When lettuce leaves are ripped, some cells are destroyed, causing the browning response to begin. Even if there is no ripping, even if we have a complete head of lettuce, browning will occur. This is due to the fact that aging of the cells causes damage and mixing of the polyphenols with the enzyme. As they mix, phenol oxidase becomes activated which produces reactive oxygen species (ROS) that cause further damage to the cells. The best way to prevent lettuce from turning brown is not to cut it at all. That's right - don't cut it up before you eat it. Leaving the whole head intact allows all the nutrients to be absorbed by the body while preventing any oxidation from occurring. Cutting it up requires more effort than just eating it whole! However, if you must cut it up, try to do so as quickly as possible after buying it. The faster it is exposed to air, the more likely it is to turn brown. If you can wait until you get home to chop it up, then this problem doesn't arise. Finally, if you are cutting up a head of lettuce and it has already turned brown, that information is useful! Knowing that it has begun to oxidize makes it easier to avoid any further color change during storage.