Can you help settle a bet (regarding diacetyl in cider)?

[AW]

Hello All, 

Consider this case study - A small batch of cider was made from pasteurized commercial concentrate.  Sanitation of equipment and vessels was rigorous.  WLP 775 was pitched along with a healthy dose of yeast nutrient.  The juice was not sulfited.  Fermentation proceeded normally, temp controlled at the cool end of recommended range.  At sg 1.009 the cider was cold crashed in a refrigerator and immediately tasted with no aging.  

The flavor of the resulting cider is buttery, presumably due to presence of diacetyl.  Subsequently, a fresh pitch of yeast was added, and the diacetyl flavor cleared after re-fermentation had proceeded somewhat.  

Question 1:  Is it possible diacetyl is a consequence of yeast metabolic activity?  Is it possibly a result of bacterial activity?  Are both possible?

Question 2:  Does the preponderance of the evidence suggest that the diacetyl precursors (acetolactate) are due to yeast activity, or bacterial activity?

Question 3:  Are LAB a likely influence in the flavor of this cider at all given the process?  

Question 4:  What process and must/wort composition conditions might result in the general observation that diacetyl is more common in beer than wine?  Or is that even an accurate observation?

  

[Nathan Shackelford]

I've noticed diacetyl in my ciders, but it was almost always with unsulfited ciders. I have read that it is more likely produced by LAB and during MLF. Sometimes I detect diacetyl in a cider that was "finished" but stored cold for a period of time. I'm not a fan of this flavor component in cider and am always put off by it, while others don't seem as sensitive. Fortunately, it is often a passing flavor that ages out... but it usually signifies that my cider is continuing to evolve in new directions which aren't always positive.
Diacetyl is spoken of in beer as a low temperature fermentation component that can be controlled by making sure you allow the yeast to metabolize it. With lager brewing there is something called a "diacetyl rest", in which increased temperature at the end of the ferment is used to encourage yeast to metabolize the diacetyl. I have never heard of such a thing in cidermaking. However, you mentioned a new yeast being pitched which helped to clear the flavor. I wonder if "cold crashing" the cider may have equated to a low temp stressor to the yeast, which may have produced diacetyl. I think cold crashing creates a new set of problems to solve.
#1 Yes, and yes.
#2 Not sure, but I'd bet more on bacteria in this case, just based on my own experiences with LAB/MLF.
#3 Maybe, but cold crashing might create new flavors that wouldn't exist in "normal" fermentations.
#4 Pasturized commercial concentrate is an unknown entity here that I can't comment on. Perhaps commercial cider makers can comment on that. In the brewing world there are specific processes that are known to limit diacetyl in the finished beer. In cider I've only seen it mentioned in relation to LAB/MLF, which probably shouldn't be happening in pasturized juice.
I'm a home cider maker, so my knowledge and experience is limited to that realm.