This is likely because of cations within the broth media that induce the formation of crystals. So what I would suggest is to test different compositions of media with different concentrations of cations within the media and different concentrations of your acid without microbes to understand what the best/optimal media is for your experiments (i.e. to prevent crystals forming).

Thank you for your valued suggestion Dr Pieter Bots

But can you help me considering the broth composition I used below what is causing the crystal formation

peptone

yeast extract

Ammonium sulfate

and phenyl valeric acid + glucose

I am not familiar with most of those from a geochemical speciation perspective, so I wouldn't be able to help with the details on that. But I would suggest to vary all of them. Although, based on my gut instinct, if you would want to narrow this down, you could start with varying ammonium sulphate, or exchanging it for another suitable cation sulphate. Next I would vary phenyl valeric acid, because from what you were saying is that the addition of this salt induces the formation of crystals.

Hello Salha,

many thanks for sharing this very interesting technical question with the RG community. As a synthetic inorganic chemist I'm absolutely no specialist in your field of research. However, I fully agree with Pieter Bots in that certain cations or bases present in your broth can form crystalline salts with your phenylvaleric acid. Thus it would be interesting to know the full composition of the broth.

Good luck with your experiments!

You can try to replace it with the other 10 isomers C11H14O2