Target Gravity and How to Get There : A Tutorial
Posted Jan 18th 2014 | By:
When I started brewing all grain a few years ago, I had been spoiled. Spoiled by the pre-packaged extract kits that come with "x" amount of extract meant to give you a specific gravity in "x" amount of volume. This meant that unless your volume was somehow over or you managed to spill half of your extract on the floor, you almost assuredly would end your brewing session at or near your expected gravity.
This made extract brewing extremely easy and to be honest, I loved it. I didn't even know that I had to worry about "Target Gravity" at that point. Hell, I'd be surprised if I could have even told you what the term meant, especially during my first few brews. However, my blissful ignorance would not last.
When I started all grain brewing my thoughts were still the same for a while: get the volume right and all will fall into place. For my first couple all grain brews, this is exactly what happened! Looking back now almost 3 year later, I have a hard time believing it. It must have been some sort of magic, as I hit my target gravity or was darn close to it both times.
However, on my third all-grain brew session, my world fell apart. As far as I remember, I did everything the same that I did with the first two brew sessions, but when I chilled my wort and finally took my gravity reading, I was shocked: I was low by almost 20%! So, I put my nose in books and scoured sources on the internet, and I learned a few things.
You like Proper Styles, Right?
Consider your next brew day for a second: just for fun let's say that you plan to brew an Imperial IPA with an original gravity of 1.080 with 82 IBUs. In other words, it's a pretty balanced beer as far as IIPAs' go. Your brew day goes well, you have a beer during the boil, get your wort cooled down; add some water to get to your proper volume, if necessary; and grab your hydrometer to take your first and only reading of the day. Low and behold! Your O.G. is way low, coming in at 1.068.
Congratulations! You've just brewed an overly hoppy American IPA instead of the well-balanced Imperial IPA that you set out to make. At that point, you're probably asking yourself "What the hell happened?" and rightly so. So, we will figure it out together.
Let's start with the basics first, and to keep things simple, let's stay with the Imperial IPA example from above. 5 gallons at 1.080 with 82 IBUs is what we are aiming for. First things first, every brewer needs to understand that every single unit of gravity (.001) is called, understandably, a "gravity unit." Nifty, isn't it? Second, when it comes to gravity, in this case 1.080, we can simplify the upcoming math and say that each gallon of wort has 80 gravity units.
Drop the decimal and the 1, and then get rid of anything behind the decimal until you have an actual whole number. Now, remember that this is gravity units per gallon, so in order to find the total number of gravity units in the whole batch; we simply take the GUs per gallon and multiply by the number of gallons, we are brewing. Therefore, 80 multiplied by 5 = 400 GUs total for the 5-gallon batch of Imperial IPA. Are you still with me? Trust me; it's an easy concept after you apply all this, hands on, while actually brewing.
So now that you know what a gravity unit is and can figure the total GUs of any given batch, where do we go from here? How does this help you keep track of your GUs to make sure that you hit your target gravity? Grab your hydrometer and let's work some things out.
With our IIPA, we have a grain bill that comes out to, say, 12 pounds. So let's use 12 gallons of water to mash (I'm keeping math easy here) and our grains absorb 4 gallons of it all. So, right before the boil you have 8 gallons of sweet sticky wort.
Right now is your one chance to calculate your gravity and make adjustments if necessary, before you get your pot on the burner and crank the heat up. Knowing that we need to be at 400 total GUs, we can figure out what the gravity of our 8 gallons of wort needs to be at. Before pulling a sample in your hydrometer, stir well.
Sometimes the heavier first runnings out of your mash like to separate from the thinner liquid that will drain at the end, so get it mixed up well. Then pull your sample and measure. Now I'm going to throw a curve ball at you: you must compensate for the temperature of the wort. Use your thermometer and take a quick temp. Lets say it comes out to 150 degrees Fahrenheit.
According to Ray Daniels in his book "Designing Great Beers," you need to add .0018 to any gravity measurement you take at the temperature. There are many sites that will do this for you, just enter your temp and gravity measurement, and it will adjust your reading automatically. BrewCalcs.com is my personal favorite.
But back to the gravity: again, you need to be at 400 GUs, so what should your gravity be? You have 8 gallons and 400 is the magical number. We multiplied when talking about the finished volume, but now we need to divide when talking about pre boil volume. So, simply, 400 divided by 8 = 50. We need to have a GU rate of 50 units per gallon of wort.
Translate that into brewing talk and our pre boil wort needs to come out to 1.050. Now, subtract the .018 to adjust for the temperature, and we need to see 1.032 on your hydrometer before doing any corrections. If you are confused, I have to assure you that once you are using these concepts hands on while brewing; they become much clearer than I could ever explain them.
How to recover if you are off
Once you take your pre boil gravity measurement and figure out where you are at you may well be off. The only thing you need to decide is how far off is acceptable to you. If our 1.080 IIPA is going to come out to 1.079, I can live with that. However, if it comes out to 1.075, that I want to do something about; so we will start with what to do if your measurement is too low.
If you are too low, I have bad news. You are going to have to add something fermentable to bring that O.G. up. However, mashing some more grain isn't really efficient, as you are going to have to heat up more water, mash for an hour, and then add the liquid to your boil. Speaking of the boil, if it's an hour-long boil, you're too late. This is why I have to recommend to every new all grain brewer that they buy a pound or two of dried light extract to have around for just this situation.
To make up for the 25 GUs that we are off for the IIPA, it's not going to take very much DME to get to where we need to be. A cup or so should do. Since different brands of malt extract are, well, different, it would be wise to look up your brand to see how many GUs varying amounts add to your beer. Personally, I've never had a problem with winging it here. If you end up too high, that's not a horrible thing.
Did your gravity come in too high? Congratulations! The god of brewing has smiled upon you and decided to give you two choices:
A: keep the volume the same and ferment stronger beer
B: up the volume to your target gravity and make more beer!
Not a bad problem to have, is it? Again, your toughest decision is figuring out if you can live with how off your measurement is. If you can't live with it, slowly add some water and check
your measurements again until you are where you need to be. It is that simple.
Do you want more beer or stronger beer? Know what? Sit down and have a beer while
you consider your options.
When brewing, especially all grain, your target gravity is everything. You either hit it or you make a beer that you didn't intend to make. It really is that simple. However, with a little know how and proper preparation, one doesn't need to feel the sting of an O.G. measurement that is
so far off that your IIPA is now a weak hoppy mess of what you intended.
Having been there myself, I hope I've given you enough information to go forth and brew your next batch without any worries.
Temp/Gravity Adjustment Chart
This exact same chart can be found on page 35 of "Designing Great Beers" by Ray Daniels:
Temp(F) (C) Add O.G./S.G.
80 (27) 0.002
90 (32) 0.004
100 (38) 0.006
110 (43) 0.008
120 (49) 0.010
130 (54) 0.013
140 (60) 0.016
150 (66) 0.018
160 (71) 0.022
170 (77) 0.025
190 (88) 0.033
212 (100) 0.040
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