agusus
Well-Known Member
First, a warning this has a fair amount of math geekery. So stop reading if you don't like that.
I just finished my 4th partial mash using the BIAB technique.
After the 1st one I started taking pre-boil gravities (PBG) so I could see how effective the mash was and be able to adjust the amount of extract I add to reach my target OG. But so far this plan has mostly failed! My PBG comes out right on target at a 75-85% efficiency, but then my OG comes out 10 to 15 gravity points lower than it should be based on the PBG + the amount of extract added.
So either the laws of chemistry have been violated (not likely ) or one of my measurements is wrong.
I've had pretty good success with measuring OG's on my pure extract batches - it comes 1 to 2 points from the expected OG - so I doubt my OGs are wrong. Which means the PBGs must be wrong, or I somehow lost significant quantities of sugar in between the PBG and OG. The only place sugar could disappear to is the hop bags, and I always shake/squeeze/drain those pretty well so I don't think I lose much liquid to them.
And if I can't get accurate PBG then the technique is useless for the purpose of adjusting my boil to reach target OG and hoppiness for the style I'm brewing. (On my first PM brew, where I didn't take PBG because I didn't know about it, I missed OG by 21 points - https://www.homebrewtalk.com/f36/missed-og-21-points-first-partial-mash-133498/ ).
Here's my process, can anyone speculate about possible causes of the incorrect PBG? Is my equation wrong?
1. BIAB mash. (the mash specifics are not important. PBG is about measuring the quantity of sugars in my wort after the mash).
2. Combine runnings into brewpot, mix thoroughly, and immediately extract 1 cup of wort by dipping a measuring cup into the wort.
3. Cool sample (which is around 160 F) to 60-80 F using ice bath.
4. Measure gravity using hydrometer. Let G = number of gravity points (G=46 if gravity = 1.046)
5. PBG = G * Pre-boil Volume * 0.97 / Fermentation Volume. (0.97 is the temperature adjustment factor for 170 F water)
Example from my last brew: PBG = 52 * 4.0 * 0.97 / 5.25 = 38.4
For my grain bill in this brew (5lb 2-row + 0.75 choc + 0.5 80L + 0.25 patent), BeerSmith says it is 85% efficiency.
6. Boil the wort (1hr), add hops in nylon steeping bags, drain the bags thoroughly when removing at the end of boil.
7. Cool wort, add to fermenter, top off with more water to get to 1 inch over the 5 gallon mark on my carboy (probably 5.1 to 5.25 gallons).
8. Aerate wort by shaking carboy, do a beer thief sample extraction and measure gravity (at 70-80 F).
In the brew that I mentioned above (PBG=38), my OG came out to 1.053, with 4.25 lb of pale LME in the recipe. According to BeerSmith the LME contributes 30 gravity points, so the partial mashed grains only contributed 23 (1.053-1.030) - much lower than the 38 points my PBG said they contributed. 23 points works out to only 50% efficiency.
Now I know there could be some slight measurement inaccuracies in my PBG formula - like my pre-boil volume might have been 3.75 instead of 4, and my post-boil volume might be 5.1 instead of 5.25. But I've tried those numbers too and that's not enough to account for a 15 gravity point difference.
Don't tell me RDWHAHB I am relaxed, and I know these batches will probably still be quite tasty (just lower ABV and hoppier than I intended). I just like math, and want to try to figure out what's wrong with my PBG process so I can correct it and use it reliably in the future.
I just finished my 4th partial mash using the BIAB technique.
After the 1st one I started taking pre-boil gravities (PBG) so I could see how effective the mash was and be able to adjust the amount of extract I add to reach my target OG. But so far this plan has mostly failed! My PBG comes out right on target at a 75-85% efficiency, but then my OG comes out 10 to 15 gravity points lower than it should be based on the PBG + the amount of extract added.
So either the laws of chemistry have been violated (not likely ) or one of my measurements is wrong.
I've had pretty good success with measuring OG's on my pure extract batches - it comes 1 to 2 points from the expected OG - so I doubt my OGs are wrong. Which means the PBGs must be wrong, or I somehow lost significant quantities of sugar in between the PBG and OG. The only place sugar could disappear to is the hop bags, and I always shake/squeeze/drain those pretty well so I don't think I lose much liquid to them.
And if I can't get accurate PBG then the technique is useless for the purpose of adjusting my boil to reach target OG and hoppiness for the style I'm brewing. (On my first PM brew, where I didn't take PBG because I didn't know about it, I missed OG by 21 points - https://www.homebrewtalk.com/f36/missed-og-21-points-first-partial-mash-133498/ ).
Here's my process, can anyone speculate about possible causes of the incorrect PBG? Is my equation wrong?
1. BIAB mash. (the mash specifics are not important. PBG is about measuring the quantity of sugars in my wort after the mash).
2. Combine runnings into brewpot, mix thoroughly, and immediately extract 1 cup of wort by dipping a measuring cup into the wort.
3. Cool sample (which is around 160 F) to 60-80 F using ice bath.
4. Measure gravity using hydrometer. Let G = number of gravity points (G=46 if gravity = 1.046)
5. PBG = G * Pre-boil Volume * 0.97 / Fermentation Volume. (0.97 is the temperature adjustment factor for 170 F water)
Example from my last brew: PBG = 52 * 4.0 * 0.97 / 5.25 = 38.4
For my grain bill in this brew (5lb 2-row + 0.75 choc + 0.5 80L + 0.25 patent), BeerSmith says it is 85% efficiency.
6. Boil the wort (1hr), add hops in nylon steeping bags, drain the bags thoroughly when removing at the end of boil.
7. Cool wort, add to fermenter, top off with more water to get to 1 inch over the 5 gallon mark on my carboy (probably 5.1 to 5.25 gallons).
8. Aerate wort by shaking carboy, do a beer thief sample extraction and measure gravity (at 70-80 F).
In the brew that I mentioned above (PBG=38), my OG came out to 1.053, with 4.25 lb of pale LME in the recipe. According to BeerSmith the LME contributes 30 gravity points, so the partial mashed grains only contributed 23 (1.053-1.030) - much lower than the 38 points my PBG said they contributed. 23 points works out to only 50% efficiency.
Now I know there could be some slight measurement inaccuracies in my PBG formula - like my pre-boil volume might have been 3.75 instead of 4, and my post-boil volume might be 5.1 instead of 5.25. But I've tried those numbers too and that's not enough to account for a 15 gravity point difference.
Don't tell me RDWHAHB I am relaxed, and I know these batches will probably still be quite tasty (just lower ABV and hoppier than I intended). I just like math, and want to try to figure out what's wrong with my PBG process so I can correct it and use it reliably in the future.