Residual sugars

[beer1965]

So I finally got around to bottling this oatmeal stout: https://www.brewersfriend.com/homebrew/recipe/view/920489/oatmeal-stout It really should be called a coffee oatmeal stout - very neat flavour with chocolate and coffee notes, but let's see when it carbonates. It's also going to be my last brew without a proper mashtun and fridge so I'm looking forward to the next.

Here's the question:

OG was 1072 and FG was 1032. I'm off a bit from what was expected from the recipe calculations. My alcohol looks to be a respectable 5.25%. I know my brew wasn't very efficient. But am I correct in saying that the higher FG means that there are residual sugars that weren't fermented? And if so, why do you think? I kept it at 68 for the last few weeks. I didn't have a super active krausen but thought it could have been the smaller batch size making it seem that way. Maybe the yeast was sluggish for some reason - I pitched at 70 I think without checking notes.

Also I cold crashed the carboy in my new beer fridge last night and all looked good when I woke up this morning showing 38F with the probe on the side of the carboy. But then this afternoon I went by and it was showing 25F = YIKES! So I unplugged it and opened the door. I don't think anything froze but it was pretty chilled. When I bottled and took the FG it was pretty cool - definitely drinkable temps! But maybe the FG reading wasn't accurate because to the cool temp.

 

[Mark Farrall]

For the attenuation, the main culprits can be oversimplified as:

• not pitching enough healthy yeast,
• whether the wort was fermentable enough or not
• some grain bills are just going to create a worth that isn't very fermentable.

The third is a given with that recipe. And most of the recipes I've tried overestimate how much attenuation you're going to get on a dark beer.

There's a heap of depth to explore on points one and two, the second point especially. Though as you're partial mash it's something for later. Did you pitch the one pack of S04? Straight into the wort?

For the efficiency, you're hamstrung on nearly every level while doing a partial mash. I'd leave that for the future batches when you're mashing

 

[beer1965]

I hydrated the yeast and then pitched. This recipe was a left over brew of sorts - I wanted to use up some left over liquid and dry malt and made up this recipe. I think I'm done with dark beers for a while. One question - given the amount of sugars extracted from the bill it's hard to imagine that wort isn't "fermentable enough".. how does thaw work? Thanks Mark!

 

[Nosybear]

I hydrated the yeast and then pitched. This recipe was a left over brew of sorts - I wanted to use up some left over liquid and dry malt and made up this recipe. I think I'm done with dark beers for a while. One question - given the amount of sugars extracted from the bill it's hard to imagine that wort isn't "fermentable enough".. how does thaw work? Thanks Mark!

Partial mashing may not convert all starches, so if you used flaked grains you get the soluble starches and glucans, carbohydrate gums that give oatmeal beers their "silky" mouthfeel. You need a glucan rest to break down the gums, I generally use a rest at 122 degrees for about 20 minutes to break them down. That would have affected gravity. I would let this one sit for a while before packaging - the FG looks way high to me. I've done partial-mash stouts and not had fermentability problems, I consider it likely you have some dextrines or gums messing with the final gravity.

 

[Mark Farrall]

The yeast amount is probably a bit low, but that's not that big a deal. There was probably just a period while it continued to grow until it had enough cells to finish the job. So then it comes to whether there was the correct food in the wort for the growth. And again it probably was.

When the manufacturers make the DME and LME they need to pick a general fermentability. They need to pick something between a big thick Belgian and a bone dry light lager or Saison. My understanding is they aim for slightly less fermentable than the middle, so that it can work for the big and medium beers. So that's probably contributing to the suggestions from Nosy, but as he mentions it doesn't explain enough.

As it's bottled just keep an eye on it for overcarbonation. One after a couple of weeks and then every week or so for the first couple of months. If you get a gusher then put the rest somewhere as cold as you can, or at least somewhere you can clean them up if one explodes.