Hi!

I would start by testing the addition of substrates at low concentrations, such as 0.2%, and 0.5%, possibly reaching 1%. I believe these values are within an average concentration of substrates in a solid medium.

I've done protease assays on plates with two different substrates on plates. One was gelatin and in order to see the proteolytic halo after growth of the colonies, you had to flood the plate with weak acid to make the gelatin turbid. Other protocols would flood with a dye like amido black or bromcresol blue. .

The other was using powdered milk as substrate (casein) which was itself turbid so no need for acid.

So in your situation if an appropriate concentration of either meal can give you a turbid plate, you can look for halos directly. Otherwise you can use dyes or acid to visualize halos.