A Whiter Shade of Pale
As I mentioned in my previous article on how red wines age, this is a complex subject that not many scientists or winemakers have a good understanding of. Such is the mystery and mystique of wine and also part of its pleasure.
But we are able to be clear on certain aspects, so for those that want a brief overview, I've summarised the main points in 'The short of it' below. For those with more appetite for the scientific details, scroll down to 'The long of it' further on.
The short of it
White wines start life pale in colour, typically anything from water-white with tinges of green towards light golden yellow.
With time, colour deepens and gains hints of amber, orange and eventually brown (or quickly for certain fortified white wines like some sherry, Madeira and Australian muscats).
Aromas change from fruity and floral towards nutty, roasted, caramel characters and in some cases, cooked vegetables, canned peas or asparagus, and even petrol or kerosene characters may appear.
It depends very much on the wine whether such changes are desirable or not, and whether the wine has the fruit concentration and freshness to carry it. For instance, petrol is generally seen as a positive character in aged riesling while canned peas and asparagus can be really unpleasant in over-mature sauvignon blanc.
White wines (and rosé) have very much less tannin or polyphenol content than reds so lack that natural preservative, and fewer white wines are deemed to be able to age well.
Acidity & sweetness
Good levels of acidity and plenty of fruit concentration are essential but then it comes down to style. Sugar seems to help wines age - noble sweet wines are often long-lived (though not ice wine which depends on its fresh fruitiness for quality) and notably acid dry wines like riesling.
Good white Burgundy can age - but even here not reliably. In contrast, many wines get their charm from youthful, fruity varietal expression and there's nothing to gain once this goes (muscat, albariño, sauvignon blanc, pinot gris and gewürztraminer would fit this mould in most cases, though of course there are exceptions and The Wine Society's buyers will let you know in their tasting notes).
The long of it
The chemistry of white wine aroma has been studied more closely than for red wines but it still is a hugely complex topic and only partially understood, with hundreds of different potential aroma and flavour molecules identified.
Readers of a previous article on taste perception in wine (The Science of Taste) will know that aroma and flavour are closely linked. Wine aroma and flavours can come from grapes themselves, or may be released from flavourless precursors during crushing and fermentation.
Other groups of flavours are produced by yeast metabolism or chemical reactions during fermentation and ageing.
And finally, there are aromas and flavours dissolved from oak in barrel-fermented and matured wines (a topic for another article).
Flavour from the grapes themselves?
Most grape varieties don't have much flavour in the grapes (which can make picking decisions tricky) but are full of flavour precursors, largely located in the skin. These are typically bound to sugar molecules like glucose in forms called glycosides and don't give any aroma or flavour until after crushing when they start to be released.
Techniques like using pectinase enzymes to break down cell walls, or freezing grapes before crushing, or extended skin contact are used by some winemakers to help release these potential flavours. As fermentation gets going, yeast metabolism not only produces that all-important alcohol, but also many of the aromas and flavours we have come to associate with wine.
The role of terpenes
In some grape varieties, particularly muscat but also riesling, gewürztraminer and torrontés, a group of chemicals called terpenes are important. They are highly fragrant and typically give aromas of flowers, rose petal and citrus.
The most important forms in grapes and wine include linalool, nerol, geraniol and citronellol. These exist in both free volatile forms (particularly high in muscat) and bound in glycosides that only become aromatic during crushing and fermentation. Unlike most wine aromas, these are relatively stable through the fermentation process. The importance of free terpenes in giving varietal character to muscat is why it is one of the few varieties where both wine and grapes taste similar.
Methoxyrazines are another important group of compounds contributing to varietal character, particularly in the sauvignon family (including both sauvignon blanc and its offspring cabernet sauvignon).
Green pepper and herbaceous aromas are typical and may be regarded as a fault if too dominant, especially in under-ripe cabernet sauvignon. These are produced by grapes, released from skins during crushing and survive fermentation to make it into the wine.
There are a few ketones found in grapes and wine that are significant in giving aromatic character. More technically called 'norisoprenoids', they accumulate in grapes during ripening but as flavourless precursors bound to sugars. They are released during fermentation and as wine ages.
B-damascenone is notable for its intense exotic flower or rose-like scent and has been identified in chardonnay and riesling among others. B-ionone has a raspberry or violet scent and is believed to be significant in the varietal aromas of several red grapes.
One significant ketone called diacetyl can be produced during malolactic fermentation. At high levels, it gives a buttery or butterscotch character, regarded as a fault if too dominant.
Another groups of chemicals that can be important in wine flavour includes aldehydes, notably hexenal and hexanal which are formed during crushing and give grassy notes (some research suggests that machine harvesting enhances green characters in sauvignon and this may be a factor).
It appears that these chemicals mostly breakdown into corresponding alcohols during fermentation though. Acetaldehyde (or more correctly ethanal) is an important flavourant in some wines, most notably sherry, but in most wines when it goes above its perception threshold, it would be regarded as a fault giving bruised apple, stale notes.
Not forgetting esters…
Esters are very important in aroma and flavour and more than 160 have been identified in wine. These are predominantly formed during fermentation in reactions between organic acids and alcohol.
They are often important in giving fruity aromas associated with young wine. Some examples include isoamyl acetate (banana-like), benzyl acetate (apple-like) and ethyl butyrate (pineapple).
Fermentation temperature influences exactly which esters form - cooler temperatures favour esters with fruitier characters, as does the technique of carbonic maceration (see More Secrets of Fermentation). I should also mention ethyl acetate here - at low levels it helps lift fragrance, but in wines contaminated by certain bacteria (especially Acetobacter), it can show an unpleasant nail-polish-remover smell.
Volatile thiols are particularly important in sauvignon blanc, but have also been found in riesling, colombard, semillon, cabernet sauvignon and merlot. These are derived during fermentation from sulphur-containing amino acids and other precursors.
A lot of research in New Zealand has gone into understanding the aroma and flavour of sauvignon blanc and it seems that three thiols are particularly important (see table). 4 MMP contributes to the blackcurrant flavour in cabernet sauvignon and green leafy notes on sauvignon blanc though it's not the same chemical as found in actual blackcurrants.
3 MH gives passion fruit, citrus or guava notes, while 3 MHA gives grapefruit, passion fruit and box leaf, but cat pee and sweat at high levels. Sulphur-containing compounds can also give unpleasant faults as a result of production of hydrogen sulphide (bad egg gas) by yeast in the absence of oxygen, which can lead onto to secondary compounds such as mercaptans noted for odours of stale garlic, rotten cabbage and cabbage.
This is just a small snapshot of the complex picture that is young wine, and it seems the relative ratios of these aromas and flavours, plus the effects of yeast strain, temperature, pH, alcohol level and bacteria can all influence the taste of wine too.
'so every time you open a bottle it will genuinely be different.'
As wine matures further, several other changes are known to take place, but in a very complex and dynamic way, so every time you open a bottle it will genuinely be different.
Youthful fruit fades…
For example, esters break down into their constituent alcohols and acids so those young fruity flavours disappear, though cool storage temperatures can slow down the speed of these reactions. Esters may also link up into longer chains so those initially fruity notes become soapy in character.
The very aromatic terpene, linalool, oxidises into alpha-terpineol which not only has a musty, pine-like aroma, its perception threshold (perhaps luckily) is at least 10 times higher, partly explaining why muscat wines don't age well.
Some ketones transform into lactones and notably, especially in riesling, a hydrocarbon appears called TDN for short. This gives that petrol/kerosene smell prized by lovers of aged German riesling but regarded as a fault in Australian versions.
Thiols are particularly susceptible to oxidation so levels of 3 MH decrease markedly during barrel-ageing and during malolactic fermentation, while 3MHA decreases significantly in bottle.
Flavours can soften
Apart from aroma modifications, other taste changes happen too. The most important grape acid, tartaric acid can undergo esterification which means it loses a carboxyl group so tastes less sour, helping to soften flavour. Tartaric acid can also transform from its L-form to the mirror image D-form. This is less soluble in wine allowing crystals to form and reducing acid levels in the wine.
It's useful to know that these reactions usually depend on temperature; so cool storage can slow down changes.
Wine aroma and flavour is such a complex subject that wine chemists will be occupied for many years to come. And for the rest of us, perhaps just opening the bottle for some 'end- use analysis' is the best solution.
Snapshot of some of the important wine aromas and flavours but no means a complete picture