Author Topic: OG Lower than Pre-Boil Gravity  (Read 553 times)

Offline Carson B

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OG Lower than Pre-Boil Gravity
« on: October 23, 2019, 09:56:46 PM »
I've seen this question answered a million times - it's theoretically impossible. I'm curious if anyone has EVER seen or heard anything that could indicate that it is indeed possible. Everything is calibrated, the wort was stirred (so no stratification), and I double-checked every measurement.

I just brewed my first batch using a new mash tun and water profile. Using a refractometer, I measured a pre-boil gravity of 1.063, which was 13 points higher than expected. Awesome, I increased efficiency with the new setup. I even checked twice because I was so shocked.

OG came in at 1.061 after boiling off about a gallon (3-gallon batches). There was no dilution from the chiller. I'm telling myself I must've somehow misread the refractometer pre-boil, but I double checked. Long story short, I'd really be surprised if I misread.

Has anyone ever heard of anything from a chemical standpoint that would cause any sort of sugars to perhaps bind to hop matter and precipitate out? I don't know if that's even possible, but I'm going crazy over here anyways trying to figure out what happened!

Offline denny

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Re: OG Lower than Pre-Boil Gravity
« Reply #1 on: October 23, 2019, 10:23:05 PM »
Obviously a mismeasurement somewhere.  Stirring does not ensure homogeneity.
Life begins at 60.....1.060, that is!

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Offline Carson B

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Re: OG Lower than Pre-Boil Gravity
« Reply #2 on: October 23, 2019, 10:26:56 PM »
Must be, right? Just shaking my head over here. Thanks Denny!

Offline Bob357

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Re: OG Lower than Pre-Boil Gravity
« Reply #3 on: October 23, 2019, 11:16:48 PM »
Yep! When you argue with the laws of physics, you always lose.
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Offline Robert

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Re: OG Lower than Pre-Boil Gravity
« Reply #4 on: October 24, 2019, 12:16:08 AM »
It is theoretically possible, and at least scientifically interesting to consider.  Some of the material dissolved in the preboil wort is, of course, precipitated out in the hot and cold break, which results in a measurable reduction in gravity.  So in a very tightly controlled commercial system, where there was very minimal evaporation, sufficiently minimal contribution to gravity from dissolved hop acids and oils (this is actually significant and measurable,) and very good break coagulation, the cast wort could potentially have a lower gravity than the sweet wort.  Obviously this will never happen to us or maybe anyone; but you may, if you measure everything carefully enough, notice that your gravity does not increase quite as much as the measured change in volume would predict.  But in this case, something was definitely not measured carefully enough!

(IIRC this was discussed somewhere around here a while ago, where Thirsty_Monk reported that he regularly sees little or no increase in gravity after the boil in his brewery.)
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Offline goose

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Re: OG Lower than Pre-Boil Gravity
« Reply #5 on: October 28, 2019, 01:38:03 PM »
I am also going to also weigh in on this.  I usually measure the kettle gravity with my refractometer when the kettle is full after stirring to see where I am.  I will again measure the kettle gravity a couple minutes after the wort starts to boil because the convection in the kettle will mix things up a bit more.  Many times I will see a change in the kettle gravity once I am into the boil and this gives me another data point to look at.  So it appears that you may have a measurement error here.  I also use the BeerSmith correction factor for the refractometer to do the brix-plato conversion.

I may have mentioned this before in a previous post on this subject but I never use these refractometer reading as gospel but just as a guide to see where I am.  If my kettle gravity measured by the refractometer is roughly 50% higher than the predicted kettle full gravity, I am usually OK.  If it is not, then I make adjustments like extending the boil or adding DME.  I can almost always get within 4% of my predicted OG using this method and obviously my hydrometer reading is the final judge at the end of the boil.  Obviously, YMMV a bit.
« Last Edit: October 30, 2019, 12:38:30 PM by goose »
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Offline denny

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Re: OG Lower than Pre-Boil Gravity
« Reply #6 on: October 28, 2019, 01:46:23 PM »
Just to add, I measure my preboil grabvity as the wort just comes to a boil.  That ensures that it's mixed.
Life begins at 60.....1.060, that is!

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Offline Robert

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Re: OG Lower than Pre-Boil Gravity
« Reply #7 on: October 28, 2019, 01:51:58 PM »
Just to add, I measure my preboil grabvity as the wort just comes to a boil.  That ensures that it's mixed.
Me too.  A bit of kicking up from the boil is more reliable than a stir with your spoon.
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Offline denny

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Re: OG Lower than Pre-Boil Gravity
« Reply #8 on: October 28, 2019, 01:56:26 PM »
Just to add, I measure my preboil grabvity as the wort just comes to a boil.  That ensures that it's mixed.
Me too.  A bit of kicking up from the boil is more reliable than a stir with your spoon.

Experience has shown me it's MUCH more reliable.
Life begins at 60.....1.060, that is!

www.dennybrew.com

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"The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts." - Bertrand Russell

Offline Thirsty_Monk

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Re: OG Lower than Pre-Boil Gravity
« Reply #9 on: October 28, 2019, 10:05:15 PM »
Just to add, I measure my preboil grabvity as the wort just comes to a boil.  That ensures that it's mixed.
Me too.  A bit of kicking up from the boil is more reliable than a stir with your spoon.

Experience has shown me it's MUCH more reliable.
Not as easy when you have to churn over 540 gallons.
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Offline denny

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Re: OG Lower than Pre-Boil Gravity
« Reply #10 on: October 29, 2019, 02:44:25 PM »
Just to add, I measure my preboil grabvity as the wort just comes to a boil.  That ensures that it's mixed.
Me too.  A bit of kicking up from the boil is more reliable than a stir with your spoon.

Experience has shown me it's MUCH more reliable.
Not as easy when you have to churn over 540 gallons.

Yep.  But this is a homebrew forum, so thats what I direct my answers to.
Life begins at 60.....1.060, that is!

www.dennybrew.com

The best, sharpest, funniest, weirdest and most knowledgable minds in home brewing contribute on the AHA forum. - Alewyfe

"The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts." - Bertrand Russell

Offline goose

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Re: OG Lower than Pre-Boil Gravity
« Reply #11 on: October 30, 2019, 12:40:22 PM »
Just to add, I measure my preboil grabvity as the wort just comes to a boil.  That ensures that it's mixed.
Me too.  A bit of kicking up from the boil is more reliable than a stir with your spoon.

Experience has shown me it's MUCH more reliable.

I agree as well.  It is amazing how much stratification occurs when collecting wort and this is way more reliable,
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