Cause of my diacetyl

TroubleShooting

Craig Fitzpatrick • Tauranga, New Zealand asks,

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.