This is a concept that on the surface seems very simple, but that turns out to be quite challenging. It is important to have some familiarity with what balance entails if you are to become a good wine taster.
Balance in wine refers to the interaction and harmony between two or more of the wine's constituents. By far the most straightforward balance is that between sugar and acidity. Not all wines, of course, have residual sugar, though all have some acidity. Sugar-acid balance is thus limited to wines which have an interplay between these two elements.
There is no accurate formula for calculating the perfect acid-sugar balance in a wine, despite the fact that there are some people who advance that very notion. In its simplest sense, a wine which has a good acid-sugar balance tastes neither too sweet nor too acidic: the sugar exists in the right quantity for the acid, and vice versa.
By extension, a wine which is out of balance has either too much acid or too much sugar. There are plenty of off-dry-to-sweet white wines on the market which are more or less out of balance. A wine with too little sugar for its acid will taste harsh, sharp and acidic; the evolution of flavors in the mouth will be interrupted by the sensation of acidity. A wine with too much sugar will taste cloying, sugary and flabby, and will not refresh the palate.
Some wines have too much sugar and acid. They are often the result of a winemaker trying to balance a high acid with additions of sugar. These wines don't work, because the other elements if the wine, especially 'extract', don't match the sugar and acid. Experienced tasters often describe such wines as having a 'sweet-tart' character.
The balance between astringency (tannins) and acidity in red wines is of paramount importance. French enologist Emile Peynaud, in his book The Taste of Wine, makes the following points:
* the less tannic a wine is, the more acidity it can support
* the higher a red wine is in tannins, the lower should be its acidity
* the combination of high acid and high tannins make for the hardest and most astringent wines
Another important balance is that between alcohol on the one hand, and acidity and astringency on the other. This is obviously most relevant to red wines. Too little alcohol will cause the acidity and astringency to dominate, making the wine harsh and thin. Too little acid and astringency will cause a wine to taste overly soft, heavy and flabby, with the spirity quality of the alcohol playing too much of a role. Back to Emile Peynaud:
* a wine tolerates acidity better when its alcohol content is higher
* a considerable amount of tannin is more acceptable if acidity is low and alcohol is high
These concepts find very useful application during the barrel-aging of red wines. It is often found that a young Pinot, for example, tastes vaguely out of balance with regard to alcohol, acid and tannins. Small additions of acid to a laboratory sample seem to improve the wine. But what it really needs is more time in barrel, to pick up some tannins from the oak. After eight months or so the low acid becomes not only acceptable but desirable.
In some wines, notably those from Alsace, there is an interplay between small amounts of sweetness and bitterness. Remove the sugar, and the bitterness becomes too apparent; remove the bitterness, and the sweetness (exacerbated by low acid and high alcohol) will play too much of a role in the finish. Alsatian wines in some ways redefine the concept of balance.
Flavor intensity, sometimes referred to as extract, exists in balance with sweetness. Good late harvest wines, as well as sweet fortified wines, have an enormous amount of extract to give the wine interest. This is how such wines can be almost syrupy sweet while still managing to finish dry - a seemingly contradictory situation. These wines also have lots of astringency to aid in this effect. Australian wine tasters refer to the flavor intensity which balances sweet wines as 'lusciousness'.
Other aspects of wines which exist in balance are oak vs. fruit and age vs. youth. As you can imagine these are almost entirely in the realm of subjective response; some tasters love very oaky wines, while others would call the same wines horribly unbalanced. Whole nations can exhibit a preference for one character over another - in Great Britain, for example, there has traditionally been a strong leaning toward wines with extreme bottle age. To these drinkers a wine showing any fruit flavors is one which needs more cellaring.
The temperature at which a wine is served can have a dramatic effect on the balance of its various elements. Low temperatures make tannins seem much more apparent - try chilling a full-bodied red wine down sometime to demonstrate this to yourself. Most people find that wine tastes less acidic at a low temperature. Sweet wines taste sweeter at higher temperatures, and by extension slightly sweet wines, served cold, will generally be perceived as dry. High temperatures tend to make the alcohol in wine more apparent. This can be a problem with red wines drunk in the summertime - the alcohol, being very volatile, will spoil both the nose and the palate of the wine.