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The Specifics of Specific Gravity

Since Newton and the apple, humans have known about the invisible force of gravity that surrounds us. Gravity is crucial for our collective understanding of certain measurements. The weight of something, in fact, is just a relative measure of an object’s mass and the force of gravity being exerted on it at a specific time. This is why you’ll weigh more on Earth than in space, as the force of gravity beyond our atmosphere is much reduced, even though your mass remains unchanged.

Gravity also comes into play in the world of beer and in certain measurements that are important to brewers and beer enthusiasts. You may have heard the term “specific gravity” in relation to beer and today we’re going to discuss the ins and outs of what it means and why it’s important when brewing and understanding beer.

The term specific gravity, in the context of beer, simply specifies the density of the wort (the sweet liquid that will become beer) in relation to water. The density is largely present due to sugars that are released from grains during the “mashing” process while brewing. These sugars become the food source for microorganisms called yeast, which go to work like tiny Hungry Hungry Hippos, eating and converting sugars to ethanol alcohol and carbon dioxide, magically turning our wort into glorious beer.

Gravity is measured in various ways. The most common method among brewers is with a hydrometer. A hydrometer is a glass tube with weights at the bottom and a small scale at the top. When a hydrometer is placed in wort, it will float at a specific point determining the present density level. There are also two basic scales used to determine the gravity. First is degrees Plato (°P), which is most common with German and commercial brewers (including Hop Valley), and is a percentage by weight of the dissolved solids present. If wort has 10°P that means it contains 10% of dissolved solids (mostly sugars). The next method is a British scale, most common in the UK and among home brewers, which takes the density of the wort in comparison to pure water. The same wort that is 10°P will also have 1.040 gravity, which is .040 denser than pure water.

Now that we know what gravity is and how it’s measured, let’s talk about how to use it. We mentioned mashing, and this is the beginning of the brewing process. Once the mash is finished (sugars being converted and extracted leaving us wort), it is then boiled, hops added, whirl-pooled, and chilled to optimal temperatures for the yeast to go to work. This is the point that the first gravity reading is taken: a pre-fermentation reading called original gravity (OG) or starting gravity. This gravity reading tells us the sugar density of the wort before the yeast is added. The higher the density, the higher the sugar content, and in turn, the higher the alcohol that can be expected in our finished beer. Comparing two Hop Valley beers helps to illustrate this point. Citrus Mistress has an OG of 14.5 and a final alcohol content of 6.5% while Light Me Up Lager has an OG of 9.2 and a final alcohol content of 4%.

The next gravity reading, a post-fermentation reading called final gravity (FG), is taken after the yeast has finished its job. This reading will show us the remaining density in the beer, which will now be less due to the yeast converting the sugars to alcohol and CO2.  The percentage of sugars that the yeast converts is referred to as the “attenuation” of the beer.  A higher attenuation means more sugars were converted (less sugar left in the beer), giving the beer a drier body, while a lower attenuation means more sugars left in the beer, resulting in a sweeter body.

Once we have this information, with some basic math we can now determine what the alcohol by volume (ABV) of our beer is. Let’s use Alphadelic as an example on how to do this. Alphadelic has a starting gravity of 16.1°P or 1.066OG and a final gravity of 2.7°P or 1.011FG. If converting with gravity, simply take your OG, subtract the FG and multiply the difference by 131.25. (1.066-1.011)*131.25 = 7.2% ABV. If using Plato, take the starting gravity, subtract the final gravity and multiply by .54. (16.1-2.7)*.54 = 7.2% ABV.

And there you have it. Gravity readings taken before and after the fermentation process tell brewers and drinkers the relative sugar density present, hints at the beer’s body characteristics and is useful for calculating ABV. Now you’re equipped with a little more knowledge to impress your friends as you explain the importance of gravity measurements and beer and how it has only a little to do with apples falling from trees.