Can gravity reduce during diacetyl rest?

[Zadkiel]

Have a beer that I thought had finished, had been sat at between 1.014 & 1.015 for a couple of days and that was about my expected FG, so I turned it up a couple of degrees, left it for a couple of days, and now it's hard on the 1.014 line whereas before it had been past it for sure. (even went back and checked pictures I took, it's definitely moved a little).

Never done a diacetyl rest before, so not clear on whether that's normal? I mean if there was diacetyl in it, then as I understand it the purpose of the rest is so the yeast can break it down into sugars and then eat them, so maybe a small gravity change in that time is OK? I know the easiest answer would be to just leave it a few days more and cover all the bases, but the timing is awkward for me, adding a couple of days at this point complicates life somewhat.

 

[dmtaylor]

Even after fermentation was complete or very nearly complete, some of the yeast is still hungry. They eat the diacetyl, AND they ate a little more sugar. Diacetyl rests are handy like this... yeast is happier at warmer temperatures, so if there's any food left to eat, they eat it. Then you'll KNOW fermentation is done-done when they stop eating again.

 

[MaxStout]

Bumping the temp is often used as a means to re-start a stuck fermentation. While yours wasn't stuck, it's analogous on a much smaller scale. There was a little sugar left, and when warmed the yeast woke up and ate it.

 

[Zadkiel]

The crux of my problem is this, I was hoping to bottle tonight, which would have been 3 days since I started what I thought was a diacetyl rest (I'm not cold crashing, it's a wheat beer so I'm not stressed about cloudiness, and I have no ability to carbonate with CO2 so want to keep the yeast active and use bottling sugar). if I can't bottle tonight I can /maybe/ do it tomorrow, but it probably means waiting a week due to IRL stuff. I can do that if needed but it will be annoying and mean it won't be ready to drink by an event I was hoping to take it to.

Is gravity dropping half a point normal for a rest period, or would it normally have stayed constant?

the first reply said it processed the diacetyl AND some extra sugar, while the second reply says it was just processing extra sugars, which implies it needs more time to get the diacetyl? I can see this isn't an easy question!

I might just go for it and bottle it. I've not had a diacetyl problem with my previous 3 brews, I'm just slowly adding in some extra steps each time as my knowledge increases (I've been following extract kit instructions, all of which say to wait until FG stable for 2 days, this was the first time I also added a temp bump). I guess if I do end up with popcorn wheat, then I answered my question

 

[dmtaylor]

If it only dropped half a point or one point in the past 3 or 4 days, it's pretty safe to say, it's ready to bottle tonight.

You're not going to get a popcorn wheat. There should be no diacetyl, and no real need for a diacetyl rest -- those come into play more with lagers and with IPAs or APAs. But with this beer, I'm 95% sure it's ready to bottle.

 

[Zadkiel]

Thanks! Didn't know that about the styles, cheers for the info, so diacetyl production is yeast-dependent?

 

[dmtaylor]

Thanks! Didn't know that about the styles, cheers for the info, so diacetyl production is yeast-dependent?

All yeast produces diacetyl. They also eat it. Slow yeast eats diacetyl slowly. So, cold fermentations tend to have diacetyl for a while until the yeast has had a chance to clean it all up. And, since hops contain enzymes that can slowly produce sugars for a while, sometimes dry hopped beers will produce more diacetyl during that slow fermentation, which the yeast then needs time to clean up again. But for most beers, where fermentation tends to be fast and furious, the diacetyl is immediately eaten up and isn't a concern. It's really only a concern when fermentation is very slow for some reason.

Bottom line: Well... it's complicated. But yeah. Most beer styles & yeasts aren't going to be a problem.

 

[Zadkiel]

Thanks! v.useful.

 

[ncbrewer]

(I've been following extract kit instructions, all of which say to wait until FG stable for 2 days, this was the first time I also added a temp bump)

I suggest doing the temp bump first, then take the initial FG reading after a suitable time. Makes life simpler. Temp bump can be after most (maybe 3/4) of attenuation has happened.

 

[505-Brewer]

1/2 point gravity could just be due to the warmer temp. You may have off gassed some CO2 during that time which would lead to a lower value. Or there may have been a little more fermentation. Im Guessing like most homebrewers you didnt degas the samples prior to testing. Not commonly done (or as necessary at the homebrew level). Diacetyl does not turn into sugar. Learn to do a forced diacetyl test and sensory analysis. It’s the only way to ‘know’. I would go ahead and bottle if you are using sugar and yeast. It can continue to clean up in the bottle if needed. There would be flavor benefits to letting it sit and clarify though even if you dont mind it being cloudy.
Cheers!

 

[Zadkiel]

I did de-gas the samples in my hydrometer, this was my first fermentation where the samples were gassy, and I just assumed the presence of CO2 would affect the reading - plus it was easier to read the hydrometer with the sample 'flat' and not having to deal with surface bubbles.

Thanks for clarifying about it not turning into a sugar, I had assumed (incorrectly!) that diacetyl was a complex sugar and the rest was to get the yeast to break it down into simple sugars, but I just looked that up and my assumption was completely wrong!

 

[VikeMan]

Just dropped in to say that yeast don't actually make diacetyl. Yeast make α-acetolactate (which is a precursor to diacetyl), which "leaks" into the beer. The α-acetolactate in the beer is then oxidized to diacetyl, which the yeast can then clean up. I realize this may sound like an unimportant distinction, but it can have practical ramifications. It's possible (and not terribly uncommon) to taste a "finished" beer, detect no diacetyl, and declare it ready to package. The problem is that the beer may still contain some α-acetolactate (which can't be tasted), which will subsequently oxidize to diacetyl. If the yeast is no longer very active (say the beer has been kegged and refrigerated), you can have diacetyl appearing in a beer and not being cleaned up. This phenomenon is the reason for some breweries doing forced diacetyl tests. It's also the reason for occasional posts saying "My beer tastes like diacetyl, but I'm sure it didn't before I kegged. What happened?"

 

[Zadkiel]

doesn't sounds unimportant at all - I'm loving learning all this stuff.

 

[dmtaylor]

Just dropped in to say that yeast don't actually make diacetyl. Yeast make α-acetolactate (which is a precursor to diacetyl), which "leaks" into the beer. The α-acetolactate in the beer is then oxidized to diacetyl, which the yeast can then clean up. I realize this may sound like an unimportant distinction, but it can have practical ramifications. It's possible (and not terribly uncommon) to taste a "finished" beer, detect no diacetyl, and declare it ready to package. The problem is that the beer may still contain some α-acetolactate (which can't be tasted), which will subsequently oxidize to diacetyl. If the yeast is no longer very active (say the beer has been kegged and refrigerated), you can have diacetyl appearing in a beer and not being cleaned up. This phenomenon is the reason for some breweries doing forced diacetyl tests. It's also the reason for occasional posts saying "My beer tastes like diacetyl, but I'm sure it didn't before I kegged. What happened?"

Very good point, thanks.