Recipe pre-boil and post-boil don't add up

[Dustydust]

I have a recipe with an estimated 1.081 post boil gravity and a 1.059 pre-boil gravity. I brewed it today and my pre-boil gravity was 1.058 (very close); however, my post boil gravity was 1.068 (>10 pts off). I did some investigation into the accuracy of the post boil gravity calculated by Brewer's Friend, In my experience, Brewer's Friend is usually very accurate with gravity calculations. It should be noted that this recipe contained a late boil addition of d-180 candi syrup. In my investigation, I copied the recipe and removed the candi syrup, in order to simplify the search. Both recipes are linked below.

I'll use the debug recipe going forward with this post because it simplifies the problem and the d-180 didn't appear to be related to the root cause. What I found was that the gravities and estimated boil off of the debug recipe doesn't add up. I used the "print" tool from the recipe view in order to obtain the following values.

Debug recipe stats (no d-180 kettle addition)
Pre Boil Gravity: 1.059 (recipe based estimate)
Post Boil Gravity: 1.076 (recipe based estimate)
Pre boil volume 7.35 gal
Post boil Volume 6.48 gal
Boil Time: 90 min

These are the pre-boil and post boil volumes I'm used to using based on my equipment profile. I've used close variations of these volumes many times and have always had good results. Something seemed fishy about a 0.017 gravity bump from a 0.87 gal boil off.

Using the dilution/boil off gravity calculator (link below), I ran the following calculation and found that the estimated post-boil gravity for the debug recipe should have been 1.067, not 1.076 like the print tool and the recipe creator tool estimated. I found this very odd because Brewer's friend has been correct in the past, but appears to be off by a lot here.

Wort Volume: 7.35 (gal / L / qt)
Current Gravity: 1.059
Target Volume: 6.48
New Gravity: 1.067
Difference: 0.008

In order to hit a 1.076 post-boil, using the volumes listed, the pre-boil gravity would need to be 1.067. Perhaps it is just a coincidence that the "New Gravity" found above is what the gravity would have needed to be to actually hit the 1.076 required for the recipe, seems like an odd coincidence though.


Original recipe - https://www.brewersfriend.com/homebrew/recipe/view/1404581/swim-trunks-and-flippy-floppies
Debug Recipe with D-180 removed - https://www.brewersfriend.com/homebrew/recipe/view/1406394/debug
Dilution/Boil Off calculator - https://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/


I am a bit bummed that this beer didn't turn out the way I wanted it, there were high hopes for this one. I brew for competitions and this one is going to be all out of whack in balance now.

 

[Dustydust]

Following up on this. I'm curious if I'm doing something wrong. I've been using BF for years and haven't encountered this issue before. I'd like to know if this is a regression or if I didn't use the tool properly.

 

[Pricelessbrewing]

Hey Dusty, sorry for the delay. @Yooper sent this over to me to help with, and I've identified the issue.

Good thinking on removing the D-180 to try and isolate, and you're thinking is in the right territory but it's in the opposite direction. The calculation that's not working is actually the pre-boil gravity, not the volumes or the post boil gravity.

If we remove the d-180, and look at the beer stats section you can indeed see that there's an issue with the preboil gravities not lining up. Note that the total "points" are nowhere near each other, this shows that either there's a calculation error, or some source of fermentables is being added between the preboil and postboil. In this recipe, that shouldn't be happening.

If you change the two roasted malts from Late:Mashtun addition to a standard mashed grain, you get the following, which absolutely lines up within itself. No mysterious sugars coming or going.

So here's where things get less clear, but if I were to take a well educated guess, I would suspect that the recipe efficiency is actually the culprit on why you missed your OG. If the volume calculations are correct, and I'm reading your recipe right, then I'm seeing an entire gallon is left behind post boil but you're still expecting 74% brewhouse efficiency? That would be equivalent to an 88% brewhouse efficiency without leaving behind any wort, which is extremely high but doubly so for an almost 1.080 OG recipe. You'll also lose a small amount of efficiency on those two late mash additions, but they're small so it probably won't make a big difference. My predictive algorithm for efficiency based on grain amounts, boil off rate, volume losses, and sparge ratio predicts an ideal gravity of about 66% at essentially what would be the upper bound. If I put that into the recipe builder it gives a gravity of 1.061preboil and 1.069 postboil, just above what you read.

So based on your gravity readings, I would suggest lowering the efficiency down on this recipe to about 64% and that should let you consistently hit your gravities for this amount of grain.

 

[Dustydust]

Thanks for providing that insight. I've overlooked the batch stats (other than BU/GU) in the drop down menu, I'm going to pay more attention to that, especially when mash capping. I'm looking forward to trying it out the next time I make a mash capped beer.

I ordinarily use around 74% BH on average strength beers, but tend to have less than a gallon left behind, not a lot less though. So in reality, I'm usually lower than that. With this beer, I wanted to see how close I could get to the target of 74 that I've used on regular strength beers; it seems like 64% would have been a better target. There were red flags that indicated I wasn't adding enough sugar, but I ignored them. If the pre-boil calculation had shown I missed my target, I would have added more fermentables to compensate.

Regarding your predictive algorithm, do you have that algorithm in a tool or some way to share? That seems like a convenient way to identify a target efficiency.

 

[Pricelessbrewing]

Thanks for providing that insight. I've overlooked the batch stats (other than BU/GU) in the drop down menu, I'm going to pay more attention to that, especially when mash capping. I'm looking forward to trying it out the next time I make a mash capped beer.

I ordinarily use around 74% BH on average strength beers, but tend to have less than a gallon left behind, not a lot less though. So in reality, I'm usually lower than that. With this beer, I wanted to see how close I could get to the target of 74 that I've used on regular strength beers; it seems like 64% would have been a better target. There were red flags that indicated I wasn't adding enough sugar, but I ignored them. If the pre-boil calculation had shown I missed my target, I would have added more fermentables to compensate.

Regarding your predictive algorithm, do you have that algorithm in a tool or some way to share? That seems like a convenient way to identify a target efficiency.

Not yet, but it's on our to do list to integrate into BF! If you Google my name you'll find a very rudimentary calculator I made over a decade ago that will spit out the expected efficiency. I hope to make it into the pipeline this year though.