I’m looking for a stainless plate chiller, and I see ones that are soldered with copper. Does this copper touch the wort, or just stainless?
The copper in a plate chiller contacts the wort. I serves as the boundary between cooling water and wort,
While wort can contact copper in a stainless plate chiller, the copper surface area is pretty small. The copper only exists at the edges of the stainless plates where the stack of plates are soldered together.
Those copper soldered plate chillers are about as good as you can get without going with an all-stainless plate chiller that is held together in a screw press with gaskets between each plate.