It's basically, how cleanable is the surface. The surface should be a material that doesn't retain anything after being wiped and ideally there are no seams for

It's basically, how cleanable is the surface. The surface should be a material that doesn't retain anything after being wiped and ideally there are no seams for bacteria to fester in. This is why stainless is so popular. I think that is actually easy to do. Ceramics are food grade, and people have been making those at home for centuries. For espresso gadgets I think the biggest risk is the heating element.