Roger, can we also look at Riesling sweetness level by the alcohol amount ?
Answer From Expert Roger Bohmrich MW
You are highlighting something which could indeed be helpful for someone wondering if a certain Riesling will be dry or sweet, based only on the alcohol content. The idea behind this is that alcohol percentage, with this variety in particular, tends to be associated with residual sugar in the wine. The lower the alcohol, say 9%, the greater the likelihood that some sugar remains unfermented; if the alcohol is near the upper limit for Riesling, around 13%, the greater the chance that no fermentable sugar remains. (All this assumes that no sweetener, a concentrate for example, has been added.) It's a handy rule of thumb and works best at the low and high extremes of ABV. But as with all such devices, it doesn't take all the nuances into account.
On the Archives page of my website, you'll find a report on a tasting of Austrian Riesling and a discussion of what "dry" actually means for this variety. On the last page, I present the technical analysis for 8 Rieslings (http://www.vintrinsic.com/images/Tasting_Notes_Report-Austrian_Riesling-NY,_Oct_15_3_.pdf). The wines range from 2.1 to 8.0 g/l of residual sugar, and from 12 to 13.55 ABV. You will see that the relationship of RS to ABV is certainly not an exact one. In fact, all of these wines would taste "dry" to most tasters, and that impression would come from the complex interplay of alcohol, acidity, pH, and sugar. Of these parameters, pH is arguably the most influential, yet often is not printed on the label of most European examples.
Let's come back to your question. The alcohol, low or high, may give you a general idea of sweetness. But whether a specific wine tastes drier or sweeter depends on many factors, not least your own taste sensitivities.
A final twist: due to U.S. labeling rules, the ABV stated on the label may not be exact since there is an allowance of plus or minus 1.5 percentage points for wines with 14% alcohol or less.