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Malt Diacetyl production

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  • Malt Diacetyl production

    During the brewing process alpha acetolactate (AAL) a precursor to Diacetyl is formed. AAL is orderless and tastless and yeast can't break this down until it converts into diacetyl, thus the reason for the Diacetyl rest. (Simple) I get it.

    Has anyone noticed more or less Diacetyl produced in different malts by different maltsers? We saw this with Weyermann Wheat. We changed to Briess wheat and the Diacetyl reduced significantly.

    For all those who know a lot more than me, during the malting process I assume the malters use malting techniques to reduce the amount of AAL that is produced thru the different amalayze processes thus reducing the Diacetyl created during fermentation? We have been testing some new malt and get large amounts Diacetyl and are trying to figure out if it is the malt or coming from somewhere else like our yeast not effectively working to consume the Diacetyl or being overwhelmed by too much to absorb. We don't think it is our yeast as we don't get it in the beer not using this particular malt.

    What techniques to the maltsers use if any to reduce AAL and other problems we see during the brewing process? Trying to understand more to figure out where the problems lies with our process or withing the raw materials we use.
    Shades of Pale
    Park City UT

  • #2
    An intriguing query. I have never heard of diacetyl being somewhat malt quality dependent, but there is always a first for everything.

    There is no alpha acetolactate in malt, or if there is, it is not measured, being a by-product of the yeast metabolic pathway, and never mentioned (at least to my knowledge) as part of the malt germination metabolic pathway. To quote from something I lifted from the internet, to save me researching my old notes:-

    Alpha-acetolactate is produced as part of the valine anabolic pathway (the reaction pathway yeast use to create the amino acid valine), so it’s clear that valine has a very important role in diacetyl formation by influencing the yeast’s ability to produce or reduce diacetyl levels. This influence is demonstrated by the fact that the enzyme which produces alpha-acetolactate is directly inhibited by the presence of valine, so when the yeast’s internal valine levels are high enough this enzyme is inhibited and total diacetyl production is reduced. Since valine is an amino acid (one of about 20 protein building-blocks), then it stands to reason that protein levels in wort will exert an influence on diacetyl formation during fermentation, and this has been shown to be the case. Worts which are deficient in free amino nitrogen (FAN; important nitrogen/protein nutrient sources for yeast which are derived from the malt) are therefor also deficient in valine, and it’s been demonstrated that low levels of FAN in worts can lead to elevated diacetyl levels in the final beer.

    So this would suggest that it is something to do with the protein levels in the wheat used by the two companies, or probably the degree to which the protein is broken down in the malting process carried out by Briess and Weyermann.

    Simply changing the malt suggests it is not due to infection, unless you also happened to change the yeast culture at exactly the same time.

    If you want to continue using Weyermann malt, then it seems you should give the diacetyl rest a little longer, or a slightly higher temperature, or a combination of the two giving the yeast more activity to reduce the diacetyl to less flavoursome acetoin.


    • #3
      What Dick said was dead on, the amount and makeup of FAN from the malt will influence the AAL and Diacetyl produced by the yeast. As he said, you can use different malts to adjust this, or use a step mash process with a protein rest to increase FAN. You can also add yeast nutrient into the boil to increase FAN and zinc. Just don't go overboard, too high of FAN can be a bad thing.