I am brewing with fermentis S-04 and from experience (three years ago) I know that this yeast is slow.

This weekend I brewed a small batch and today the attenuation is 83% (according to the yeast's specification it should be 75% at most) only 4 days have passed.

I suspect it's contamination, could it be something else?

Edit: Its a 18 liter (4.7 gal) batch and the recipe is 5Kg (11 pounds) of 6-rows base malt and 1 Kg (2.2 pounds) of cane sugar

Note: I brew in México's Michoacan state, my temp ranges from 15 to 26

  • Is it an all grain batch or a kit?
    – Philippe
    Commented Sep 20, 2018 at 20:19
  • It is an all grain, 5Kg (about 11 pounds) of base malt and 1Kg of sugar (about 2.2 pounds) Commented Sep 21, 2018 at 17:02

3 Answers 3


I have never found S-04 to be slow. Even on 500L batches, I've had beers with S-04 ferment dry in under 5 days. It also flocs out like a ton of bricks. Are you confident you don't have a contamination with another, more attenuative yeast? Are you also confident you got your OG and FG readings right, and calculated the AA right? As jsolarski says, without a recipe and more details, it's hard to say what's going wrong for you.

  • I haven't brewed in almost 4 years now and all I had were my old notes, there I wrote that primary fermentation for S-04 was 12 days. Maybe I didn't aerated the wort enough back then and what I am observing in this batch is normal. Thank you for sharing! Commented Sep 21, 2018 at 17:07

smaller the batch, usually the faster the fermentation will happen. my 1.5 gal batches took less then 5 days to reach F.G.

also with out a recipe posted, a more ferment-able wort will attenuate more. the given attenuation on yeast are for lab conditions.

if it continues then yes, it could be contaminated, but if it stops, then its just the yeast doing its thing.

  • Why if it continues fermenting means it could contaminated?
    – rondonctba
    Commented Sep 20, 2018 at 20:28
  • "could be" is the the key phrase, i have had yeasts continue past the given attenuation, nearly 90% because of my wort. you will have to look at other factors when trying to determine contamination, like common off flavors and smells, like plastics, bandaid, medicinal, vomit...etc.
    – jsolarski
    Commented Sep 20, 2018 at 21:18
  • Oh, it must be the cane sugar then. Thank you very much I feel less worried now Commented Sep 21, 2018 at 17:09

The mash temperature can also have an effect on attenuation (fermentability).

I will quote Denny Conn, from this post: Yeast attenuation and fermentable sugars

Yeast attenuation is determined by the yeast lab under laboratory conditions, and it is only a way of comparing one yeast relative to another using the same wort. You may or may not achieve the attenuation that is listed for the yeast. It may be less, it may be greater than listed. That depends on the fermentability of your wort.

Also, the fermentation temperature can affect the speed of fermentation, the colder the slower. Many variables that can explain your concerns, so contamination is possible, but not necessarily it.

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