Not rice flour, just plain old brown rice.

I thought this would work based on my knowledge of biology, but mostly based on my extensive first-hand experience of the various means in which food spoils.

I think regular brown rice is a better substrate for fungus because it has a lot of gaps between the grains that the mycella can move through, whereas flour produces a goo that can be hard for the filaments to penetrate. Also, bacteria would have a tougher time colonizing rice because they can't move through the gaps like fungi can.