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Thread: diacetyl rest and off flavors

  1. #1
    Join Date
    Jun 2018
    Galesburg, IL

    diacetyl rest and off flavors

    I just finished my second lager. And everything I read on the advice level is all over the page.

    Short story: I brewed a bock about 6 weeks ago. Two weeks into fermentation, the beer tasted amazing. Amazing! Its time for diacetyl rest, let it free rise to the low 60s, but it never made it to 65-68. The buttery diacetyl-flavor slowly faded but was replaced by an odd medicinal flavor. I panicked, and cold crashed immediately, but I never got rid of the flavor. The only thing I could figure was that maybe it was because I didnt use RO water, and the chlorine in our water is high

    Two weeks ago to the day, I brewed my Mexican Lager and used RO Water. Same thing. Primary fermentation for 8 days, then slowly raised to 65 degrees which it hit yesterday (3/23), and I dumped trub, and then began diacetyl. Tasted it Thursday, strong buttery flavor, but it was good. Tasted it yesterday (3/22) and the buttery flavor is slowly being replaced with the more medicinal flavor. Then today, (March 23rd) airlock activity has picked back up. Rather than panicking and immediately cold crashing, I'm just letting it go, Im wondering if this off flavor is not medicinal, but part of the buttery diacetyl leaving and at this stage (only 36 hours in), this happens to be what the taste is. Ive read many posts that say, diacetyl could take up to a week. Suggestions?

    At this point, Im tempted (and scared) to leave it at diacetyl rest temps, hoping this off flavor will leave.

    Both were fermented on Saflager W-34/70

  2. #2
    Join Date
    Oct 2013
    Diacetyl is created by spontaneous oxidation of a-acetolactate (AAL) which is secreted from the cell during the lag and exponential phases after yeast inoculation. The AAL is produced as a precursor to valine and extra will exit the yeast cell and decarboxylate in the beer to form Diacetyl. During the stationary (maturation) phase, yeast cells will allow diacetyl back in, where it is acted upon by diacetyl reductase (and other enzymes) that will convert it to Acetoin and eventually 2,3-butanediol. The flavor threshold of 2,3-butanediol is something like 4500ppm, so its essentially tasteless at the amount we would see.

    The reason a temp raise is suggested is because the yeast metabolism will increase with temperature. This means faster reduction at higher temperatures. You can slow Lager at lower temps for longer times and not raise temps at all if you like. I think there was a study from Vienna on Lagering methods and diacetyl production/reduction somewhere. The key is that it seems yeast wants to uptake diacetyl with other sugars, so rest should be performed while some fermentable sugar is still available. 0.5-1*P above target final is a good rule of thumb. The yeast must also be in contact with the diacetyl (beer) to react, so High flocculation strains can be prone to leftover diacetyl. You dont need to worry about the higher temperatures after primary fermentation is complete. You can rest a week to be safe, but it really shouldnt take more than 48hrs.

    Based on your description I think perhaps the diacetyl is masking another issue, and as the diacetyl is reduced the other flavor issue is being unveiled. Diacetyl would not change into a medicinal flavor. Medicinal is more closely aligned to phenols and would not change much after primary fermentation. You want to double check your carbon filter and your flow rate to ensure you are removing all chlorine/chloramine. Start bench testing for diacetyl on all your batches to become more familiar. Ales too. Check them in mid fermentation too so you know how the changes take place.

  3. #3
    Join Date
    Oct 2002
    Chesterfield, UK
    I've not used this yeast, but Fermentis claim this is Weihenstefan yeast, and so typically these are fermented colder than you are fermenting at. Try fermenting colder, with the diacetyl rest temperature no higher than 15 C (59F). Some lager yeasts I have worked with work well at up to 18 or 19 C, but others I have used produce horrible overpowering sulphury notes, so presumably this yeast may produce other flavours I have not experienced, perhaps including the ones you are finding.

    Copied from the Fermentis data sheet:-

    FERMENTATION: ideally 12-15C (53.6-59F)
    PITCHING: 80 to 120 g/hl for fermentation at 12C 15C (53.6-59F).
    increase pitching for fermentation lower than 12C (53F), up to 200 to 300 g/hl at 9C (48F)

    Personally I would go for a colder, slower fermentation with slow cold diacetyl rest rather than a "high" temperature fermentation - but of course, I don't know exactly what flavour profile you are looking for.

  4. #4
    Join Date
    Dec 2010
    Nevada City, CA
    This is our house lager yeast. I pitch it ~ 51 and let it go in the low 50's until it gets to about 4p. I then let it free rise to ~60 and once it is ~that and sits at terminal for ~3 days, I slowly lower it 3 deg/day until 31. It sits there for another 3 weeks before transfer. I have never detected diacetyl form this yeast doing it this way.
    Dave Cowie
    Three Forks Bakery & Brewing Company
    Nevada City, CA

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