Specific Gravity
A measure of the density or mass of a solution, such as must or wine, as a ratio to an equal volume of a standardized substance, such as distilled water. Before fermentation, the density of the must or juice is high because sugar is dissolved in it, making it thicker than plain water. As the sugar is converted by the yeast into alcohol and carbon dioxide, the density (specific gravity) drops. A hydrometer or refractometer is used to measure specific gravity (s.g. for short), with an s.g. of 1.000 being the calibrated density of distilled water at a specific temperature (usually 59 or 60 degrees F.). Because alcohol is actually less dense than water, the finial s.g. of a wine can be less than 1.000, or lighter than water. See hydrometer and refractometer.