The road less travelled

Winemaking: how red wines age

Caroline Gilby MW investigates how red wines age, looking at tannin, colour and flavour development.

The magic in ageing red wines

There's widespread belief among wine drinkers that wine needs to be matured and indeed many people believe that all wine can, and should, be aged before drinking. How many of us have a bottle or two tucked away waiting for the 'right' occasion, safe in the belief that it will be quietly improving while it is waiting?

In reality, most wine is ready to drink when it is released and winemakers understand that the most cellaring a wine will receive is in the boot of the car on the way home from the shop. Few of us have the luxury of a cool cellar available, while a wine rack in the kitchen or under the stairs is less than ideal for long term storage. Some Italian research on sangiovese found that wine stored in a kitchen environment aged around four times as quickly as bottles stored in a cellar.

It turns out that ageing wine is an incredibly complex subject that not even wine scientists or winemakers have fully got to grips with. Wine is made up of hundreds of compounds that vary with each grape, vintage and winemaking style, so there are almost endless possible interactions too.

Colour change: one of the most noticeable signs of ageing

One of the most obvious changes as a wine ages is in its colour, with reds going from pinkish-purple to ruby, garnet and then brick, and whites darkening through gold towards brown.

The Short of It

Young red wine is pinkish-purple in colour with typically fruity aromas and flavours (berries, cherries, black berry, cassis, plum and strawberry come to mind). These are derived from grapes and from changes during fermentation. Sometimes (depending on the grape and winemaking, the wine will have dry or astringent characters from young tannins (think stewed tea and the drying sensation in your cheeks) and noticeable fresh or crisp acidity.

As red wine ages, colour changes to ruby, garnet, brick-red then brown. Fruity notes may gradually fade and other aromas appear - giving what the wine trade likes to call complexity (really just shorthand for lots of aromas and flavours that are too much effort to describe). Typical aged aromas include pencil shavings, violet cedar, tobacco, caramel, toast, coffee, leather and vegetal notes. Tannins also appear to mellow and become softer to taste and acidity is perceived as less sharp.

A wine that has aged well has enough fruit concentration to benefit from these softer textures and more complex aromas and flavours. Many wines don't age well - either because their joy is in those youthful fruit characters, or because the wine does not have the fruit intensity, along with tannins and acid, to develop in an appealing way so that brown, fruitless and dull is the result.

From the grape
During fermentation and ageing
In the wine
Anthocyanins (pink/purple pigments) Lots of reactions involving yeast, oxygen and by-products of fermentation pyruvic acid, acetaldehyde) Red, orange anthocyanin/tannin complexes
Tannins   Larger tannins
Other polyphenolic compounds   Perceived softer acidity and tannins
Acids and sugars   Alcohol

The Long of It

Red wines have been studied most closely and one of the most important groups of compounds involved in wine ageing is the flavonoids. These are phenolic compounds found in plants and include flavan-3-ols (in wine the main ones are catechin and epicatechin - bitter compounds also found in tea and dark chocolate); the similar sounding flavonols (not believed to give flavour but do form pigment complexes), anthocyanins and tannins. These contribute to colour, texture and flavour in wine.

The base of phenolic compounds
The base of phenolic compounds

Phenolic compounds are based on a ring-shaped molecule consisting of a benzene ring with a hydroxyl group (an oxygen atom bonded to a hydrogen atom) as shown here.

In wine, the phenolic compounds are much more complex than this but will always contain some of these ring structures. And a couple of definitions - a polymer is a large molecule made up of chains or rings of smaller molecules (or monomers) of the same kind, and polymerisation is the process of linking these monomers to form a polymer (other common polymers you might have heard of include PVC polystyrene, nylon and Teflon).

A good place to start is with the anthocyanins because these come directly from grape skins where they give colour, typically the very pinkish-purple hue of young wines. They have cheerfully floral names like Malvidin (the most important one in grapes), Peonidin, Petunidin and Delphinidin.

These grape-derived pigments start to disappear very quickly, even during fermentation, as they react with and combine with other components of the wine to form more complex and stable anthocyanin-derived pigments, which are more red in colour than simple anthocyanins.

Scientists believe that there are probably a whole series of mechanisms involved in these changes, such as adsorption by yeast; oxidation and breakdown; reaction with proteins or tannins leading to precipitation, and so on.

Kékfrankos from Heumann - showing that lovely pink anthocyanin colour
Kékfrankos from Heumann - showing that lovely pink anthocyanin colour

Some changes that are known include a reaction between anthocyanin and pyruvic acid (produced by yeast during fermentation) to produce a brick-red pyranoanthocyanin, also named vitisin A.

Acetaldehyde (produced from oxidation of alcohol) is another molecule that is believed to be involved in polymerisation of anthocyanins along with those flavonols and catechins mentioned earlier. And many more by-products of fermentation are believed to be involved too.

It seems that around 25% of the anthocyanins have disappeared by the end of the alcoholic fermentation, 40% after a year of ageing and then the process of polymerisation continues gradually as a wine ages, so that the purple tones fade and the brick and orange-toned polymers come to dominate.

Barriques : CIVB/Philippe ROY
Barriques : CIVB/Philippe ROY

Techniques like barrel-ageing and micro-oxygenation are used by winemakers partly to develop and stabilise colour with the help of tiny amounts of oxygen. So, you can see the picture is already complicated - one research paper on tempranillo identified 129 pigment molecules.

Tannins are the other important phenolic polymers

The other important group of phenolic polymers, or polyphenols, in wine are the tannins. These polyphenols get their name from their traditional use for tanning leather - because they bind to and cross-link proteins (useful for turning floppy animal skin into firm leather for instance).

You can sense tannins on your gums and around the sides of your mouth
You can sense tannins on your gums and around the sides of your mouth

In wine, these are formed of at least two and often many flavan-3-ols (or catechins) joined together and they give astringency and dryness in the mouth. This is more of a tactile sensation than a flavour as tannins react with proteins in saliva and make the mouth seem dry, something that probably originally evolved as a defence against grazing animals. Tannins are very hard to study in wine, as there is a dynamic mixture of tannins reacting, linking up and breaking apart constantly, so measuring exactly what is present is very scientifically challenging.

You can sense tannins on your gums and around the sides of your mouth

Then another complication is that the source and amount of tannin will be a factor. Tannins mostly come from grape skins or seeds, and possibly stems too depending on the winemaking method. Seed tannins are regarded as tasting rougher and usually show more noticeable bitterness than the longer chain tannins from than skin, possibly due to presence of gallic acid units.

Winemakers typically try to be careful to harvest only when grape pips are ripe and brown, and use techniques that don't extract these bitter seed tannins. Tannins have a role in ageing potential of red wine - in complex combinations with anthocyanins. These seem to act as a 'sink' to mop up oxygen as well as undergoing slow oxidative changes themselves. However, the detailed science still needs more research to try and understand why and how perceptions of tannin change as wine ages. Oxidation may have a role as well as hydrolysis of tannin polymers into smaller units.

Do tannins taste 'softer' in aged wine?

One frequently repeated story about how wine tastes softer as it ages is being disproved. The old myth was that tannin chains get longer and longer until they are too big and heavy to stay in the liquid, so they fall out as sediment, making the wine literally taste softer by removing a portion of the tannins.

Unfortunately, research has found this to be wrong and it is short-chain tannins that taste softer, with longer ones tasting more astringent and drying. If the myth were true, wine would get more astringent as it aged, as tannins polymerised into longer chains but that's not what we see. Tannins do appear to taste softer as a wine ages, but no one has quite worked out why yet, and it seems they don't disappear either - analysis of 50-year-old wine in Australia still found high tannin levels.

Plunging in further

So maturation of red wine is still largely a very complex mystery, with plenty to keep wine scientists busy. And winemakers too don't know for certain how to predict if a wine will age well. In the meantime, perhaps the best thing to do is a bit of personal research by wielding a corkscrew on some older bottles.

Caroline Gilby MW

Guest Writer

Caroline Gilby MW

Caroline Gilby is a Master of Wine and a scientist by training. She is a wine writer with a passion for the wines of Central and Eastern Europe and contributes to several wine books, magazines and websites.

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