Diacetyl

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bobk34

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What is it? What causes it? I made my first batch and there is a little apple taste to my beer the first couple were good but few in the middle weren't had a couple after week 3 than week 5 worried about next go around? Advice plz
 
Diacetyl is usually a buttery flavour/slick mouthfeel. Green Apple flavours are usually associated with Acetaldehyde.
 
Diacetyl is a yeast by-product generated during fermentation, and is usually reabsorbed during secondary fermentation. The compound is identified as an off-flavor in beer, but is sometimes appropriate in small quantities depending on the style. Diacetyl is identified as having a butterscotch/buttery popcorn flavor. Most beers contain some amount of diacetyl, but at a level below the threshold for detection. A beer may have an elevated level of diacetyl if the yeast separate out of the wort too early. This can be caused by cooling of the fermentation vessel at an early stage of fermentation, by racking to the secondary too early, or other (mostly uncommon) reasons.

You describe your beer as having an apple taste. Would you say it has a green apple taste? If so, you may be picking up on acetaldehyde. This is also a by-product of fermentation, which is later converted to ethanol. Acetaldehyde can be overly present in beer if the yeast are not sufficiently active, or if there is a presence of bacterial infection.

There may be other factors to consider as well. What style did you brew? What yeast did you use? How long, and at what temp was your primary fermentation? What was your OG and FG?
 
I brewed a pumpkin spice as I didn't have a hydrometer when starting however took one at end it held at1.021 for four days. I fermented at about 67 with saf04
 
Diacetyl is mostly an issue with lager yeasts and certain strains (mostly English ones like 1968ESB/WLP002 and Ringwood) of ale yeast. Above detectable amounts, it begins producing what some simply call "butter beer".

During ferment, yeast can produce varying amounts of a compound called alpha acetolactate. It doesn't have the butter flavor (it's more of a slick mouthfeel), but it's a precursor to and converts to diacetyl at higher temperatures. Yeast can't eat acetolactate. They can, however, convert diacetyl to some compounds that have a much higher detection threshold (they're present, but you won't taste them).

For lagers, when they get 80-85% of the way towards FG, you simply raise the temp from around 50*F up to about 62*F or so. This causes the alpha acetolactate to go ahead and convert to diacetyl which the yeast can take care of for you.

For ales, you can simply pitch/start the ferment on the lower end of the optimal range, hold it there until activity begins to slow and then begin ramping up the temp so that it finishes at the upper end of optimal.
 
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