Q: Is it safe to eat the edges of lettuce leaves that have turned brown?
A: Yes. It's not a health issue and does not indicate a loss of nutrients. Still, most people find brown lettuce unappealing and will want to cut off the discolored parts.
Browning usually results from oxidation caused by exposure to air when the leaves have been bruised or cut. Another cause is "tipburn," which occurs in the field because of climate or soil conditions. Exposure to the ripening chemical ethylene (produced naturally by most fruits, but also used commercially as a ripening agent) can produce browning as well.
Bacteria and fungi can also cause browning, but these do not pose a health hazard. Though lettuce and other produce can harbor harmful bacteria, of course, these do not cause browning.
Select lettuce that looks fresh (not wilted), since it will taste better and last longer. To reduce browning at home, keep lettuce in the coolest part of the refrigerator, away from high-ethylene fruits (such as apples, pears, peaches, tomatoes, and cantaloupe), and keep it dry.
Source: Originally published in The University of California, Berkeley Wellness Letter (May 2011)