I grabbed a beer from the fridge last night and it was a butter bomb. I know you are thinking why would I post about it, well this came out of a six-pack and was the last bottle which had been hanging out for a while. None of the other five bottles had diacetyl, but they had been consumed month ago. Typically in ale fermentations the diacetyl is scrubbed by the yeast during fermentation. With lager yeast the temp is typically raised at the end of fermentation to allow the magic removal of diacetyl to happen. So if the fermentation does not remove the diacetyl you may have it at a perceptible level and that level will not change appreciably over time. Wild yeast can cause diacetyl, but again the level really doesn’t change over time. Because the other five bottle consumed earlier had no diacetyl and the last bottle was a butter bomb the diacetyl was formed by bacterial contamination, probably pediococcus. So if you have a beer post fermentation which has no diacetyl, but the level of diacetyl increases over time then you have a bacterial contamination which is causing the change. Diacetyl doesn’t appear out of nowhere, it is either present due to not being removed during fermentation or it is being formed by bacteria. In most cases it is not desirable in a beer and all of us perceive diacetyl at different levels, some people cannot taste it at all, but it does provide a slickness on the palate which can be felt in the mouthfeel. At any rate, diacetyl is not your friend, avoid it if possible.