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The question: What makes some wines dry and others sweet, and how can I tell which is which?

The answer: It's a paramount consideration for most wine consumers, yet the industry likes to keep us guessing.

The relative dryness of a wine is measured in terms of residual sugar, or RS in the wine geek's argot. This is the level of natural grape sugar left after fermentation. Once grapes are crushed, yeast feeds off grape sugar to produce alcohol and carbon dioxide. It's hard to predict exactly how yeast will behave, and they rarely finish the job completely, mainly because some sugars are not easily fermented. There's always a little sugar left, even in the case of "dry" wines, though the level is pretty trivial. Technically, a wine is considered dry if it contains less than two grams of sugar per litre of fluid. But even here the perceived dryness of the wine depends on a host of other components, most notably acidity. If there's a lot of acidity in the wine (as in the case of, say, riesling), it can still taste pretty dry even if it contains much more than two grams per litre.

To produce off-dry or sweet wines, winemakers will intentionally halt fermentation prematurely, usually by controlling temperature. Chilling the vat paralyzes the yeasts, halting them from completing the job. Alternatively, many dessert wines are produced from dried grapes, essentially raisins, which contain a higher sugar-to-juice concentration. The yeasts will gorge till they get their fill, then die off as the alcohol rises, leaving behind lots of extra sugar. Many dessert wines contain much more than 100 grams of sugar per litre. Sweet port is made in yet another way, by halting fermentation halfway through the process with the addition of high-alcohol spirit. That extra alcohol instantly kills the yeast, once again leaving behind lots of natural grape sugar.

In a few cases, sweetness will be added to the wine from the outside, sometimes in the form of natural, unfermented grape juice. Many sparkling wines and German rieslings, which tend to be high in acidity, will be balanced in this way so as not to taste too sharp. But this can be considered an exception to the "residual-sugar" norm.

How do you know the sugar content? As I say, winemakers can be cagey. They rarely list whether a wine is dry, off-dry or sweet, preferring to let consumers make an educated guess. This can be frustrating, because with some wine styles the spectrum can range from bone-dry to quite sweet. Vouvray, an appellation of France's Loire Valley, is a classic example. You rarely know in advance how sweet the wine will be because only some Vouvray producers list the terms "dry" or "sweet" on the label. Look for "sec" if you want to be certain it's dry ("moelleux" and "doux" denote sweet).

German labels tend to be descriptive, but it helps to know German as well as something about wine laws. "Trocken" means dry, though "dry" wines often contain much more than two grams-per-litre of sugar. Those wines are rendered essentially dry-tasting by the high acidity prevalent in German wines. The popular label term kabinett often is mistaken to mean dry when in fact it refers to the relative sweetness of the grapes at harvest, not the wine itself. That said, kabinett rieslings usually are on the drier side (no guarantees). One big and laudable exception is Canadian riesling. Often producers will helpfully label them as "off-dry" to distinguish slightly sweet styles from the dry riesling.

The best rule of thumb (and I'm sorry to say it's sorely inadequate) is to check for alcohol content. If the wine weighs in at 11 per cent or lower, chances are it's at least a little sweet. Low alcohol tends to mean the yeast did not finish the job (of converting sugar to alcohol). Less alcohol usually equals more RS.

In the end, sugar is just a rough gauge of whether the wine will taste sweet or not. It's a question of balance. Some 14-per-cent-alcohol wines can taste subtly sweet, not because they contain much sugar but because they're either very fruity (a flavour often confused with sweetness) or because they lack sufficient acidity to create a sensation of total dryness. Australian shiraz is a good example; it can taste vaguely sweet even when it's technically dry.

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E-mail your wine and spirits questions to Beppi Crosariol. Look for answers to select questions to appear in the Wine & Spirits newsletter and on The Globe and Mail website.