Confused on Post Boil vs. OG...brewersfriend glitch?

[luckybeagle]

It's my understanding that Post Boil gravity and OG are the same thing.

I brewed a Mocktoberfest (Unconventional Marzen-ish, Recipe: https://beerandbrewing.com/make-your-best-mocktoberfest/) yesterday and exceeded my preboil gravity target of 1.043 (I hit 11.1 brix, or 1.045), which is fine as I can just adjust my brewhouse efficiency from 72% to 75%:

After the boil (60 minutes, fairly vigorous, 12% evaporation rate accounted for), my refractometer read ~11.8 brix (1.048). I double checked this with my hydrometer and confirmed 1.048/1.049.

My brewer's friend app says I should hit 1.049 for my "post boil gravity." 1 point off is close enough to me. However, it also states the Original Gravity should be 1.054... what gives?

I can't figure out why OG and post boil gravity are different. Does anyone know? Which one should I be shooting for, typically? Haylllp?

 

[luckybeagle]

Ok, I might've had a few wrong calculations! I ended up with 11 gallons post boil. I followed my kettle etchings, which may be off a bit. Everything was calculated for 10 gallons in the fermenter. My bad. When I adjust for new batch size, the OG and Post Boil gravity are both the same now, which matches my results. I STILL don't understand why OG and post boil gravity are ever different, since I'm not adding plain water to anything post boil....

Anyway, it's looking like this will produce a pretty weak beer at 4.9% ABV with the typical attenuation level of WY1056 (75%). I'm afraid of it lacking in flavor or body, having a bad hop balance etc. Do you think I should add some table sugar to the fermenters to bump it up a few points? Or would that run the risk of drying the beer out/thinning out too much for the style? Or should I boil some DME on my stovetop and add it to the fermenter at high krausen? Or should I just leave it alone and RDWHAHB??

 

[CascadesBrewer]

I don't use Brewer's Friend. Are all the values shown ones estimated by BF, or are some of them values that you entered? Yeah, in general Post Boil Gravity and Original Gravity should be the same number (unless you are adding fermentables directly to the fermenter).

I would go the RDWHAHB route myself, especially once I have the beer cooled and in the fermenter. There is not that much difference between a 1.048 beer and a 1.054 beer. Sometimes I will make adjustments if my pre-boil gravity or volume are off.

Only 4 gravity points difference between pre and post boil seems low. For a typical beer in that gravity range I usually see around a 10 gravity point change (5 gal batches, 1 gal boil off...so 1.043 to 1.053). Was your boil off rate low?

 

[luckybeagle]

I don't use Brewer's Friend. Are all the values shown ones estimated by BF, or are some of them values that you entered? Yeah, in general Post Boil Gravity and Original Gravity should be the same number (unless you are adding fermentables directly to the fermenter).

I would go the RDWHAHB route myself, especially once I have the beer cooled and in the fermenter. There is not that much difference between a 1.048 beer and a 1.054 beer. Sometimes I will make adjustments if my pre-boil gravity or volume are off.

Only 4 gravity points difference between pre and post boil seems low. For a typical beer in that gravity range I usually see around a 10 gravity point change (5 gal batches, 1 gal boil off...so 1.043 to 1.053). Was your boil off rate low?

After racking my brain about it, this is what I think happened: I think the main cause for me being off from what I thought I'd see for OG (1.054) was that I sparged with too much water and had a higher preboil volume. My efficiency was plugged in at 72%, but it must've been quite a bit higher than that and was simply diluted down with the extra sparge water. That coincidentally made me hit the target 1.043 preboil volume, but gave me more volume in the kettle which didn't evaporate out as much as it would've needed to in order to hit the recipe's 1.054 estimate. That's my best guess, anyway! Next time I will triple check volumes! My oversight cost me valuable booze units

I'm always amazed at how mistakes domino in brewing. Really makes me proud when I brew something good!

 

[sicktght311]

If i remember correctly........isnt post boil gravity the amount before chilling and racking to your fermenter? Literally the moment you shut the boil down. And then OG is what ends up in your fermenter after wort shrinkage and final evaporation?

 

[bucketnative]

I was confused by this before, and I worked through a couple of scenarios to come to the realization that The Brewer's Friend Recipe Calculator uses a different set of numbers to give the Post-Boil Gravity (PBG) and the Original Gravity (OG). The PBG is based on the post-boil volume in the kettle. The OG is based on the volume in the fermenter (with a catch - see below). I think this set up is to account for brewers who do a partial-boil process, and is only valid going the direction of diluting the wort from the kettle with added water in the fermenter.

Case 1 (Partial Boil): Mash 10 lb pilsner, resulting in 4 gal runnings in the kettle (pre-boil volume). Boil to yield post-boil volume of 2 gal. Set batch target size to 5 gal in fermenter. [Essentially adding 3 gal water to fermenter to dilute wort from kettle]
Pre-Boil Gravity = 1.093 -> Post-Boil Gravity = 1.139 -> OG = 1.056

Case 2 (Full Boil): Mash 10 lb pilsner, resulting in 6 gal runnings in the kettle (pre-boil volume). Boil to yield post-boil volume of 5 gal. Set batch target size to 5 gal in fermenter. [Essentially emptying the entire kettle into the fermenter]
Pre-Boil Gravity = 1.046 -> Post-Boil Gravity = 1.056 -> OG = 1.056

Now Case 3 is where I was getting caught up in the calculator. I was mashing 10 lb pilsner and getting 6 gal in the kettle. My post-boil volume was 5 gal, but I was only transferring 4.5 gal to the fermenter to leave enough headspace and to leave some hop matter and cold break behind. So, I set my target in the fermenter to 4.5 gal... and it spit out the following numbers:
Pre-Boil Gravity = 1.046 -> Post-Boil Gravity = 1.056 -> OG = 1.062

The OG is artificially high in Case 3, because the calculator is assuming that I am concentrating my post-boil volume of 5 gal down to 4.5 gal and raising the OG... This is incorrect. My wort in the fermenter should still be 1.056 even when I leave 0.5 gal in the kettle, because the wort is assumed homogeneous.