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A few days ago I brewed a 5-gallon recipe with the following fermentables:

  • 8 oz Crystal 20
  • 8 oz Crystal 40
  • One 3.3 lb can Briess Munich LME
  • 2.5 lb Light LME

Beer Calculus predicts an OG of 1.046.

For the first time in years, I got distracted for a moment and there was a boil-over. This is an annoying complication to my question (which is coming below). I don't believe too much was lost during the boil-over. I know I started with about 1.5 gallons of water when I mashed the grains. After removing the (waterlogged) grains and sparging with maybe a quart of water, I added the syrups. After the boil-over, I noted that I still had about 1.5 gallons in the pot. Now, having cleaned up the mess, I'd say about a pint of wort was lost to my stove top. When I consider the water that would have been removed with the grains, I feel like these numbers add up, but I'll let the experts weigh in on that. If I'm right about losing a pint when there were 2 gallons total, that would reduce Beer Calculus's OG estimate to about 1.043

So I finished my brewing, and topped off the carboy to 5 gallons. After I pitched the yeast, I took a gravity reading: 1.030. That seems incredibly low! At first, I told myself that the combination of losing some concentrated wort during the boil-over and a low efficiency from the mashing might explain it. So I told myself whatever happens will happen with this batch, and I went to bed.

But this morning I had another memory/realization. My girlfriend and I both noticed a ring about 1/3 of the way up in the carboy after the brewing. It was a ring of particulate inside the wort. I thought that was neat - there was a little meteorogical system in the carboy. Liquid of a heavier density had settled to the bottom and the particulate was floating on top of it. Possibly relevant - I used Irish moss in this batch (1/2 tsp for 10 min) which I do not normally use.

So I wonder: could my low OG reading be (in significant part) because I sampled from the top, where the less dense liquid was? Or is this just wishful thinking? I've never before seen the layering I saw with this batch. Does anyone else see that in their recipes?

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2 Answers 2

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Yep, you've got it. I've seen it and had it happen many times before I started doing full boils. Even when you think you've got it well mixed, you probably haven't! Since you use extract, it's going to be more accurate to calculate the OG than measure it.

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It can make a huge difference. The wort is much higher density than the top-up water, so it will happily sit on the bottom of the carboy pretty much undisturbed until it's mixed well with the top up water. Shaking the carboy can help remove the stratification, so you get a more reliable reading, and also helps introduce oxygen into the wort, which will improve yeast growth and fermentation.

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    Having broken several carboys, I urge you not to shake it. Too dangerous. Find another way to mix, like using a MixStir.
    – Denny Conn
    Commented Jul 28, 2012 at 16:01
  • I should have been more clear. I definitely shook the carboy a lot simply to oxygenate the wort. I now suspect that density layering will happen anyway on rare occasions, depending on the recipe, even after homogenized mixing. @Denny Did you drop it? I can't understand how you'd break one by shaking it. I either hold mine sideways and shake it in the air, or I rock it a lot on a linoleum floor.
    – alex.jordan
    Commented Jul 28, 2012 at 20:05
  • Small fractures can develop that you can't even see. I had one crack diagonally as I was rocking it on carpet. I only got minor cuts, but lost all the beer. I've also dropped them when I was holding them sideways. I hate them so much I went to buckets 10+ years ago and never looked back. If you want to keep using carboys, i REALLY urge you to get a MixStir. Very effective aeration, very inexpensive, VERY safe!
    – Denny Conn
    Commented Jul 28, 2012 at 21:00
  • I forgot there are people still using glass, I only use stainless or plastic. Naturally, be careful when rocking a glass carboy, but for other materials, rock on! :D
    – mdma
    Commented Jul 29, 2012 at 9:49

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