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Knowledge Drop: Gravity

Welcome to the first part of a new series on beer where I’ll discuss facts, history, and science behind beer. I’ll cover beer terminology, the brewing process, and beer styles, and whatever else comes to mind. Figured we’d break up the monotony of just reviews. We begin with Gravity.

Hold on tight, ‘cause thar be math ahead!

I’m not talking about why letting go of your beer makes it fall to the ground. I’m talking about the specific gravity of beer. Specific Gravity (SG) refers to how dense a liquid is in relation to water. Density is basically an object’s mass as it relates to each unit of volume it’s in. So, like if you have a 1lb rock and drop it into a 5-gallon bucket of water (whose volume is roughly 0.77 cubic feet), the rock’s density is 1.3lbs/cu ft. It’s important to know that the temperature of the water affects the water’s own density.

Now, since we’re measuring the SG of our beer in relation to water, the water we’re measuring against will have a SG of 1.000. There are a variety of ways to measure gravity, but the tool Rhea and I use in our homebrewing is called a hydrometer. It’s a glass tube with measurements on it and a weight inside at the bottom that causes it to float upright. The more it sinks into the beer, the lower the density of the beer. This is a pretty simple concept to understand. If you drop a baseball into a swimming pool of water, it’ll sink to the bottom, but if you drop that same baseball into a swimming pool of pudding, it would slowly sink and eventually stop somewhere in the middle, since the pudding is obviously more dense than the water. Most hydrometers are calibrated to measure in relation to 60° water.

Photo from A Life Content

Specific gravity is measured in two different units these days. Most homebrewers use the ratio of density to water density, often called “brewers points”, which is expressed like 1.052, but many breweries use degrees Plato, expressed as 14°P. There isn’t a really direct conversion for °P to specific gravity ratios, but the closest approximation is about 1°P is equal to approximately .004 brewers points. So, 14°P is a specific gravity of about 1.056 ((14 x 0.004) + 1, the 1 is the density of water).

So, now we know that specific gravity just tells us how dense beer is in relation to water. Now back to what the hell specific gravity is and why you would even care about it when drinking your beer.

When you brew beer, you have to then ferment it, as you likely know. The mix of grains and hops and water you just boiled is called the wort (pronounced: wert). Once the wort cools, you transfer it from the kettle to the fermentation vessel, in homebrewing, usually a big glass jug called a carboy. Once in the carboy (and before the yeast is added), you take a sample of the liquid and measure its specific gravity and record it. This is called the original gravity (OG) (not O.G.). This is so you know how dense your liquid was before fermentation. As the yeast begins to devour the sugar from the grains in your beer, the beer loses density as there is less stuff in the liquid. The more sugars the yeast eat, the more alcohol they produce.

The beer’s recipe will generally have a target gravity that the brewer will aim for. This is calculated by the size of the batch, the various grains, hops, and other ingredients in the recipe, plus how much yeast you’ll be adding and the temperature at which it’ll be fermenting. Periodically during the fermentation period, the brewer will take a sample and check the specific gravity of it. Once the specific gravity stays the same two or so times in a row, fermentation has finished, and hopefully it’s at the intended gravity. This final reading is called the final gravity (FG). It will be a lower reading than the original gravity since the yeast ate the sugar from the grains.

Now, here’s where it turns into something with which you’re no doubt familiar — the percentage of alcohol by volume, ABV. That number on the bottle that’ll tell you whether you should have one for a few.

The ABV can be calculated fairly simply by subtracting the FG from the OG. For example, if the original gravity of your beer was 1.056 and the final gravity was 1.01, the difference between them is 0.046, times 131 for a percentage and some for error correction in the non-linear relationship, and you get 6.03%ABV. This isn’t 100% accurate, as temperatures need to be taken into account, but this is a quick and dirty way to calculate the ABV of a beer.

You may hear people refer to high-ABV beers as “high-gravity” beers. This refers to the high original gravity. When there is more grain and other sugars in the wort of a beer, there is more for the yeast to consume, giving off more alcohol. The yeast will try to eat all of the sugars, getting the beer’s density closer to that of the water. So, a beer with a lot more grains in it would be more dense and have a higher gravity.

For instance, we had a fairly high-gravity beer in our mathy example up there, and you can see what a difference a higher gravity makes in alcohol content by increasing that number. The first example had an OG of 1.056. If you use 1.08 and the same final gravity, you’d have a 9.17%ABV beer, a big beer indeed!

Your typical beer off the shelf will have anywhere between 4.2% and 5.3% ABV, which means it didn’t have a crazy amount of fermentables, like grains or fruit. This also makes the beer cheaper to produce because you’re using fewer ingredients, hence why a 22oz bottle of a 9% beer might cost you $10+.

So, in layman’s terms: you put a bunch of grain and stuff into water and boil it and then measure the gravity of this new liquid and that’s the original gravity. Then you put that new liquid into a big ol’ jug to ferment it for a few weeks and check the gravity after a little while and once the gravity stops changing, you now have your final gravity. You do some simple math of subtracting the original gravity from the final gravity and multiplying that by 131 and now you know your ABV percent for your beer.

Gravity isn’t super exciting, but it is pretty interesting and I only just scraped the surface. Here are a few sites with a ton more information.