some tiny suggestions:
. the protein should be a short peptide, or it can be easily denatured by cooking so that it can do no harm to human
. the protein had better have the ability to combine some essential enzymes of humans to make them invalid, or the protein will be very harmful to human's nerve system. I think only these two ways can prevent humans from eating those vegetations (and choose to starve to death, oh my god )
. artificial mutation will maybe do help with your 'evolution'. That is, the human beings on the plate are obliged to mutate their genes frequently (eat pills, shine the sun, whatever), and select those mutants to do the relevant tests. It will accelerate evolutionary procedure, i suppose.
i hope my suggestions will be helpful to you