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Can a lack of oxygen in wort generate an increase in fusel alcohols given that a fermentation is still healthy/vigorous?

Background for those who are interested

I have been struggling to determine the cause of strong fusel alcohol presence in my brews. My most recent beer that I brewed was done in my chest freezer at 17-18.5C ambient temp using Saf s-04 yeast. This is a 5% beer. Upon opening the pail after two weeks I noticed oil slicks on top and a medium fusel alcohol taste. I brewed 1.5 Gallons for a 2 gallon pail.

I don't shake or rock the wort before pitching. I just pour my wort at a high height into the pail. I then add a packet of dry yeast without creating a starter. This method has worked for me in the past, finishing off a vigorous fermentation in a week.

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Do you rehydrate your dry yeast? I get fewer unwanted fermentation byproducts (but not hot alcohol yet) when rehydrating compared to sprinkling dry yeast into the fermenter. Either way, Fermentis and Danstar recommend allowing the yeast to sit on top of the liquid for 15-30 minutes to prevent osmotic shock, then mixing it in. If you want to aerate but not rehydrate, then you should sprinkle first, wait 15-30 min., and then aerate, so you don't end up sprinkling the dry yeast onto a head of foam. –  Chino Brews Dec 19 '13 at 18:58
    
Also, some unsolicited advice from a fellow small-batch brewer here: if you are using a whole packet for a 1.5 gal batch, consider adjusting your pitching rate. I use the Mr Malty pitching rate calculator, which has a tab for dry yeast. There are other good pitching rate calculators. My understanding is that overpitching leads to decreased ester production in styles where esters are desirable, so it is not directly applicable to your problem here. –  Chino Brews Dec 19 '13 at 19:01
    
Thanks, that's helpful advice since I do tend to sprinkle the yeast right on top of the foam. –  fthinker Dec 20 '13 at 15:27

1 Answer 1

up vote 1 down vote accepted

Hot, fusel alcohol flavors generally correlate to stress in the yeast — insufficient population, not enough nutrients, or insufficient oxygen levels in your starting wort. If you're getting these off flavors for even a smaller beer at 1.050, in the very least do the following:

  • in your boil, add yeast nutrients in the last 15 minutes of the boil, and

  • before pitching the yeast in your cooled wort (between 18-21ºC for ales, around 7ºC for lagers) aggressively swirl/rock your carboy for a few minutes. You want to swirl it aggressively enough so that you see swaths of small bubbles churning and mixing with your wort. A representative from Wyeast has recommended shaking for about a minute.

I don't brew very often with dry yeast, but you can dry pitch, and for the most part do OK. However, if you're taking the above two steps already, then you should look into making starters. Starters ensure that you've got a healthy, viable population of yeast sufficient to consume the long-chain sugars in your wort. If you've got a healthy culture and population of yeast and produce an environment rich in oxygen and nutrients, then you're a lot less likely to get the fusel alcohol biproducts in your fermentation.

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(1) To clarify, I assume @michael_t means making starters with liquid yeast. Dry yeast is manufactured with optimal, internal energy and nutrient reserves, so making a starter defeats the purpose of using dry yeast. (2) On rehydration, it is clear that sprinkling into wort leads to lower viability, and different beer flavors, but taste preference seems split: link 1, link 2; (3) Anyway, the problem with a 1.5-gallon batch is not underpitching if you use 1/2 packet or more of dry yeast. –  Chino Brews Dec 19 '13 at 21:14
    
You can make a starter with dry yeast - but when there's sufficient cells then there's no need to. I brew 10 gallon batches, and when using a dry yeast, I'll step it up using a starter so the pitching rate is correct. What you have to be careful of with dry yeast is when rehydrating/proofing in warm water+optional sugar that you pitch to the main batch within 30-45 minutes or the yeast will start to starve. –  mdma Dec 20 '13 at 9:48
    
"...viable population of yeast sufficient to consume the long-chain sugars in the wort". Yeast cannot consume sugars with more than 2-3 glucose units (depending on species). Dextrins with 4 units or more are left alone. –  mdma Dec 20 '13 at 10:07
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Thanks. I think the take away message here is that my yeast are stressed and that I need to do what ever it takes to alleviate that, whether that's adding more oxygen or rehydrating the yeast. –  fthinker Dec 20 '13 at 15:26

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