins are condensed, large phenolic polymers with a strong affinity for proteins. It is precisely this tendency to bind to proteins that gives the astringent sensation we know as tannin: the feeling of the mouth drawing together because the tannins bind to the proteins in saliva.
In the grape, tannins are found in three places: in the skins, in the stems and in the seeds. The three sources are not equally attractive. The seeds make up around four to five percent of the cluster's weight and are rich in both tannins and lipids, but seed tannins in particular are something the winemaker often wants to avoid extracting too strongly. The stems make up a few percent of the weight and contain proanthocyanidins, which can give the wine a sharp, pungent character.
When it comes to the individual compounds, catechin is the dominant one. Catechin makes up around 73 % of the grape's total tannins, which makes it the backbone of the wine's tannin structure.
Why the source matters
Because the tannins from skins, seeds and stems have different characters, the technical choices in the cellar become decisive. How gently the fruit is handled, and how much the seeds are subjected to abrasion, directly affects which tannins end up in the wine. A gentle pressing tears less at the skin and thereby releases fewer tannins and phenolics, which is central in white wine production, where one typically wants minimal phenolic influence.
Anthocyanins and colour
If the tannins are the structure, the anthocyanins are the colour. These phenolic flavonoids sit in the skins of red grapes and are what determine a wine's hue. They occur in combination with sugar residues, that is, as glucosides.
This also explains why colour extraction is tied to the fermentation process itself. The anthocyanins are released easily from the skin either by heating to 70 °C or above, or during fermentation as the alcohol is formed. This is where ethanol's role as a co-solvent comes in: alcohol increases the solubility of the non-polar polyphenolic compounds and thereby draws both colour and tannin out of the skin.
Since the colour compounds sit in the skin, it also follows that a white wine made with minimal skin contact stays pale, while a red wine with several weeks of maceration draws deep colour and powerful structure.
Extraction and polymerisation
Extraction is about controlling what migrates from the solid parts into the juice, and to what degree. Here the winemaker works with temperature, time and mechanics.
Red wine must is typically macerated at temperatures between 24 °C and 27 °C to promote the extraction of pigments and phenolic compounds. White wine goes the opposite way: the juice is cooled to around 10 °C to promote fruity esters and slow the uptake of phenolics. The duration of maceration spans widely, from around three days for quickly drinkable wines to two to three weeks for wines built for ageing. The longer on the skins, the more phenolic material.
The mechanics play a part too. A batch press is gentler than a continuous press, tears less at the skin and therefore extracts fewer phenolics and tannins. Pneumatic batch presses distribute the pressure evenly over a large surface and allow lower pressure and better control.
The role of enzymes
Extraction is not just a matter of physical pressure. Pectinases break down the pectin-containing substances in the cell walls of the grape skin. These enzyme preparations affect the cell wall structure, ease the extraction of phenolic compounds and accelerate the natural breakdown of the skin during maceration. This makes enzymes yet another tool for controlling the phenolic profile. If you want to go deeper into the world of enzymes, we return to them later in the series.
Polymerisation
Extraction is only the beginning. The phenolics do not remain sitting as isolated molecules. The tannins are themselves condensed polymers, and over time they bind together and enter into new compounds. It is this polymerisation that gradually transforms a young, angular tannin structure into something rounder. A relevant detail is that a higher alcohol concentration reduces the tannin-protein interactions, which affects how astringency is experienced in a powerful wine.
Phenolics over time
The beautiful thing about phenolics is that they do not stand still. During ageing the polymerisation continues, and the ratio between the different phenolic groups shifts. Colour and structure develop in step: the young, free anthocyanins become part of ever larger molecules, and the fresh, often angular tannin sensation softens as the polymers grow.
Oxygen plays a dual role. The phenolics are part of the wine's defence against oxidation, but they are also the very thing that oxidises. Remember that the flavonoids contribute more to browning than the non-flavonoids do. In red wine production, micro-oxygenation is used precisely to control this process: small, dosed amounts of oxygen over a period from a few days to a month help to mature the tannin structure in a controlled way instead of letting the wine oxidise uncontrollably.
The result is that a well-built wine over time moves from primary, fruit-driven youth towards a more integrated whole, where colour, tannin and body have melded together. It is phenolic chemistry that makes ageing meaningful.
In brief
- Phenolics make up a small part of the grape, but govern colour, structure, mouthfeel and resistance to oxidation.
- Polyphenols are divided into flavonoids (colour and structure) and non-flavonoids (including stilbenes such as resveratrol).
- Tannins come from skins, seeds and stems, and catechin makes up the majority of the grape's tannins.
- Anthocyanins sit in the skin as glucosides and are released by heating or during fermentation with alcohol.
- Temperature, time, pressing and enzymes control the extraction, and polymerisation softens the structure over time.
Frequently asked questions
Why does one avoid extracting seed tannins?
The seeds are rich in both tannins and lipids, and their tannins have a character the winemaker often wants to limit. That is why one works with gentle handling and pressing, so that the desired structure is drawn from the skin without over-extracting the harder seed tannins.
Why does a young tannin become softer over time?
Because the tannins continue to polymerise during ageing. The smaller molecules bind together into larger ones, and this changes the mouthfeel from angular to rounder. Controlled oxygen supply, as in micro-oxygenation, can support this maturation.
Ready for the next step?
Now that you have the colour and structure of phenolics in place, it is the obvious moment to move on to what makes each wine recognisable on the nose. In the next part, Wine's aroma compounds, we look at the volatile compounds that shape the scent, from varietal aromas to what arises during fermentation and ageing.
Do take your new knowledge out into the range and taste how tannin and colour play together across different styles. And remember that the finest pairing is still the wine you like with the food you like. The rest are nuances you now have the tools to understand.