Hitting your Target: Original Gravity

If you’re anything like me, your brew day rarely goes off without a hitch. There’s always something to tinker with, correct, or watch out for. Perhaps you’re really like me and forget to do something fundamental, something you already know you should be doing, something like forgetting to measure your pre-boil gravity because you were too busy focusing on how your first time batch sparging was going!

Inevitably there are always things to watch out for. The trick is to know which of those things are critical, and which are not. Hitting your target starting gravity is one of those critical components.

The last few batches I’ve brewed, I’ve missed my starting gravity considerably, like 0.007 pts worth. It’s never been a problem before, but I now have 3 different set ups I brew with, 10 gallons, 5 gallons and 2.5 gallons. All of which, I’m finding, have different efficiencies, evaporation rates and trub amounts. These last few batches have been 2.5 gallon ‘test brews’, if you will, to try out new recipes, specialty malts, and hops. It looks like my numbers have been off and that’s why I’ve been missing my target original gravities. So how can I fix that?

First off, I think I’ve been focusing too much on my efficiency percentage and not on what’s important = good quality wort. Zack Weinberg at Toronto Brewing helped to get me thinking about it the right way.  “Think more about collecting the high quality wort from the the first part of your runnings. Getting a high efficiency is like squeezing every last drop of sugar out of the grain, and those last drops don’t have that much in them. You’re better off to aim for a lower efficiency, like 80-85%, or even lower if you want, and get all the good juicy stuff from the beginning of your sparge, that’s where the flavour lies.” If you’re a multimillion dollar brewer who brews 2500 Hectoliters a batch, then efficiency will come into play, but as a home brewer, what’s an extra pound or two of grain going to cost you? Not that much. Besides, isn’t it’s about making great tasting beer? Thanks Zack!

Aside from that, hitting your target starting gravity is quite important. Why? because your recipe is based on it. Your ratio of sweetness in the malt to the bitterness in the hops (bitterness ratio) is dependent on you getting the appropriate amount of sugars out of your mash. If you extract less, the bitterness will be too high, and the intensity of your malt profile will suffer, extract more, and your beer will not be as hoppy as you’d like. Balance is a very important factor in good beer. Not the kind of balance that means ‘boring’, but balance for whatever complexity you have going on in your beer.

When brewing from extract, this isn’t an issue because the mashing has already been done for you. With all grain, it’s a different story. Depending on your equipment setup, how close you are to your target temperatures, how good your lautering system works, how fast or slow you sparge, and of course, how well you crush your grains will all have an effect on how much sugar you obtain from your mash.

If you’re used to only measuring your starting gravity once it’s in your fermentor, you may be wondering, “How can I measure it before that and adjust it accordingly?”

The answer is gravity units.

Specific gravity is inversely related to volume. Meaning that if you add water to your wort, the less dense it will be (ie: a lower specific gravity). The opposite is also true, if you concentrate your wort by boiling it, the higher the specific gravity will be. Good thing is, the overall sugar content in the wort doesn’t change, only the density. This gives us a handy way to adjust our starting gravity using gravity units.

specific gravity x volume = gravity units

So how in the world can you tell if you’ve extracted enough sugars into your wort? Well, take a gravity reading of course! Collect your target volume of wort, and be sure to give it a decent stir in order to get an accurate reading (the heavier wort may sink to the bottom during your sparge). One thing you need to keep in mind when taking readings is the temperature of the wort. Hydrometers are usually calibrated to read at 60°F (sometimes 68°F). So if you measure your wort at say, 135°F, then the reading will be different than at 60°F. I forgot to take this into consideration on my first all grain brew. I ended up adding dry malt extract to what I thought was a lower gravity wort, which gave me a beer with 1.2% more alcohol than I was planning for! If you don’t use BeerSmith or another brewing software program that has a temperature adjustment calculator, here’s a handy one from Brewheads.

Let’s walk through an example to make things a little clearer:

A couple of weeks ago, I was making a standard Pale Ale on my 2.5 gallon setup. BeerSmith calculated that my pre-boil gravity was to be 1.045 @ 3.47 gallons . After I was done sparging, I took a reading (adjusting for temperature of course) and found that I had a S.G. of 1.038,  a difference of -0.007. In order to find out how to compensate for this, we need to make a few calculations to get things into gravity units (G.U.).

BeerSmith calls for a gravity of 1.045 @ 3.47 gallons.
Take the 1.045, drop the 1 and decimal place to give you 45,
then multiply that by the volume of 3.47 gallons.
45 x 3.47 = 156.15 Gravity Units (G.U.).
Then take my reading of 1.038, drop the 1 and the decimal
place to give you just 38, and multiply it by the volume
(3.47 gallons).
38 x 3.47 = 131.86 Gravity Units (G.U.)

Therefore, we’re off by 24.29 Gravity Units.  (156.15 – 131.86 = 24.29)

At this point, we have 2 choices. We can either A) boil longer to concentrate the wort to the gravity we need, or B) add some more fermentable material, the easiest being Dry Malt Extract (DME). In this case, I chose to add DME as it is easier, faster and doesn’t require the extra time and energy needed to boil for a longer period. Exactly how much do we need? Well, DME will give you an average of 45 G.U. per pound/gallon. (Liquid malt extract will generally yield 35 G.U. per pound / gallon) We need 24.29 G.U., so it’s as easy as dividing 24.29 by 45 = 0.5398, or 0.54 lbs of DME. Add this to your wort and boil as normal.

If you want to instead boil for longer because you don’t have any DME on hand, or you just want to hang out in the brew house a little longer, then we can figure that out too. You need to calculate at what volume would our 1.038 wort need to be to give us a S.G. of 1.045. In order to do this, we can take our original equation of 38 x 3.47 = 131.86 G.U. and divide that by our desired gravity of 45: 131.86/45=2.93 gallons. You could then boil your wort down to 2.93 gallons and then start your 60 minute hop additions from there.

The reverse is also true. Say our wort instead had a S.G. of 1.050 as an example. We would then have 50 x 3.47 = 173.5 G.U. Our option then would be to add some more water to reach a volume that would give us a S.G. of 1.045. Taking our equation from above: 50 x 3.47 = 173.5, divide that by 45 to give you 173.5/45= 3.86 gallons. Therefore you would add 0.39 gallons of new water to your pot (3.86 – 3.47 = 0.39), giving you more beer than you had planned for. A nice bonus! (You could also mix in the water and then remove 0.39 gallons to save for yeast starters and still end up with your target final volume)

So there you have a breakdown of how to compensate if you don’t hit your target original gravity. This is of course dependent on you being able to accurately measure the volumes you have in your brew pot, but that is for another post….


4 Responses to Hitting your Target: Original Gravity

  1. First off, wanted to say that I, and a number of my friends, are quite thankful for your Tankhouse recipe. Side by side in a blind test, everyone is picking my brew. Thumbs up. Thanks for this article though. I recently missed an IPA by .012 points and I wasn’t happy. I’ve been tweaking things and have been getting better results, but it’s good to read more science behind the matter. Looking at a refractometer now as I’ve heard it is pretty helpful. Winter brewing also does strange things to those boil off rates. Looking forward to your upcoming articles! Cheers.

    • Thanks Joe! very kind of you to say. I appreciate it!
      I’m glad you guys are enjoying the Tankhouse recipe. If your brew is winning out over Mill Streets’, then you’re definitely doing something right!
      I really need to brew another batch myself as I’m just about finished my last one.
      It is frustrating when you miss your target OG, but as with every brew, you learn something. Now that I’ve figured it out, it’s quite easy to remedy, so hopefully this post will help you correct it if you happen to miss it again.
      and I agree, winter does a number on a lot of things during the brew day, I’m looking forward to warmer days!
      Cheers! and thanks again!

  2. Thanks for putting together this detailed and well-thought-out guide! We’ve had similar experiences with various issues popping up on brew days, so this provides a nice element of perspective. In our first few batches, we were happy if we made anything moderately potable, but now we’re determined to refine our technique, and OG is something we’re really trying to focus on. So your timing was perfect!

    • Awesome! I’m glad it will help!
      I, like yourself, was happy with getting something decent in the beginning.But the more I dig into it, the more I want to make it better every time. Good luck with your future batches!

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