Author Topic: does gravity change (increase) during the boil?  (Read 286 times)

Offline KCguy

  • Cellarman
  • **
  • Posts: 78
  • Drinkin Tripels, Seeing Double, Feelin Single!
does gravity change (increase) during the boil?
« on: July 01, 2019, 10:45:12 PM »
Took a sample after draining my bag but before it hit a boil.  Gravity read as 1.066, an hour later 1.070.  Likely my readings are incorrect,  or does it move by points during the boil?  I havent added anything but hops.  no sugars.  edit - room temp samples.
Michael B
Kansas City

Offline Robert

  • Official Poobah of No Life.
  • *
  • Posts: 3681
Re: does gravity change (increase) during the boil?
« Reply #1 on: July 01, 2019, 10:48:26 PM »
Yes, you're boiling off water but keeping all the sugar, etc. so it's more concentrated.  That's one of the reasons for the boil.  You just don't want to overdo it.  A gentle simmer that results in the kind of evaporation rate you're seeing is fine.
Rob Stein
Akron, Ohio

I'd rather have questions I can't answer than answers I can't question.

Offline KCguy

  • Cellarman
  • **
  • Posts: 78
  • Drinkin Tripels, Seeing Double, Feelin Single!
Re: does gravity change (increase) during the boil?
« Reply #2 on: July 01, 2019, 11:17:44 PM »
Yes, you're boiling off water but keeping all the sugar, etc. so it's more concentrated.  That's one of the reasons for the boil.  You just don't want to overdo it.  A gentle simmer that results in the kind of evaporation rate you're seeing is fine.

Dang, I knew that was a dumb question.  I guess the evap is obvious...Im just tired today. 
Michael B
Kansas City

Offline Robert

  • Official Poobah of No Life.
  • *
  • Posts: 3681
Re: does gravity change (increase) during the boil?
« Reply #3 on: July 01, 2019, 11:19:11 PM »


Yes, you're boiling off water but keeping all the sugar, etc. so it's more concentrated.  That's one of the reasons for the boil.  You just don't want to overdo it.  A gentle simmer that results in the kind of evaporation rate you're seeing is fine.

Dang, I knew that was a dumb question.  I guess the evap is obvious...Im just tired today.

No problem.  Have a  beer and all will be good.
Rob Stein
Akron, Ohio

I'd rather have questions I can't answer than answers I can't question.

Offline Megary

  • Cellarman
  • **
  • Posts: 99
Re: does gravity change (increase) during the boil?
« Reply #4 on: July 02, 2019, 12:02:59 AM »
I'm sure you know this already, but one of the things I have learned early is to take that gravity reading pre-boil.  If you are off an "expected" pre-boil gravity you can make adjustments on the fly.  Low gravity...add extract.  High gravity...add water or boil a little longer (or just enjoy the extra kick!). 

I wish I knew to take that reading on my first batch back to brewing.  I only took it after the boil and ended up with near beer.

Offline KCguy

  • Cellarman
  • **
  • Posts: 78
  • Drinkin Tripels, Seeing Double, Feelin Single!
Re: does gravity change (increase) during the boil?
« Reply #5 on: July 02, 2019, 12:29:25 AM »
I'm sure you know this already, but one of the things I have learned early is to take that gravity reading pre-boil.  If you are off an "expected" pre-boil gravity you can make adjustments on the fly.  Low gravity...add extract.  High gravity...add water or boil a little longer (or just enjoy the extra kick!). 

I wish I knew to take that reading on my first batch back to brewing.  I only took it after the boil and ended up with near beer.

Reason for taking the gravity reading in the first place for me as well!  Couldnt agree more. 
Michael B
Kansas City

Offline Thirsty_Monk

  • Brewmaster General
  • *******
  • Posts: 2383
  • Eau Claire WI
    • Lazy Monk Brewing
Re: does gravity change (increase) during the boil?
« Reply #6 on: July 02, 2019, 01:45:32 AM »
Now do not ask me how that happened but my pre boil gravity is always bigger then my post boil gravity.

Granted it is on 15 BBL brewhouse. I whirlpool the whole time I sparge to avoid stratification.

It is mystery to me.
Na Zdravie

Lazy Monk Brewing
http://www.lazymonkbrewing.com

Offline Robert

  • Official Poobah of No Life.
  • *
  • Posts: 3681
Re: does gravity change (increase) during the boil?
« Reply #7 on: July 02, 2019, 01:54:23 AM »
^^^^
Two possibilities come to mind, both admittedly longshots:  1) Stratification leading to a false sample (but whirlpool should homogenize the wort I would think;) or 2) virtually no evaporation and the decrease in extract is in the material precipitated out in the break (surely there is evaporation.)  As to the second point, my increase in gravity after the boil is always less than the decrease in volume would predict, if loss of break were not taken into account.  But that cannot account for a very big difference such as you describe, can it?
Rob Stein
Akron, Ohio

I'd rather have questions I can't answer than answers I can't question.

Offline goose

  • Brewer
  • ****
  • Posts: 350
Re: does gravity change (increase) during the boil?
« Reply #8 on: July 02, 2019, 05:37:05 PM »
The other possibility could be instrumentation error if you are using a refractometer pre-boil and a hydrometer post-boil.  I have run into this as well even though I whirlpool the heck out of my pre-boil wort to try to eliminate any stratification and get the most accurate reading on my refractometer.  Although mine reads pretty close to my hydrometer, my measured pre-boil gravity is always gives me a bit different predicted OG than what I actually measure once the wort is cooled post-boil.  However, it is always within about a couple percent of what the brewing software predicts and that is close enuf for me.
Goose Steingass
Wooster, OH
Society of Akron Area Zymurgists (SAAZ)
Wayne County Brew Club
Mansfield Brew Club
BJCP Certified
AHA Governing Committee Member

Offline Thirsty_Monk

  • Brewmaster General
  • *******
  • Posts: 2383
  • Eau Claire WI
    • Lazy Monk Brewing
Re: does gravity change (increase) during the boil?
« Reply #9 on: July 02, 2019, 09:17:53 PM »
It looks like I successfully hijacked this thread :)

I do not have a lot of evaporation in my BK. It is quite common to be below 3% in commercial brewhouses. Just the volume to surface area. On top of that my evaporation stack is sideway and I do not get a good draft.

One would assumed that if there is no reduction in volume the gravity would stay the same.

I take preboil gravity  from side port. I take a sample about 16 Oz, dump it and take another sample to measure.

I use fine scale Plato refractometer so that would eliminate the error on sample part as well as instrument part.

Why would hot break make make a difference? Hot break usually coagulate protein.

Looking into the records I am off about 0.3-0.5 Plato. My next step would be to take pre boil sample from top of BK.

Thank you for reading.
Na Zdravie

Lazy Monk Brewing
http://www.lazymonkbrewing.com

Offline Robert

  • Official Poobah of No Life.
  • *
  • Posts: 3681
Re: does gravity change (increase) during the boil?
« Reply #10 on: July 02, 2019, 09:44:22 PM »
Hot break is mostly protein.  But the density of wort is a measure of all dissolved material -- of course mostly sugar, but also small contributions from soluble proteins, and even hop acids and oils, and the mineral content of the water.  I presume that the removal from solution of a portion of the proteins would be measurable,  at least with a hydrometer.   I don't know how or if a refractometer calibrated with a sucrose solution would read this.  (Maybe  I should know, because I too use a digital Plato refractometer.)  On the homebrew scale, I have always assumed there is a real effect which accounts to some degree for my observed difference in increase in density as actually measured and as predicted by the evaporation rate.  But I also assume that this is all within the margin of my measurement accuracy and resolution (if my eyeball reading of my dipstick is off by a few fluid ounces, and my refractometer has an accuracy of +/- 0.2 °P, any real effect will probably be obscured by error.)  But perhaps on a larger scale, you are able to more reliably observe such small variations?  This has always been in the back of my mind, and I am curious to know if there is any good scientific study of this.  It might even in some way provide an index  index of effective boiling, insofar as the precipitation of undesirable proteins is a primary goal of the boil.
Rob Stein
Akron, Ohio

I'd rather have questions I can't answer than answers I can't question.

Offline coolman26

  • Brewmaster
  • *****
  • Posts: 847
Re: does gravity change (increase) during the boil?
« Reply #11 on: July 02, 2019, 09:48:34 PM »
Now do not ask me how that happened but my pre boil gravity is always bigger then my post boil gravity.

Granted it is on 15 BBL brewhouse. I whirlpool the whole time I sparge to avoid stratification.

It is mystery to me.
15BBL fantastic stuff!!
You have came a long way from when I visited your brewery years ago. You were trying to get the canner operational. Bravo Sir!!
One day I hope to make it back and see your “new” location. 


Sent from my iPhone using Tapatalk
Jeff B

Offline Thirsty_Monk

  • Brewmaster General
  • *******
  • Posts: 2383
  • Eau Claire WI
    • Lazy Monk Brewing
Re: does gravity change (increase) during the boil?
« Reply #12 on: July 02, 2019, 10:41:07 PM »
Now do not ask me how that happened but my pre boil gravity is always bigger then my post boil gravity.

Granted it is on 15 BBL brewhouse. I whirlpool the whole time I sparge to avoid stratification.

It is mystery to me.
15BBL fantastic stuff!!
You have came a long way from when I visited your brewery years ago. You were trying to get the canner operational. Bravo Sir!!
One day I hope to make it back and see your “new” location. 


Sent from my iPhone using Tapatalk
Yes. It had been nine years already. Time flies when you have fun.

Let me know when you are around and I will show you how it is going.
Na Zdravie

Lazy Monk Brewing
http://www.lazymonkbrewing.com

Offline Thirsty_Monk

  • Brewmaster General
  • *******
  • Posts: 2383
  • Eau Claire WI
    • Lazy Monk Brewing
Re: does gravity change (increase) during the boil?
« Reply #13 on: July 02, 2019, 10:44:03 PM »
Hot break is mostly protein.  But the density of wort is a measure of all dissolved material -- of course mostly sugar, but also small contributions from soluble proteins, and even hop acids and oils, and the mineral content of the water.  I presume that the removal from solution of a portion of the proteins would be measurable,  at least with a hydrometer.   I don't know how or if a refractometer calibrated with a sucrose solution would read this.  (Maybe  I should know, because I too use a digital Plato refractometer.)  On the homebrew scale, I have always assumed there is a real effect which accounts to some degree for my observed difference in increase in density as actually measured and as predicted by the evaporation rate.  But I also assume that this is all within the margin of my measurement accuracy and resolution (if my eyeball reading of my dipstick is off by a few fluid ounces, and my refractometer has an accuracy of +/- 0.2 °P, any real effect will probably be obscured by error.)  But perhaps on a larger scale, you are able to more reliably observe such small variations?  This has always been in the back of my mind, and I am curious to know if there is any good scientific study of this.  It might even in some way provide an index  index of effective boiling, insofar as the precipitation of undesirable proteins is a primary goal of the boil.
Very interesting. You are correct we are measuring liquid density and assuming that it is all sugar.

Thank you for the explanation.


Sent from my iPhone using Tapatalk
Na Zdravie

Lazy Monk Brewing
http://www.lazymonkbrewing.com