Question about sugars and volume

Ok maybe someone can explain to me how this happened.

Est Pre-boil Gravity: 1.063
Est OG: 1.087
Est Pre-Boil Vol: 8.5 gal
Est Post-Boil Vol: 6.5 gal

I go through my normal 90 minute mash and with a starting temp of 150 and ending of 148. Pull my grain (BIAB) and take a sample.

Pre-Boil Vol: 9.6 gal
Pre-Boil Gravity: 1.061

I'm thinking this is great. I have a gallon more of wort and got damn close to my estimated gravity (85.3% mash eff. calculated by Beersmith).

Start my 90 minute boil and go through my normal process there.

Boil-off: 2.1 gal
Post-Boil Vol: 7.5 gal
OG: 1.076
Vol. into fermenter: 6 gal

I had a full 1.5 gallons of hot/cold break and hop gunk (5oz pellet in recipe) in the bottom of my kettle.

My question is could there really have been that many sugars trapped in the trub to where I would have come in 10 points shy of my Est OG? I thought the sugars were constant regardless of volume and boiling down should concentrate them at a measurable rate.

All gravity readings were adjusted for temp of the sample and taken with a hydrometer. Both hydrometer and thermometer have been calibrated.

Thanks in advance for any help.

You extracted a given amount of sugars from the grains with the assumption of having 6.5 gallons of wort at a specific gravity of 1.087. You still extracted the same amount of sugars (approximately) except you diluted it with an extra gallon of water to get to 1.076. My calculations come out pretty much dead on to what happened. If you were to keep boiling and boil off another gallon of water until it was down to 6.5 gallons you would have had your expected OG. My guess is you need to dial back your sparge water slightly.

Remember, the most common way to measure sugars in your beer is points per pound per gallon

It looks to me like you still have the standard grain absorption rate set in beersmith. When you BIAB, it's a lot lower which accounts for the extra gallon of preboil you ended up with.

Thanks for the help everyone. I think for future brews I will definitely use a hop sock to reduce the trub some.

I have also adjusted down my biab grain absorption in Beersmith based on my numbers from this brew and the numbers from my past 4 brews. Hopefully I wont have the same problem again.

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There also seems to be something strange with the calculation for your expected values.

The expected values say you planned to boil down 8.5 gallons of 1.063 wort to 6.5 gallons. This is a concentration factor of 8.5/6.5 = 1.031, so you'd expect to get (8.5/6.5)*63 = 82 points, i.e. an OG of 1.082. To reach 1.087, I'd calculate you'd need to boil it down to 6.15 gallons.

Did you get these numbers from Beersmith?

In the event, you had 9.6 gallons of 1.061 wort boil down to 7.5 gallons, which gives a concentration factor of the slightly lower 1.28. You should then get (9.6/7.5)*61 = 78, so 1.078. Like others have said, this is close to the 1.076 you report, and probably within reasonable measurement error on the volumes and so on.

It's worth noting that the volume is 4 % higher at boil temp than room temp, so if you're measuring one volume hot and one cold, you could need to adjust your math a bit. For instance, if your 9.6 gallons were at boil temp, and 7.5 at room temp, the difference is really between 9.6*0.96 = 9.22 and 7.5. Incidentally, using this, we get ((9.6*0.96)/7.5)*61 = 75, which is even closer to your 76.

Re-reading this answer, I'm not sure if I'm clearing things up, ur muddying the waters. At least I got to do some calculations!