Correcting Low Original Gravity in All Grain Brewing

Have you ever missed your original gravity (OG) target when brewing all grain beer? Its not an uncommon problem for beer brewers, particularly when you start all grain brewing or change you equipment setup. This week we’re going to examine some of the possible causes of low OG and also how to mitigate them.

Predicting Your Original Gravity

Before we can talk about “missing” an original gravity we need to look at getting a good estimate for OG in the first place. The best way to predict your OG is to use software likemy program BeerSmith with a good equipment profile set up for your particular brewing system. With your equipment set up and recipe entered it will give you a good estimate.

Alternately you can calculate the OG estimate by hand. This calculation is covered in general here, or for the all grain case here, or alternately you can use a variety of original gravity calculators online. One challenge with doing it by hand, however, is that you need to know the potential for every grain used as well as be able to calculate losses accurately.

Probably the worst way to estimate your original gravity is to use the figure that came with the recipe or kit. While it may be an accurate figure for the beer brewed by the manufacturer, the fact is that every brewer uses slightly different equipment with different losses.

Missing Your Original Gravity

Once you have your OG estimate and brew the beer, you often miss. Occasionally the OG comes in high, but more often your original gravity will be lower than you predicted. So what do you do next?

I’ll cover how to adjust your OG using dry malt extract or water below, but as you plan your next batch here are some of the potential issues to look at:

  • Grain Milling – For all grain brewers, poorly crushed grains can have a large effect on original gravity. Your grains should be crushed finely – with the center of the grain forming a fine powder. Large pieces of the husk should be left intact, however, to form a bed for filtering during the lauter. A dual roller mill like the Barley Crusher (full disclosure – we sell these) works best for crushing your grains.
  • Sparging – If you sparge too quickly, have a poorly designed mash tun filter, or sparge the wrong volume you can get a low OG. Take your time when sparging, which will let the wort extract as much as possible from the grain bed. Also design of the mash system can have an impact. A cylindrical (Gott or Igloo style water cooler) with a full size false bottom (screen type) will generally yield higher efficiency than a flat picnic cooler with only a few copper tubes acting as the filter system for example.
  • Measure your Volumes – If you have not accurately measured your volumes at each stage in the brewing process and accounted for losses along the way, it can have a huge effect on your OG. For example, a 10% increase in final volume can result in a 10% decrease in OG. So to hit your target OG, you also need to hit your volumes at each step – into the boiler, post boil and also into the fermenter. If you find your volumes don’t match your equipment profile – then adjust your equipment profile until you have a good match with your brewing style.
  • Additional Tips: Here are some more tips to improve your all grain efficiency.

Correcting Your Original Gravity

Once you’ve missed your OG, and looked at potential causes, you are usually faced with the problem of what to do with your existing batch? Fortunately it is not hard to adjust the OG of wort as it goes into the fermenter by using some water or some dry malt extract. I usually keep a few pounds of light DME around just in case.

Note that there is a simple Adjust Gravity Tool in BeerSmith which will do this calculation for you. I’ve included the calculations in English units below (as it is easier to calculate) but you can convert metric volumes to english pretty easily (1 gallon = 3.785 liters, 1 kg = 2.2 lbs)

If your gravity is too low, the calculation for the amount of dry extract to add is:

  • Calculate the difference between your target and actual OG, then multiply by 1000. For example if you were targeting 1.056, but only hit 1.048 this would give us (1.056-1.048) x 1000 = 8 points
  • Now we need to raise our gravity by 8 points which means we need to add 8 points/gallon of dry malt extract (DME) equivalent. Assuming a 5 gallon batch size, we need a total of 40 points of DME.
  • DME has a potential of 1.046 which means it contributes 46 points/lb added, so we simply take the 40 points and divide it by 46 to get 0.9 lbs of DME to add.

If the gravity is too high, we can dilute it by adding water:

  • This time we’ll assume our target was 1.056 but we overshot and came in with a gravity of 1.064, again using a 5 gallon batch. We’ll use the fact that the number of points times volume should be a constant to do the dilution.
  • So we start by taking our starting points of 1.064 = 64 gravity points, and multiplying by our original volume of 5 gallons: 64×5 = 320 points
  • Now we divide by our target points which is 1.056 = 56 points which will give us the target volume: 320 / 56 = 5.71 gallons
  • Since we started with 5 gallons, we need to add 0.71 gallons of water to dilute our gravity to achieve the target of 1.056

That covers some potential causes of missed gravity as well as how to adjust your gravity if you miss your target.

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from BeerSmith

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