Cu is the chemical symbol for copper, an essential trace element used by enzymes involved in iron metabolism, energy production and connective tissue formation. Shellfish, nuts, seeds and whole grains are good dietary sources, while both deficiency and excess can harm health. In food processing, copper surfaces can catalyse the oxidation of fats and the destruction of ascorbic acid, causing off-flavours and nutrient loss, which is why stainless steel has largely replaced copper equipment and why migration limits protect acidic and canned foods.