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Diacetyl from an infection?

On my last two batches (10g) I am seeing a weird issue.

I ferment in 2 seperate 5g fermenters. 10g wort split into the 2 fermenters. Same yeast pitch. Same time and temp in primary. Same time and temp in secondary. Primary in plastic buckets, secondary in glass.

Both batches were kegged. In both cases, one keg tastes and smells right on and one has what I perceive as diacetyl in the aroma and flavor. I have ruled out it being in the keg as a different keg had the issue each time.

On one batch, I did notice the issue out of secondary at the time of kegging. Is it possible for an infection in one of my fermenters to cause diacetyl? Or am I mistaking some other off flavor/aroma as diacetyl?

Any help would be appreciated. I am hoping a good clean and sanitizing will take care of this.

It can be.

If you are leaving the beer on the healthy yeast in primary for a least 4-7 days after primary ends then the yeast should have cleaned up any normal VDK and diacetyl that is produced in normal course. Some strains produce more than others and post ferment activity is differed also. If not infection and you used the same strain for both ferments then it could have either been that the one ferment was stressed due to lack of pitch count being high enough, Lack of initial O2, lack of other vitamins or glycogen/sterol reserves if this was a repitch etc…causing excess initial high levels of VDK/diacetyl or not cleaning up due to the nutrition/stress factors also.

Here is two quotes from Michael Lewis and Charles Bamforth/ Essays in brewing science(2006)
“Yeast can mop up the diacetyl, if it is healthy and remains in contact with the beer.”
“Persistently high levels of diacetyl may indicate an infection by Pediococcus or Lactobacillus.”

Does it smell like butter…butterscotch…or popcorn…then yes it is diacetyl. If not, then it could be the yeast health (as noted earlier) or another wild organism.

I could be a lactobacillus infection in one of your fermenters…but if it was in there, any plastic items down stream of your fermenter most likely are infected as well and evidence would turn up in any finished product that came in contact with those items(other beer kegged using the same). Lacto is everywhere and is the most common brewery infection…the wild stuff (not the strains produced for brewing) can produce quite a bit of diacetyl and low levels of lactic acid. Pedio usually takes longer to grow and, given time, will produce ropy strands and a pectile…acetobacter produces a whitish pectile-like layer on top and gives flavors and aroma of vinegar. Brett can usually be remedied with good cleaning and sanitizing practices as the cells are larger and cant hide in pores and scratches as easily.

if it continues, you might consider replacing your plastic beer items…you’ll find it very difficult to remove a bacterial infection from plastic.

My guess is lacto or unhealthy ferment.

hope that helps.

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