Several factors affect the impression of a Riesling wine’s sweetness on the palate. As residual sugar increases, the impression of sweetness increases. The sweetness from the sugar interacts with:
The acidity (malic and tartaric acids) and the level of dissolved carbon dioxide, which offers a sense of freshness
The phenolic content, which offers astringency and body
The ethanol content, which brings a feeling of weight, warmth and dryness on the finish
The pH, which accentuates the expression of the acids and reduces the sensation of sweetness as it goes down
For example, a Riesling with 1.5% (grams/liter) residual sugar can appear either dry or sweet. If the wine is 12.5% alcohol with a pH level below 3.0 and an acid level above 9 g/L, the wine will appear to be quite dry. If the wine is 10% alcohol with a pH level of 3.3 and an acid level around 6 g/L, the wine will seem relatively sweet.
This information is provided, with gracious permission, from the Riesling Rules Book of IRF Friend Pacific Rim Winery.