Ricotta is an Italian whey cheese made from sheep (or cow, goat, or Italian water buffalo) milk whey left over from the production of cheese. Ricotta (literally meaning "recooked") protein can be harvested if the whey is first allowed to become more acidic by additional fermentation (by letting it sit for 12–24 hours at room temperature). Then the acidified whey is heated to near boiling. The combination of low pH and high temperature denatures the protein and causes it to precipitate, forming a fine curd. Once cooled, the curd is separated by passing through a fine cloth. Ricotta curds are creamy white in appearance, slightly sweet in taste, and contain around 13% fat. In this form, it is somewhat similar in texture to some cottage cheese variants, though considerably lighter. It is highly perishable. However, ricotta also comes in aged varieties which are preservable for much longer.