Many thanks to Laurence Mueller for suggesting adding
propionic acid to my cultures. I was already adding
cream of tartar, so I had an acid, but propionic acid
has a special property of interfering with the metabolism
of molds and bacteria. Straight propionic acid isn't
really suitable for my formulation, but reading on
Wikipedia I found that calcium propionate is acceptable.
It's readily available to amateurs because it's a
widely used food additive in baked goods. According to
Wikipedia, the calcium also helps the yeast amylase
break down food. Not mentioned in Wikipedia is that it
acts as a firming agent. I'd been having some problems
with my cultures becoming too liquid, but that's no
longer a problem.
My yield has greatly increased. I'm harvesting maybe
1.5X to 2X as many flies as before. I still have some
bad batches and bad cultures within a batch, but that
problem is greatly reduced. I suspect using bananas
that are not fully ripe enough is one of the remaining
causes, and hot weather caused many of the problems
I had over the summer.
An odd phenomenon is that with calcium propionate the
flies seem to have trouble climbing up the sidewalls.
Before using the c.p., when I disturbed a culture,
nearly all the adults would climb to the top. Now,
a minority climb to the top, and the majority remain
at the bottom. They seem to be agitated and attempting
to climb, but for some reason they can't do it. Maybe
they are less athletic or maybe the sidewalls are more
slippery. This doesn't seem to hurt yield -- I notice
the phenomenon in my best cultures. I am curious what