Post boil gravity doesn't match post boil gravity

Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum

Help Support Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Kaiser

Well-Known Member
Joined
Nov 1, 2005
Messages
3,895
Reaction score
171
Location
Pepperell, MA
That is correct.

Today I measured the post boil gravity before chilling the wort by cooling down a sample in a large stainless steel bowl and then using a hydrometer. The gravity I measured doesn't match the gravity I got when I measured the same wort after chilling with an immersion chiller.

post boil and pre chilling ~ 16.0 Plato or 1.064
post boil and post chilling ~ 17.0 Plato or 1.068.

That is to large to be caused by looking at the hydrometer in different ways. I even compared both samples at the same temperature.

What's going on here? Is it the precipitation of cold break. But 1 Plato difference is a lot. That is 1% of the weight of the wort. No water was added by a possibly leaking immersion chiller as the volumes pre and post chilling matched once corrected for temperature

Kai
 
I have always been curious whether break and hop material affects the hydrometer reading. I usually forget until I have the carboy full, so I measure with the last from the kettle which is sometimes very uh... thick. I let it settle and make sure the hydrometer isnt resting on a compost pile in the bottom of the tube.
 
Kai - Just a thought, but perhaps your wort wasn't properly mixed during one of the samplings?
 
What's going on here? Is it the precipitation of cold break. But 1 Plato difference is a lot.
I can't think what else it could be. I assume that your measurement practices are sound, since I know you're pretty experienced with the scientific side of brewing... Having not added or subtracted anything from the wort between the two samples, it has to be some physical change that occurred during chilling, and cold break is the very obvious culprit.

Considering how much cold break you can get with proper chilling, I'm not surprised that it would produce some noticeable change, and the numbers you've quoted don't seem COMPLETELY unreasonable to me as being attributable to cold break, but I certainly don't know enough to be sure.
 
Full boil and all grain.

I think I have a possible explanation: evaporation. The sample I pulled before chilling must have lost about 5% of its water while it was cooling. Seems like an awful lot for something that is not boiling, but it was a large bowl to speed up cooling.

But the pre-chill gravity and volume make sense with the pre-boil gravity and volume, whereas the post-chill gravity and volume don't make sense with the pre-boil volume. (everything temperature corrected of couse).

Cold break is reported to be only 40-350 mg/l, but to cause the difference I'm seeing it must have been 10,000 mg/l.
 
I do not understand how a full boil not be mixed well? I guess it settles a bit during whirlpool etc, but I would not expect it to make a 4 point difference?
 
aside from evaporation, my only other guess is a little bit of water from washing the hydrometer and cylinder didnt get shaken/dried out and it diluted one of your samples. Ive had this happen to me when I didnt dried it carefully.
 
Just a thought, but perhaps your wort wasn't properly mixed during one of the samplings?

The odd thing is, that the pre-chill gravity would be the one that I could have messed up with insufficient mixing. But I did mix it well and it also makes sense if I look at the numbers:

pre boil gravity = 15.3 Plato; volume = 25 l
post boil/ pre chill gravity = 17.0 Plato; volume = 22 l

If I predict the post boil gravity from the preboil I get 17.4.

The post chill gravity of 16.0 Plato for 22l just doesn't fit into the picture.


Kai
 
I don't think the difference would be caused by break material. The heavier solids precipitating out would leave a less dense wort - wouldn't it?.
 
It could be your measurements are off some, ie temerature, gavity and volume, causing the final prediction to be off some too. Also, the wort should shrink about 4% from boiling temperature to 60 F.
 
I'm inclined to think evaporation, but from your kettle, not the sample. If the sample was evaporating much, it would have had the higher gravity. How long between taking the sample & chilling the wort? Also, how do you cool the wort, CFC or chiller? The 20 minutes it takes me to cool a batch would allow a fair amount of evaporation.
 
I'm inclined to think evaporation, but from your kettle, not the sample.

I put a lid on the kettle 2 min after taking the sample. So no significant evaporation. And even if there was evaporation, shouldn't I have seen a corresponding 5% drop in volume in addition to the 4% wort shrinkage? But I only saw a 4% smaller wort volume after the end of the boil.

This one is a real puzzler to me.

Kai
 
This problem still exists for me. Last night I brewed a batch for which I used more water for chilling than usual as I wan’t able to move the IC as often as I generally do. I lost a whopping 6% efficiency between pre-boil and post chill:

4.2 kg grain bill:
25.8 liter at 100C pre-boil volume at 11.5 Plato -> 88% efficiency in kettle
21.9 liter at 7C post-chill volume at 12.0 Plato -> 82% efficiency in kettle

B/c this calculation was done with efficiency, it would not matter if the chiller leaked water I to the wort. Something it didn’t do since the pre-chill volume was about 4% more than the post chill volume. All extract measurements were temperature corrected.

I have the suspicion that the chiller allows for an exchange of extract from the wort into the chill water. Not sure how it does this w/o changing the volume in the kettle. Maybe water is pushed in at one place and pulled out from another though the venturi effect. But the chiller doesn’t leak when I just run water though. The outside stays dry. It has been suggested that there might be pin-holes that open when the chiller is hot.

In one case I even measured the gravity of the spent chill water and (corrected for temp) it seemed slightly higher than that of tap water. This would support the idea of the missing extract ending up in the spent water.

If that is truly the case, I should also be getting some “hose” flavors into the beer

Once we have snow I’ll brew one batch chilled by submersion in ice water only. That will remove the chiller from the equation.

This is puzzling and annoying me.

Kai

 
I'm resurecting this old thread to give an update as I now think I know what was the problem:

The preboil samples were cooled in an open bowl which allowed for evaporation. Enough evaporation to increase the gravity of the sample by 2-4 points (0.5 – 1 Plato). This lead me to believe that my efficiency was higher than it actually was and hence the discrepancy to the post chill gravity.

I now always chill samples in a 100 ml baby food jar than I close with a lid and place in ice water. As a result my preboil and post chill measurements are now making sense.

Kai
 
Back
Top