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Homebrewer using a floating hydrometer to test the specific gravity of his beer. (Only the top of the hydrometer can be seen.)

Gravity, in the context of brewing alcoholic beverages, refers to the specific gravity of a beverage. It is an indirect measurement of the amount of sugar, and thus ethanol, present in a given alcoholic beverage. The gravity of a beverage is measured using a hydrometer.

Types of gravity measurements

For a given alcoholic beverage, there are two types of gravity measurements, taken before and after fermentation:

• Original gravity (OG), which is a measure of the specific gravity of an unfermented beverage. (OG is the "sugar before fermentation" measurement.)

• Final gravity (FG), sometimes called Terminal Gravity, which is a measure of the specific gravity of the fermented beverage. (FG is the "sugar after fermentation" measurement.) Typically, FG is measured only once fermentation is completely finished - that is, all of the fermentable sugars have been turned into ethanol.

The difference between OG and FG is a measure of the amount of sugar consumed in the fermentation, and therefore it can be used as a rough, indirect measurement of the alcohol content of the beverage. It is a "rough" measurement of alcohol content for several reasons:

• Each yeast has a certain power of attenuation, which is the ability to consume sugars down to trace amounts.
• Different beverages have higher or lower proportions of non-fermentable sugars (trisaccharides and longer polysaccharides).

Thus, final gravity is not a simple function of original gravity - that is to say, FG cannot be accurately calculated from OG. However, it is nonetheless a common practice to "design" a beverage based on the theoretical final gravity. This is done by summing the GU/lb of the beverage's various starting materials and dividing by the volume of liquid being fermented. This gives the expected final gravity, but is rarely, if ever, an accurate calculation of actual final gravity. (For example, dry malt extract has approximately 45 GU/lb. If five pounds are used to make two gallons, you have: 45GU/lb x 5lb = 225GU ; 225GU / 2gal = 112.5 GU per gallon, yielding a product with a gravity of 1.1125.)

Definition

In the case of alcoholic beverages, gravity is defined as the ratio of the density of fermentable sugars to the density of water at a specific temperature and pressure. (Technically, at 4°C (39°F) and 1 atm (29.92 inHg), though this is often ignored by homebrewers.)

where G is the gravity of the beverage, is the density of the sugar in the beverage, and is the density of water. (By convention ρ, the Greek letter rho, denotes density.)

To put it more simply, the gravity of a beverage is a measure of the sugar content, and by extension, the alcohol content (ABV) of the beverage. The indirect measurement of the alcohol content of the beverage is possible because sugar is fermented into ethanol in a (fairly) consistent and predictable manner.

The density - and thus the gravity - of the beverage drops during fermentation. The main reason for this is that the fermentation process breaks down sugar molecules into ethanol and carbon dioxide, the majority of the latter escaping as gas. The density of ethanol is 790 kg/m³, which makes it less dense than water at 1000 kg/m³. The density of the unfermented beverage (basically a solution of sugar) is less than the density of the fermented beverage (basically a mixture of ethanol and water).

Specific gravity vs. specific density

Specific gravity is a special case of specific density, and thus the terms are often used interchangably by brewers. In terms of brewing, gravity is a special case of specific gravity, in which the reference material is water and the substance whose density is being measured is sugar.

Gravity units

Since gravity is defined as a ratio of density per density, it is a dimensionless quantity (the units cancel each other out in the fraction). Despite this fact, the term "gravity unit" (GU) has come into use as a way of defining how much the gravity of a beverage will be raised by a given quantity of sugar-containing material (e.g. malted grain, malt extract, fruit juice, pure sugars such as glucose, etc.). By itself, a "gravity unit" is a meaningless measurement; it is always expressed as GU per unit of weight, e.g. GU/lb or GU/kg.

Measuring gravity

Brewers record OG readings with a hydrometer after cooling the beverage to a reference temperature (usually about 15°C). After fermentation is complete, an FG reading is taken to compare the relative densities of the beginning solution and the finished product. Because liquids are less dense at higher temperatures, it is important to take the OG and FG readings at the same temperature.

Often a hydrometer will list alcohol content as well as gravity, and occasionally it will also show the appropriate gravity ranges for the original and final gravities of beer/wine. (Wine has much higher original/final gravities than beer does.) Otherwise, a table showing gravity and ABV equivalences is used to determine alcohol content from gravity measurements. In addition, the following formula gives the approximate percent alcohol by volume (ABV) as a function of the original and final gravities:

, for example

See also

• Plato scale
• Gravity (chemistry)

Wikipedia content modification information:

• This page was last modified on 1 October 2008, at 06:11.

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