Pre-boil and post-boil gravity problem/question

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JamieP

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I need some help figuring out what is going on with my gravity. For several sessions in a row I've measure very good pre-boil efficiency taken as a sample from the kettle after sparging. However when I complete my brew day, my final sample is only a few points higher than the pre-boil. Given the boil off it makes no sense that the gravity isn't increasing more.

In my most recent brew session I changed my process a bit on the mash side and ended up high on volume: 7.2 gal at 1.057. I boiled long (before hopping) to get my volume down. After boiling and losses from 9oz of hops I had 5.5 gal at 1.060. I saved my pre-boil sample and tested it multiple times to ensure no temperature issue. My final gravity was measured with both my refractometer (also used pre-boil) and my Tilt.

I'm at a loss for what is going on. Any thoughts?
 

McKnuckle

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It's very common to poorly mix, and hence incorrectly measure, pre-boil gravity. For some reason it's always an overshoot, which is odd since that should be more random. But I digress.

If you ended up with 5.5 gallons at 1.060, it's easy to reliably calculate what your pre-boil gravity was. Let's add back about 0.1 gallon since you didn't really lose that to hop absorption for the purpose of this exercise.

So 5.6 gallons at 1.060 would mean you had 7.2 gallons at 1.047.
https://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/

What happens is that the wort is stratified, with heavier sugars falling to the bottom. If you sample unevenly, especially from the lower part of the kettle, you will end up with higher gravity wort.

Try taking a sample once the boil starts, since it tends to churn enough to eliminate stratification.
 
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JamieP

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It's very common to poorly mix, and hence incorrectly measure, pre-boil gravity. For some reason it's always an overshoot, which is odd since that should be more random. But I digress.

If you ended up with 5.5 gallons at 1.060, it's easy to reliably calculate what your pre-boil gravity was. Let's add back about 0.1 gallon since you didn't really lose that to hop absorption for the purpose of this exercise.

So 5.6 gallons at 1.060 would mean you had 7.2 gallons at 1.047.
https://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/

What happens is that the wort is stratified, with heavier sugars falling to the bottom. If you sample unevenly, especially from the lower part of the kettle, you will end up with higher gravity wort.

Try taking a sample once the boil starts, since it tends to churn enough to eliminate stratification.
Interesting and I'll definitely try mixing and taking samples at different times. However I'm taking my sample from the top with a dropper.
 

McKnuckle

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I’ll admit that’s odd, since a drop from the top would be lower gravity. The other thing that can happen is that your tiny sample may evaporate enough on the plate to concentrate, and hence inflate, the Brix reading.
 

CascadesBrewer

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Do you know how much you boil off during your boil? Did you take a gravity and volume reading at the end of the boil. The gravity should be the same as your 1.060 number, but the volume would help to validate your numbers.

If you had 7.2 gals of 1.057 beer, then you have 410.4 "gravity points" (7.2 x 57). If you boiled off 1 gal of that, you would have 6.2 gals of a 1.066 beer (the same 410.4 gravity points divided by 6.2 = 66.2). If you boiled off 1.2 gals you would be in the 1.068 range.
 

Bobby_M

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You have to stir before taking a sample. Once you know gravity and volume, you know the total sugar. When you boil off some water, that fixed amount of sugar is now diluted with less water so the gravity goes up a calculable amount.

6 gallons @ 1.050 = 300 "sugar things"
300 divided by 5 gallons = 1.060
 

balrog

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I BIAB, full disclosure etc.
I stir the snot out of it after mash and use refractometer, and rough calc 6gal-->5gal change in SG and over the last 25-30 batches average 3 pts calculated higher, than what multiple hydrometer+refractomer OG actually hit. Given my inaccurate vol readings (+/- 1.5qt) I'm happy. But I stir. Like I'm trying to dislodge, oh, say, a pair of reading glasses dropped into the kettle. :rolleyes:
 
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