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Why does specific gravity decrease in wine?

Answer»

Specific gravity is relative to water (S.G. = 1.0) so unfermented wines should be heavier/denser than water, eg S.G. 1.090, YMMV. As the wine ferments the S.G. decreases because the sugars are consumed. This MAKES the S.G. fall. When wine is finished fermenting it can have a S.G. close to or even below 1.000.

To calculate the (approximate) ABV one must know the original gravity and the FINAL gravity of the fermented wine. One must record the starting/original gravity (O.G.) before/at pitching the yeast and SEVERAL successive readings of the specific gravity at later dates. If the specific gravity is the same for three days in a row then the primary fermenting is deemed to have stopped. The LAST specific gravity reading is then the final gravity (F.G.)


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