- natural sugar that remains in a wine after the fermentation process or that is added to a sparkling wine as a dosage to cause secondary fermentation in the bottle. The natural sugars remain in a wine either because the original grape juice had so much natural sugar to start with that it is not all used up in fermentation, e.g. when producing sweet wines; because the fermentation process was stopped by adding alcohol (raising the alcohol level to one that prevents the yeast from working), e.g. when producing fortified sweet wine such as port; or because they are of the small proportion of the types of sugar that do not easily ferment. Residual sugar is usually measured by percentage, weight or volume.