Certain recipes call for nonreactive bowls (or other cookware) because many foods—especially salty or acidic ones—react with untreated surfaces such as iron, copper, and aluminum. When the foods react, the metals dissolve, and the foods pick up a metallic taste, says Harold McGee, author of On Food and Cooking: The Science and Lore of the Kitchen. (The metals absorbed don’t reach dangerous levels, though according to the FDA copper “in sufficient quantities can cause nausea, vomiting and diarrhea.”)
The reaction can also affect a food’s color. “Red pigments in fruits and vegetables turn a greenish-blue color when they come in contact with those metals,” says McGee.
But in the case of whipping egg whites, using an unlined copper bowl is a good thing, McGee explains: “Copper interferes with the process of protein coagulation just enough to prevent curdling.”
In terms of bowls, it’s pretty hard to find anything but nonreactive versions these days; most are made of stainless steel, plastic, ceramic, or glass, none of which are reactive. For more information on cookware safety, check out these tips from Clemson University’s Home & Garden Information Center.