Why Lettuce Browns

• Lettuce browning is caused by a reaction between the enzyme polyphenol oxidase and oxygen. Yellowing quinones formed by enzyme interactions with oxygen continue to interact, forming melanins, which turn tissue brown or black. Even the greenest lettuce shows an example of this oxidation process where the stem was cut at harvest. While some consumers blame browning on the use of metal knives, neither plastic nor ceramic blades prevent browning. Bruised or torn tissues react similarly. The process can be slowed by reducing exposure to oxygen. One reason that bagged salads stay fresh as long as they do is that carbon dioxide is a by-product of enzymatic oxidation. Torn lettuce edges release carbon dioxide into the sealed lettuce bag, reducing oxygen and oxidation, thereby slowing deterioration.

Ethylene Production

• Ethylene gas is a by-product of the ripening, and eventual decay, process for some fruits and vegetables. Only fruits and vegetables that continue to ripen after picking emit large quantities of ethylene. Apples and bananas are particularly well-known for the levels they produce while continuing to ripen after harvest. Placing them close to ethylene-sensitive fruits and vegetables can accelerate their ripening as well. Home gardeners, for example, often enhance the ethylene produced by ripening tomatoes by placing green tomatoes in a paper bag with a few riper ones or an apple.

Shared Vulnerability

• A few fruits and many vegetables, however, are highly sensitive to ethylene. They are usually harvested only when they have become ripe and are not expected to continue ripening after they are picked. Heavy exposure to ethylene gas accelerates their post-harvest state from ripe to rot. Along with broccoli, asparagus, potatoes and watermelon, cucumbers and lettuce are both ethylene-sensitive vegetables. Storing them close to ethylene producers, like apples, stone fruit, bananas or kiwis, can drastically shorten their durability, even in the refrigerator. Cucumbers and lettuce do not interact with each other, but it may seem so if they are stowed in the same vegetable drawer with heavy ethylene-emitters like apples.

Browning Prevention Strategies

• Use several strategies to keep produce fresh and durable. Wrap and store different produce varieties separately to diminish oxidation and ethylene vulnerability. Examine produce carefully before storing. Remove any browned or bruised areas before they can spread. Buy only what you plan to use within a few days keeps produce in prime condition. Maintain storage conditions similar to those in the grocery store: do not wash root vegetables, berries or stone fruits until you are ready to use them. Keep salad vegetables chilled and slightly moist but never wet. Excess moisture can increase bacterial activity and other deterioration processes.