Summertime & Fruit Flies

There isn’t much brewing going on right now because I’d like to avoid moving a carboy full of fermenting beer into a new apartment, but last month I brewed through two very quick batches which has meant plenty of bottling has been happening. Around the time I was bottling Colleen’s brew, we had an outbreak of fruit flies in the apartment. They happily buzzed about my fermenter, safely kept at bay by the airlock. However, I couldn’t be so sure that they wouldn’t venture a dive into the defenseless bottling bucket.

My friend Cato recently had his shot at a saison spoiled by these bugs. Fruit flies are a common vector for acetobacter infections in beer. As the name suggests, acetobacter produces acetic acid, and unfortunately for any beer around the bacteria does it by metabolizing ethanol. That means a delicious batch of beer may just mutate into malt vinegar. Like lactobacillus and lactic acid, this is desirable in a few styles (Flanders Red comes to mind), but is largely avoided in most brewing.


It’s not uncommon to see a fly take its time contemplating this deadly dip.

Fortunately Colleen has had plenty of experience dealing with these creatures from her time on the hot and humid Jersey shoreline. Not only do these flies commonly turn alcohol into vinegar, they seem to be attracted to vinegar itself, perhaps because it smells like a food source that other flies may have already visited. Setting out a small dish of apple cider vinegar with a drop of detergent added to disrupt the surface tension of the fluid has proved an effective trap, netting a few fruit flies a day. Not bad given that our visible population seems to be only about a dozen, though it is very hard to estimate. The trap certainly culls the numbers such that a place previously abuzz with three or so constantly in the air will only host one that infrequently appears, which made me much more comfortable with my bottling operations.

UPDATE: Another pro-tip I have read is if you have produce discards from cooking, don’t put them in a bin that you will not be emptying immediately. Instead, put them in a plastic bag and pop it into the freezer. You can continue to discard into this bag until you are ready to take out the trash.


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