I want to screen strains of lactic acid bacteria (LAB) for the production of exopolysaccharides (EPS) using Ruthenium red milk agar.

I already tried to make a stock solution of Ruthenium red (ThermoFisher) using 10 mL of distilled water to solubilize 1 g of Ruthenium red powder as indicated on the ThermoFisher website and I sterilized it by filtration on 0,45 um filter, but I encontered issues with solubility (the solution was very dark at first, but I noticed a lot of small particles after filtration). Therefore, my stock solution was not concentrated enough after filtration and my agar plates were barely pink after adding 0,008% (0,08 g/L) of Ruthenium red.

I now ordered the Ruthenium red Technical grade from Sigma, which I heard is easier to solubilize, but I would like some advise on exactly how to make a stock solution with this product since it is pretty expensive. Did you let it solubilize overnight? Did you heat up the solution to solubilize it? Did you use filtration to sterilize it? Did you store it at -20C or 4C?

Any advise would be greatly appreciated.

Thanks in advance!!