27 CFR § 24.177 - Chaptalization (Brix adjustment).
In producing natural grape wine from juice having a low sugar content, pure dry sugar or concentrated grape juice may be added before or during fermentation to develop alcohol. In producing natural fruit wine from juice having a low sugar content, sugar, or concentrated juice of the same kind of fruit may be added before or during fermentation to develop alcohol. The quantity of sugar or concentrated juice added may not raise the original density of the juice above 25 degrees Brix. If grape juice or grape wine is ameliorated after chaptalization, the quantity of pure dry sugar added to juice for chaptalization will be included as ameliorating material. If fruit juice or fruit wine is ameliorated after chaptalization, pure dry sugar added under this section is not considered as ameliorating material. However, if fruit juice or fruit wine is ameliorated after chaptalization and liquid sugar or invert sugar syrup is used to chaptalize the fruit juice, the volume of water contained in the liquid sugar or invert sugar syrup will be included as ameliorating material.