[For the last three months, I've been busier than a long-tailed cat in a room full of rocking chairs so please forgive my recent lack of contribution here.]

Diacetyl, that butterscotchy/toffee flavour. It's pretty much always mentioned as a fault (correct me please if I'm incorrect on that) however, I often really like that flavour.

A few batches ago, on one brew day, I brewed an APA and IPA, that, for reasons I still can't work out, were really heavy on diacetyl. Besides not being able to fathom what caused that diacetyl in those beers (I have very good temp control), the other amazing thing was that some non-brewers thought it was the best beer they have ever tasted and want to know when I will be brewing it again.

I get that. What's not to like about butterscotch or toffee?

So, my question is, "Why does diacetyl have such a bad wrap?"

Welcome back PP!
We missed you.

Just to get the conversation started:

Diacetyl = two acetyl groups (duh!) and in more technical nomenclature, it is 2,3-butanedione. That's two ketone carbonyls side-by-side in the middle of a chain of four carbons. Therefore it must be an oxidation product and gets a bad rap simply because oxidation is supposed to be bad. Even when buttery/toffee flavors might be good, oxidation is generally considered bad and it is guilt by association.
There are beers where the flavor would be awful and maybe those beers are more susceptible to revealing this oxidation in the worst way. Extrapolating that a butter flavored pilsner is tainted must have had an effect on the idea that it is a fault in more (all) styles.

Whatever sequence of events PP had in his brews resulted in a good off-flavor. There are other undesirable oxidation reactions that did not screw things up, and they didn't overpower the good this time. Maybe next time they will.

Afterthought - maybe its bad rep is from people who did not like the Harry Potter stories where Hogwarts students drank butter beer?

This comes from a member on the low oxygen website, which I am too.

quote=stpug post_id=6616 time=1509030173 user_id=69]
Fix wrote about the potential for a combination of marginal yeast coupled with mechanical abuse during packaging, coupled with oxygen in headspace, to lead to spontaneous diacetyl production in packaged beer:

"[]... a widely observed but little discussed phenomenon occurs when diacetyl appears spontaneously in a beer that seemed to have normal flavors. Strong evidence indicates that this can occur when marginally dysfunctional yeast have been used in the main fermentation -- they tend not to metabolize all the acetolactic acid in the wort. The acetolactic acid spills over into the finished beer and later is oxidized to diacetyl. Mechanical abuse of packaged beer can promote this; headspace air is the oxidizing agent. Elevated temperatures augment the effect. I have seen cases in which wort constituents (melanoidins and tannins), oxidized on the hot side in wort production, were passed on to the final beer, only to play the role of oxidizer there."

Source: http://www.morebeer.com/brewingtechniqu ... 2/fix.html

Since you already feel that your packaging needs some attention, then I would give it the attention you think it needs, and ensure you're always using plenty of healthy yeast.
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Many thanks for the welcome back SP - much appreciated

And thank you for yet another of your high quality posts. That goes for you too MS Those sort of posts I read and re-read.

Of course, I did not take enough notes when brewing those beers so how on earth I produced so much diacetyl is beyond me. MS quoted "abuse during packaging" but, honestly, it was a pretty smooth transfer from fermenter to keg. I didn't swear once!

I think though that there may be an untapped beer market for this flavour and probably a few others that are regarded as faults

That's the idea, turn an Oops into a Eureka! Can you do it again?
Home brewing presents lots of opportunities for surprises but they certainly aren't all going to be desirable outcomes. You have hit upon a combination of conditions that gives a good beer - due to some oxidation with or without strained yeast or abuse in packaging. It can be a trap in that you could try to reproduce it and never get it to be the same. If your recipe was set up to make good beer and you like the results, you can repeat what you did and still get something different. Embrace the difference and appreciate home brewing for providing the possibility of making a unique beer or making the same thing over and over again.
Did you know that there is a Journal of Irreproducible Results? http://www.jir.com/ Perhaps we need a Journal of Irreproducible Beers?

I'd just call it a secret recipe and a limited edition brew.

No, I'll never reproduce that one. I can't even remember if it was the APA or IPA that had it. I think it actually increased over time too! Very poor record-keeping by me