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Cause of my diacetyl

TroubleShooting

Craig Fitzpatrick • Tauranga, New Zealand asks,
Q

I seem to have a common issue with excessive diacetyl. I am quite wary of certain yeasts being more prevalent to this issue and the need for a temperature rest post primary fermentation. At first I thought it was a Wyeast 1968 issue, however I have since had issues with the Wyeast strains 1084, 1318 & 1272.

I am brewing all-grain beers using a stainless set-up with good fermentation temperature control to the nearest one degree. I typically ferment my ales between 18 and 20 °C (64 and 68 °F) with a rest of at least 2+ days. The beers usually spend a total of 2 weeks in the fermenter before I keg with yeast drawn off sometimes early and sometimes late. Note: I aerate the wort using pure O2 for 30–60 seconds for a 35-L (9.2 gal.) batch and use a cone-bottom fermenter.

At the time of kegging, I always taste the green beer and cannot detect noticeable amounts of diacetyl. I then put the kegs in a smallish under-bench keg chiller and pressurize to ~15 psi at approximately 4-6 °C (39-43 °F). I then wait a couple of weeks. The beers all seem to develop diacetyl over this period. At first I thought it could be the forced carbonation that was causing it so I tried kegging early to naturally pressurize. This however didn’t seem to make a difference.

Now I’m wondering if it is the chiller, which constantly vibrates — considerably more than your average fridge. Could the yeast be flocculating prematurely and stressing or is it potentially something else, for example, bacteria?

I am on non-chlorinated country water and use Proxitane as my non-rinse sanitizer. I am fairly confident it shouldn’t be bacterial, however I have had the 20-L (5.3-gal.) drum of Proxitane for about five years now. Does it have a shelf life in its concentrated pre-mixed form? I am basically looking for any advice regarding potential cause of my consistent diacetyl issue, particularly how it seems to develop once in the keg.

A
  At first glance I suspect that the yeast strains you like may be the culprit. Most of the strains are described as highly flocculent on the Wyeast Web site and these
Response by Ashton Lewis.