Gravity, in the context of fermenting alcoholic beverages, refers to the specific gravity of the wort or must at various stages in the fermentation. This article focuses primarily on the brewing industry. The concepts and equations are basically the same in the wine making industry.
At various stages in alcohol fermentation, the density of the wort varies. Depending upon the depth that the hydrometer falls into the wort, the percentage of alcohol can be determined.
Initially (before alcohol production by the yeast commences) the specific gravity of a wort is dependent mostly on the amount of sugar present and, therefore, specific gravity readings can be used to determine sugar content by the use of formulae or tables (see article Plato). This sugar content is expressed in units of grams of sugar per 100 grams of wort equivalent to % w/w and called, in the brewing industry, "degrees Plato" (abbreviated °P) and in the wine industry "degrees Brix". Even when specified in terms of °P it is not uncommon to refer to the pre-fermentation reading as the "Original Gravity", (abbreviated OG) though it is more correct to term it the "Original Extract" (abbreviated OE). It is, of course, correct to refer to the original specific gravity reading as the OG. By considering the original sugar content the brewer or vintner obtains an indication as to the probable ultimate alcoholic content of his product. The OE is often referred to as the... Read More