Down on diacetyl
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.
Craig Fitzpatrick
Tauranga, New Zealand

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 types of strains often drop out so effectively that a diacetyl rest is difficult because much of the yeast is on the bottom and contact between diacetyl in the beer and yeast cells is limited. I have had problems with diacetyl using flocculent strains in uni-tank fermenters and solved my problem by simply changing yeast strains, in particular selecting less flocculent yeast.   

To test this idea I suggest giving Wyeast 1056 a try. This yeast is popular among homebrewers and US craft brewers. A feature of this strain is that it is not very flocculent and if you like clear beer you will need to do more than simply wait for the beer to clarify. Finings or filtration is really required for a bright beer. Clarity aside, you should be able to produce a very clean and diacetyl-free beer using 1056. If you are unsuccessful with this strain, then you may be picking up your diacetyl from bacteria as you suggest.  

The sanitizer you are using contains peroxyacetic acid (PAA) and this is one of my favorite sanitizers. PAA, although sharp-smelling and wicked on skin and mucous membranes, is very beer friendly since it decomposes into water and vinegar. This decomposition occurs when PAA, a strong oxidizer, comes into contact with soils and also occurs over time with storage, especially if stored hot. It is important to recognize the fact that PAA, like other peroxide-based cleaning products, does not have an indefinite storage life.   

While the storage life of PAA does depend on how it is stored, I have seen data sheets that indicate a reasonable shelf life of 6–12 months at room temperature. If your 20-L (95.5-gal.) jug of PAA is five years old, and it is the only sanitizer you are using, you may be unknowingly omitting your sanitize step. PAA has a strong odor because of the acetic acid portion of the molecule and concentrated acetic acid is just as sharp as PAA.   

There are other diacetyl remedies that I did not mention because I don’t think they are the cause of your problems. Increasing the length and/or temperature of the diacetyl rest and minimizing air pick-up after the diacetyl rests are two things that brewers use to combat diacetyl. But I really think your problem is either due to yeast selection or the efficacy of your sanitizer. Good luck!