Answer :Lettuce does contain a small amount of iron; according to the USDAs nutrient database, for example, a cup of green leaf lettuce has 0.31 milligrams of iron. But thats not why lettuce turns brown. Rather, as with most fruits and vegetables, this browning is the result of oxidation catalyzed by a naturally occurring enzyme called polyphenol oxidase. When exposed to oxygen-as when lettuce is cut-this enzyme assists in the formation of yellow-colored compounds called quinones. These then further react with oxygen to eventually form melanins, the same type of compounds that give humans a tan.
Bagged lettuce prevents this browning by taking advantage of the carbon dioxide that lettuce naturally produces as it breathes. Transparent bags of ready-to-eat lettuce are specially designed to partly keep oxygen out while letting extra carbon dioxide escape. By maintaining the right ratio of oxygen to carbon dioxide, the oxidation that results in browning can be slowed and lettuce kept green.