So why do the little sods get into my homebrew?
Fruit flies seem to be everywhere this year - in fact at my sons school they had a hunt for rotten fruit as there were so many in one of the classrooms. Just how bad are they for the home brewer?
Well, the scientific name for this little beast is Drosophila melanogaster, they are 2-3 mm long and carry the bacteria that turn alcohol in vinegar hence their other name, the vinegar fly.
Reading through some old homewbrew books it is suggested that in the blink of an eye just one fly will ruin a batch and the container in which it was fermented for ever.
Lets bust a few myths.
Fact: To turn alcohol into vinegar we need bacteria and oxygen as well as alcohol. So what happens if a dozen fruit fly get into your brew?
Before you have added the yeast there will be loads of bacteria and oxygen, but no alcohol, so no vinegar.
Alcohol will start to build up once fermentation is underway, but there is now no oxygen, so no vinegar.
After fermentation, racking and bottling we have bacteria, alcohol, and oxygen. Hurrah, at last. Hold on though, those campden tablets you added at the end produce SO2 which kills bacteria - so no vinegar.
In short, don’t panik. You have to work hard to make vinegar and will not do it using good brewing technique - even if a dozen fruit fly do get in. Just pick them out with a spoon and enjoy the extra body.