Gravity Questions Pre/Post Boil

[Boston85]

This may sound like a dumb question, but I am trying to figure out how and when to best take a gravity reading pre/post boil. I brewed the Zombie Dust recipe listed on this site, and had a targeted batch size of 5.5 gallons

Yesterday I was brewing AG and got 7 gallons into my boil kettle. At the time it was very hot, so I took a sample and put it in the freezer for about 20 minutes to cool off while I began the boil. When I got it down to about 65 degrees I took a reading and it showed 1.055, which is exactly what Beersmith called for. This showed as a mash efficiency of 73.6% in beer smith.

I then went through my boil, and after the hour was left with 6 gallons in the boil kettle. I cooled to 72 degrees and transferred to a fermentation bucket. I let it sit in there for about 10 minutes while I cleaned and before I pitched yeast. I then took a sample using a racking cane from the top of the bucket, and it only measured 1.056 (or 58% measured efficiency), which I feel is impossible if I started at 1.055 pre-boil. One thing I noticed was after putting it in the carboy, I had a lot of stuff at the bottom. I then used one of those wine airation things on a drill to get some oxygen into the wort. While doing this, it naturally kicked up all the stuff at the bottom. I then decided to take another sample, and this one now measured 1.061 (or 63% measured efficiency).

My questions are how exactly should I take a reading post-boil? Should I stir everything in the carboy up and do it? Also, my efficiency of 63% is calculated on the 5.5 gallons I put into my carboy. I still had an extra half gallon in my boil kettle that I decided not to transfer because of junk in it. If I change my measured batch size in Beersmith to 6 gallons, which is what was left in the boil kettle after boil, I get a measured efficiency of 70%, which is right on what the calculated one was.

Even though I only transfer 5.5 gallons, should I still use 6 in my measured batch size? Should I have kept boiling more at the end of 60 minutes to get the full kettle down to 5.5 gallons? If I did that though then I would only likely end up with 5 in the carboy because of some lost to junk.

 

[brewkinger]

I will try and get the ball rolling to answer your questions, but I (we) are gonna need to probe with more questions....

1) What instrument are you using to check gravity, hydrometer or refractometer?

2) There are a couple different "efficiencies" that we can calculate while brewing.
Mash efficiency is a representation of the percentage of the total available sugars that are actually extracted from the grains during the mash/sparge.

Brewhouse efficiency is a representation of how much of the extracted sugars make it from the grains into the wort, through the boil and into the fermenter.

It seems that you are referring to mash efficiency in your discussion, although there are some points that can still be made.

My questions are how exactly should I take a reading post-boil? Should I stir everything in the carboy up and do it? Also, my efficiency of 63% is calculated on the 5.5 gallons I put into my carboy. I still had an extra half gallon in my boil kettle that I decided not to transfer because of junk in it. If I change my measured batch size in Beersmith to 6 gallons, which is what was left in the boil kettle after boil, I get a measured efficiency of 70%, which is right on what the calculated one was.

Even though I only transfer 5.5 gallons, should I still use 6 in my measured batch size? Should I have kept boiling more at the end of 60 minutes to get the full kettle down to 5.5 gallons? If I did that though then I would only likely end up with 5 in the carboy because of some lost to junk.

The 1/2 gallon remaining in your kettle is important... think of it as sugars that you extracted BUT did not make it into the fermenter (to more than likely increase the OG of your beer.)

As far as taking the post boil, makes no difference if you stir it up or not, the gravity is the amount of dissolved SUGAR, so suspended "junk" does not alter the reading.

I hope this helps a little bit (or a lot)

 

[Boston85]

Thanks Brewkinger.

I was using a hydrometer. My mash efficiency was 73%, while brewhouse efficiency was 63%.

What I am wondering is whether I should include that .5 gallon in beer smith when calculating the batch size to get my efficiency. When I use 1.061 at 5.5 gallons I get an efficiency of 63%, but if I use 6 gallons (what was left in the boil kettle) I get 70%. Should I use 6 gallons in the calculation, or am I "punished" for lack of a better word, for not capturing the whole boil kettle into carboy?

As for the comment of stirring it or not, what was weird was I got 2 different readings before I stirred and then after. It was 1.056 before stirring, and then 1.061 after stirring. I suppose there could be a slight difference in temperature of the wort, but I wouldn't think it would make a difference of that much.

 

[brewkinger]

Thanks Brewkinger.

I was using a hydrometer. My mash efficiency was 73%, while brewhouse efficiency was 63%.

What I am wondering is whether I should include that .5 gallon in beer smith when calculating the batch size to get my efficiency. When I use 1.061 at 5.5 gallons I get an efficiency of 63%, but if I use 6 gallons (what was left in the boil kettle) I get 70%. Should I use 6 gallons in the calculation, or am I "punished" for lack of a better word, for not capturing the whole boil kettle into carboy?

As for the comment of stirring it or not, what was weird was I got 2 different readings before I stirred and then after. It was 1.056 before stirring, and then 1.061 after stirring. I suppose there could be a slight difference in temperature of the wort, but I wouldn't think it would make a difference of that much.

So as far as hydrometers are concerned, they are temperature dependent.
In your case I do not think that was the issue as it sounds like your samples were at the right temps.
Around 68 degrees is the norm, and there are small variations but not too bad if you are close to the calibration temp.

One thought that I have from a scientific law / physical world reality...
When you place a sample aside to cool, you have to cover it to account for evaporation loss.
Otherwise you lose a little bit of water and that can skew the reading.

The stir vs no stir debate is a tricky one. Suspended gunk should not make a difference as a hydrometer measures sugar in solution (dissolved), so the suspended crap makes no difference.

As far as including the 1/2 gallon, my thoughts are yes, include it in your batch volume and dump it all in.
The gunk and junk that you are leaving behind are gonna sink to the bottom and not affect the taste or outcome of the final product.
You aren't being punished by the BS progam, to me you are punishing yourself by not dumping it all in. If you are really concerned, get yourself a a stainless steel mesh strainer and strain out some of the gunk if you want to.

It sounds like you just have some tweaking to do to BS to fine tune it to your setup.

 

[Natdavis777]

I like to take my "post boil" sample right before I kill the boil. I pull a cup with a pyrex dish, and set it in the freezer. One thing I have experienced in the past, and this may have happened to you as well, was stratification. I have had a pre-boil OG read higher than a post-boil. In those instances, the wort had sat in the kettle postboil for some time, and I drew my sample from the top. Hence, why I take my sample now at the end of boiling, while the boil is keeping the wort "homogenous" per say.