You hear it all the time, "high gravity beer" or "the OG was 1.072!", etc. But what's it mean? Its really cool and really easy science and it will all make great sense in just a minute.
Ok, before we start with the brewing specifics, some background.
"gravity" is short for "specific gravity" and it is the measure of density. In our case, the density of a fluid, but solid objects are also measured with specific gravity. Heavier things have a bigger "specific gravity" number than lighter things...and I'll show you, down below, why this is important to you.
The whole concept was invented / discovered by Archimedes back about 200 B.C. You probably remember, from sixth grade science class, the story of the guy that jumped out of the tub and started running down the street naked yelling "Eureka! Eureka!". That was Archimedes and buoyance / density what what he was shouting about.
Brewers use a device called a hydrometer to measure the specific gravity of their wort and beer (wort is nothing more than the fluid that hasn't fermented yet but is going to be beer in a week or so when the yeast get done doing their thing).
The hydrometer, as far as we know, was invented by Hypatia of Alexandria somewhere around 400 A.D. She was an early scientist / physicist (they called them "philosophers" in those days) that was finally killed by a Christian mob in one of those periodic science versus religion things that humans seem to go through from time to time.
It works like this:
- The specific gravity of water is 1.000.
- As you dissolve sugar in water, it gets denser and the specific gravity rises.
- Then, as the yeast turn the sugary water into alcohol, the gravity lowers.
Just to give you some quick frame of reference. A medium strength beer might start fermentation with an OG (original gravity) of 1.050 and might end, after fermentation is complete, with an FG (final gravity) of 1.008.
As a brewer, you measure your gravity while the wort is fermenting into beer and you see the gravity get lower and lower as the days go by. When the gravity stops getting lower, that means the yeast are done and you can bottle your beer.
Another quick aside here...sometimes your gravity stops getting lower way too soon...that's called a "stuck fermentation"...after brewing a few batches, you'll get a feel for when the fermentation is done and when it's stuck.
Just for some extra jargon, how much the gravity drops is known as attentuation and different yeasts attenuate differently. In the example above, a low attenuation yeast might be completely finished when the FG is 1.014, but a high attenuation yeast might drop it as far as 1.004.
So there's two things going on here you need to think about...a low attenuation yeast, all other things being equal, makes a sweeter, less-alcoholic beer (because it stops turning sugar into alcohol while there's relatively more sugar left in the fluid). Conversely, a higher attenuating yeast will make a drier, more alcoholic beer, all other things being equal.
One more term and then we'll be ready to take on a specific example. "Gravity points". To keep things simple, we're going to use LME (liquid malt extract - the thick syrup you get at the homebrew store to make your beer). A basic LME might have 36 gravity points. What that means is that, if you dissolve 1 pound of LME in 1 gallon of water, the gravity will be 1.036.
We typically brew in 5 gallon batches, so 1 pound of LME in 5 gallons of water will make an OG of about 1.007 (36 points per gallon spread out amongst five gallons is about 7 points per gallon...or put arithmetically, 36 divided by 5 is about 7).
So here we go with an example:
We want to brew a beer and we buy 6 pounds of LME. We boil 5 gallons of water and stir in the LME, boil for an hour, cool and use our hydrometer to measure the OG. We expect to see somewhere around 1.043 (6 pounds of LME times 36 gravity points is 216 gravity points in ONE gallon....so we divide that by 5 because we used FIVE gallons.)
We write down the date and the OG because we are, after all, master brewers keeping accurate records.
We pitch our yeast, put on the airlock, put the bucket or carboy into a place where it doesn't get too cold and it doesn't get too hot, lets say 64-68F, and we just let the yeast work for a couple days. Then we measure again...and we wait a few more days and measure again...wash, rinse, repeat.
Eventually, usually about a week or two, depending on a ton of variables, the gravity will stop getting lower. When the gravity stays the same for 3-4 days, the yeast have done all they can.
Congratulations. It's now beer.
No comments:
Post a Comment