Rice flour, and water.

[ The rice flour is the one he's not sure of, since he can't really tell the difference between the multitude of white powders that's found in any usual pantry, but it's easy enough to tell if it's the right one after adding some warm water. ]

I was thinking we'd make something like mochi. Fill it up with medicine and swallow it whole- it'll release in the stomach, and nobody will have to taste it.