The Maillard reaction is an oxidative degradation and I don’t think changing to microwave heating would help. You could perhaps exclude oxygen from the steaming process, firstly by carrying it out in a controlled (i.e. enclosed) environment and secondly by adding at least 50% nitrogen gas to your steam supply. Alternatively, you could perhaps try to bleach out the discolouration using a bleach/rinse approach using hydrogen peroxide.

David Oulton Thank you sir for your kind suggestion.