Brewing, at its core, is little more than a chemical reaction. So it makes sense to apply some chemistry terms to the process. The one I wish to focus upon today is that of gravity.
I’m not talking Newtonian gravity. Instead, let’s examine the ratio of the density of a substance to the density of water.
Specific gravity is a measure of concentration. It is the weight of a substance, as compared (as a ratio) with that of an equal volume of water. The specific gravity of pure water is 1. If you have a beer with a specific gravity of 1.030, it is 1.030 as dense as pure water.
Why is this important? In brewing, specific gravity is used to tell us how much dissolved sugar is in our wort. And sugar leads to two very important characteristics of beer – alcohol content, as well as sweetness.
There are two points in the brewing process where it is important to measure the specific gravity of the beer (or would be beer). The first is called the Original Gravity, often represented by the initials OG. The OG of a wort (pre-beer) is the specific gravity reading prior to fermentation.
Why is Original gravity important? This number helps tell a brewer that they are on the right track for their brewing recipe. For example, a brown porter should have an original gravity somewhere between 1.040 – 1.052. Fall outside these numbers, and you risk getting something other than a brown porter.
So how does the brewer of brown porter ensure that they end up with the proper original gravity? By choosing, not just the right ingredients, but the right amount of the ingredients. Creating beer is no different than following a recipe for any other food product.
A brewer looks at the PPG (points per pound per gallon) of the malt they wish to use for their beer in order to help determine OG. PPG is the amount of sugar in brewing ingredients. As an example, a malt which gives you 30 PPG will result in a wort with a specific gravity of 1.030, when mixed with 1 gallon of water. When a brewer considers the PPG of all of their ingredients, then can be reasonable sure that the OG of their wort will be close to what they expect.
And when I say “a brewer considers”, what I mean is “a brewer does the math”. It’s not a coincidence that many of the home brewers and professional brewers that I’ve come across over the past year or so have backgrounds in engineering.
Final Gravity (FG) is the second time that specific gravity is measured, and it is taken after fermentation is complete. This measurement helps determine how much sugar is left over after fermentation is done. Why is this important? It tells us how much of the sugar that had existed at OG had been converted into alcohol. If you get an ABV or an FG that you’ve expected, then life is good. However, even if you get results that fall outside the range can help the brewer make changes to their brewing techniques later on.
Going back to the brown porter example, the FG should be between 1.008 – 1.014. Too low, and it likely either means that the brewer had too low a temperature during the saccharification rest portion of the fermentation process, or it could mean that the brewer let some wild yeasts get into the fermentation tanks. Too high of a FG, and it could mean that the saccharification temperature of the mash was too high, or that the brewer is underpitching or underaerating the wort. The larger point here is that the FG can demonstrate to the brewer where to improve on their brewing techniques.
Each type of beer has their own range for OG and FG. By looking at these gravities, a beer drinker can determine how how strong the beer should be, as well as how sweet the final beer will taste.
For example, American Barleywine has an OG range of 1.080 – 1.120, which is on the high end of OG range. But their FG is only 1.016 – 1.030, which is compares to the FG of Russian Stouts and Eisbocks. The conclusion here? American Barleywines will be higher in Alcoholic content than most beers.
Additionally, Strong Scotch Ales have an OG range of anywhere between 1.070 – 1.130, which puts it in the range (and in some cases, exceeds the range) of American Barleywines. However, their expected FG runs much higher: 1.018 – 1.056. Not only should one expect Strong Scotch Ales to be less in alcohol content than American Barleywines, they should also taste much sweeter. In fact, the BJCP’s overall impression of the Strong Scotch Ales states that they can be “Rich, malty and usually sweet, which can be suggestive of a dessert.”