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Measuring residual sugars after fermentation

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  • Measuring residual sugars after fermentation


    I'm trying to figure out why some batches as of late haven't reached the target attenuation when the yeast was able to do it in the past. Since the pitching rate hasn't changed and I have already employ a new pack of the same yeast, I want to confirm if the sugar profile of the wort has changed in favor of unfermentable dextrins.

    Is there a standard method to profile the sugars on the wort? From analytical chemistry I remember there where some old procedures to titrate sugars using copper sulfate (Fehling reagent), although if I remember correctly those merely quantify the number of extreme aldehyde groups of the saccharides, so there isn't a 1-1 relationship between the result and the sugar distribution in the wort.

  • #2
    If the yeast was able to achieve greater attenuation in the past, then there is a good chance that there is a problem with the wort. For future brews, you could try doing a forced wort test where you pitch a small amount of wort with about 10x the normal pitch rate of yeast. Your beer fermentation should reach the same final gravity as the forced wort test. If the forced wort is higher than normal, then this points to a problem with your mash and wort fermentability.

    The best method for detecting dextrins is HPLC, but the equipment is expensive so you would have to send a sample to a lab for analysis. I'm not aware of any low-tech dextrin quantification methods.

    You could try this though... re-pitch first with Nottingham. You didn't mention which strain you used, but Nottingham will consume all of the maltose and maltotriose leaving behind only dextrins. If you then pitch with a diastaticus yeast such as Belle Saison then this will start consuming the dextrins. It won't be terribly quantitative, but you will get an idea for dextrin quantity by the change in density due to dextrin metabolim.
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