Welcome to the second instalment in our six-part series on wine blind tasting.
(In case you missed it, here is the first instalment: Why Blind Taste Wine?)
Wine is estimated to contain over one thousand different flavour compounds, half of which are made by yeasts during the process of fermentation.
Some aromas leap out of the glass while others need to be coaxed out by stirring—a reflection of their relative volatility in solution.
Over time, some compounds bind to one another, become insoluble, and precipitate out of solution, resulting in tannin sediment or tartrate crystals.
Apart from water, the most important component of any wine is ethyl alcohol, or ethanol, which is formed by the fermentation of sugars by yeast cells.
Although ethanol does not taste of much, it does provide body or density to the wine and also alters the perception of other compounds. For example, a wine with a modest alcohol can come across as more savoury than a similar wine with a higher alcohol, while excessively high alcohol can obscure fruit flavours and aromas.
Alcohol ought to be in balance with the other components of the wine, in which case it is perceived as unobtrusive or ‘integrated’.
Next come organic acids. Wine grapes contain malic and tartaric acid and also a small amount of citric acid.
Tartaric acid stabilizes the finished wine, but some of it may precipitate out in the form of tartrate crystals resembling shards of broken glass.
Malic acid, named after the Latin for ‘apple’, gives green apples their characteristic bite.
During the winemaking process, malic acid may be converted to lactic acid through a decarboxylation reaction variously referred to as secondary fermentation, malolactic fermentation, or malolactic conversion. This can occur naturally, but is often initiated by an inoculation of desirable lactic acid bacteria, usually Oenococcus oeni.
Lactic acid is also present in soured milk products, whence the name, and is softer and richer than malic acid, leading to a rounder and fuller texture.
With some fruity and floral white grape varieties, for example, Riesling and Gewurztraminer, the malolactic conversion may be inhibited to conserve a tarter and more acidic profile.
Other acids in wine include succinic acid, which is a by-product of fermentation, acetic acid or vinegar, and butyric acid.
Excessive amounts of acetic acid or butyric acid (which smells like spoiled milk or rancid butter) are bacteria-induced wine faults.
Apart from preserving it, the acids in a wine contribute to freshness and depth or contrast; balance alcohol, sugars, and flavour components; and help to dissolve fats in accompanying food.
A wine lacking in acidity can appear flat, dull, and uninteresting.
Grapes contain near equal amounts of glucose and fructose sugars, which are converted to ethanol during the fermentation process. Sometimes, the fermentation process is inhibited so that the wine is left with a certain amount of so-called residual sugar.
During the fermentation process the yeast preferentially eats up glucose, such that most residual sugar is the sweeter tasting fructose.
Dry wines have a residual sugar of 4g/l or less, which, in general, is undetectable or only indirectly detectable as offset acidity or a slightly fuller body. At the other end of the scale, some sweet wines can contain more than 100g/l of residual sugar.
The sweetness of a wine can be masked by acidity and, to a lesser extent, tannins.
Polyphenols (Anthocyanins and Tannins)
Polyphenols are a broad group of chemicals that are principally found in the grape skins. They account for much of the taste of a wine, and, over time, interact with other chemicals in the wine to form a vast array of secondary and tertiary flavour compounds.
Anthocyanins are a class of red, blue, and purple polyphenols that leach into red wine through skin contact during fermentation. They are unstable and, in the presence of oxygen, react with tannin molecules to form larger compounds that precipitate out of the wine, leading to some colour loss. At the same time, anthocyanins are antioxidants that preserve the wine and, according to some scientists, also the drinker.
Tannins are a group of polymerized polyphenols found in grape skins, pips, and stems. The tannin levels of a wine are related to, among other things, the degree and duration of contact with the skins and other solid matter. Although tannins are mostly associated with red wines, some white wines undergo a degree of skin contact to give them a slightly astringent texture. Oak barrels can represent an additional source of tannins for both red and white wines.
Tannins are detectable as a textural or structural element together with a certain astringency and bitterness. They interact with saliva proteins to form large compounds that prevent the saliva from lubricating the mouth—experienced as a drying, puckering sensation. With increasing bottle age, they come across as softer and gentler, sometimes almost silky or velvety. The process by which this occurs is poorly understood.
The aroma and most of the flavour of a wine is perceived not on the tongue but in the nose, triggered by volatile compounds that escape the surface of the liquid and reach the olfactory bulb.
These volatile compounds originate either in the grape itself or as a by-product of chemical reactions during fermentation or maturation. They include higher alcohols (or fusel oils), esters, aldehydes, lactones, and pyrazines.
Short chain esters are responsible for fruity and floral notes, and long-chain esters for notes of perfume and soap. Aldehydes give rise to nutty, sherry, or oxidized notes; lactones to vanilla and butter notes; and pyrazines to vegetal, leafy, grassy, and green pepper notes.
This article has outlined the principal components of wine.
But wine is so much more than a soup of molecules: it is the fruit of a soil, climate, and vintage, digested by a fungus through a process guided by the culture, vision, and skills of an individual man or woman.
Wine is one of the most complex and harmonious of all beverages. It speaks a language, and it has many stories to tell—if only we can read them.
Next week, we will be talking about the perception of wine, covering both the physiology and the psychology of wine tasting.