Beer Varieties: The Origins (Part Two: Gravity)
Original Gravity, Specific Gravity, Final Gravity find out what it all means as we continue on our beer variety journey.
By David Nuttall on Aug. 17, 2016
In part one of this series, it was pointed out that there are hundreds of styles of beers. While yeast is the most important determinant of beer style, two of the other main ingredients (hops and malt), and how the beer is brewed, among other things, also play a part. It is these factors that create the inherent qualities of the beer, which formulates each category or style. The main factors are the original and final gravities (from which the alcohol by volume (ABV) is calculated), the colour (expressed by Standard Reference Method (SRM)), and the bitterness (expressed in International Bitterness Units (IBU)). In this column, we will explore gravity, how it is measured, and how alcohol is calculated.
So what exactly is gravity? Gravity in brewing is a way of describing the density of the liquid. In the first step of brewing, when the first grains, or malt, are combined with hot water, a cereal mash is created. During the mash, the starches in the grain are converted to simple sugars via a conversion process called saccharification. The result of this process is a sweet sugar rich liquid called the wort. Gravity refers to the specific gravity (SG), or relative density, of this wort during the next step in brewing, which is the fermentation process. The density of the wort is compared to water, which has a specific gravity of 1.000. Measured by a hydrometer, anything added to pure water may make the hydrometer float higher or lower in the solution, altering the gravity. For example, a specific gravity of 1.050 is 5% denser than pure water. This ratio depends on the temperature and pressure of both the wort and water, so consistency is key. The pressure is always considered to be 1 atmosphere and the measurements are taken at the same temperature, which is usually between 16 ° – 20 °C.
The original gravity (OG) is the specific gravity of the wort measured at the end of the boil, after the cooling, but before fermentation. When the yeast gets added to the cooled wort, fermentation begins. As the yeast cells eat the sugar in the wort, they create carbonation (CO²), alcohol, and other by-products that can contribute to the beer’s flavour. As the sugars are consumed, the density drops, causing the SG to decline. The brewer monitors this decline in SG to gather information about the fermentation and to determine when it is complete. When the gravity stops declining (and that could be days, weeks or months), they take another hydrometer reading and record what is called the final gravity (FG).
Of course, not all yeast strains are the same. Not only do they work at different temperatures and different locations (top or bottom) within the wort, they have other characteristics as well. The ability to decrease the original gravity of wort, called apparent attenuation, is unique to each strain. Also, different strains have different alcohol tolerances, which affects how well the yeast will continue to ferment as the alcohol concentration increases during fermentation. For example, many lager yeasts can only ferment up to about 8% ABV, while some ale strains can ferment up to 12% or more.
After fermentation is complete, the brewer has two main numbers, the OG and the FG. To calculate the ABV, simply subtract the FG from the OG and multiply by 131.
The formula is this:
(OG-FG) X 131= ABV
For example, if the OG measured at 1.060, and the beer stopped fermentation with an FG measurement of 1.013, the formula looks like this:
1.060 – 1.013 = 0.047 x 131 = 6.157%
So this represents a 6% ABV beer.
There are other scales to measure alcohol, one of which is alcohol by weight (ABW), which is used primarily in the United States. To convert ABW to ABV, simply multiply the ABW by 1.25. To convert from ABV to ABW, multiply the ABV percent by 0.80.
Common in Europe and with craft brewers, the Plato scale (°P), is also used. Somewhat similar to the Brix scale used by the wine industry, this scale is derived to measure density of beer wort in terms of the extract’s percentage by mass. First developed in Europe in 1843, and later refined by German Fritz Plato, this scale expresses the density of the wort as the percentage of sucrose by mass. Because it is only concerned with the amount of fermentable materials and not any other solids which may be found in the wort, it is considered more accurate than specific gravity. Therefore a wort measured at 14° Plato has the same density as a water/sucrose solution containing 14% sucrose by mass. Degrees Plato are more popular in central European brewing, and are commonly noted on the labels and sometimes even in the names of beers.
The relationship between degrees Plato and SG is not linear, but a good approximation is that 1° Plato equals four “brewer’s points” (4 x .001); thus 14° Plato corresponds to an SG of 1.056 [1+(14 x 4 x .001)].
It should be noted that ABV is the most important measurement of a beer in Canada as it is the only beer quality that is mandatory to be shown on every label in this country. In other countries, most notably the United States, this is not the case. Other qualities, such as colour (SRM) and bitterness (IBU) are rarely shown on labels. These measurements are derived by other methods, and that is where we are headed to in the next column.
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