I am experimenting with a banana based fly food for better larval synchronization. Instead of propionic acid, I am going to use (added) citric acid as fungicide. Is there any downside to this approach?
I have no data on the fungicidal and insecticidal properties of citric acid at various concentrations (and you may be hard pressed to find a good study on this), but I have worked in some fly labs and I understand the composition of fly food only peripherally. The most commonly and most abundantly used fungicide in a lot of fly labs is methylparaben (trade names Tegosept or Nipagin in Europe). I imagine food recipe-makers have arrived at this solution through trial and error over the years; this is as much an art as it is a science, in balancing the ingredients with humidity, temperature, species and genotype with desired generation time. Methylparaben is used with and without propionic acid for Drosophila melanogaster. Do you have methylparaben in your fly food? Also, I think citric acid is not commonly used, again I imagine because technicians probably found it inferior to its alternatives.
The National Drosophila Species Stock Center at Cornell uses methylparaben with its banana food recipe.
Alternatively, please take a look at the variety of recipes Bloomington Drosophila Stock Center uses. To my knowledge they supply and stock the largest collection using the most recipes.
There is still some dispute over the exact cause of the antimicrobial effects seen with organic acid. One of the theories is that the protonized acid crosses the cell-membrane and destroys the pH-balance inside the cell. This would explain why these acids are more effective at low pH (a higher fraction will be protonized). Citric acid is a significantly stronger acid (pK = 3.1 for the most acid proton) than propionic acid (pK = 4.87), meaning that the pH of the food has to be around 2 units lower to obtain the same fraction of uncharged molecules.