Why does lettuce turn red
Before going into the details of this answer, one must note that there certain lettuce varieties which are originally red in color instead of being of the common green variety that we are accustomed to seeing. Even the green lettuce may become reddish or pinkish if it is kept out in the open air for long or if it is cut with a knife made out of metal. The change in color is due to a chemical reaction which is quite similar to rusting. As the enzymes inside the lettuce leaves come in contact with the open air and begin to break down through oxidization, the change in color is observed, however, it is not harmful for the body to consume these leaves simply because of this, but it may just taste a tad bit bitterer. One should keep in mind that breaking up of the enzymes is also a sign that it is time to finish the lettuce leaves quickly because waiting any longer may result in complete oxidization which would make the lettuce leaves inedible.
Although it is not harmful, you may want to avoid the change of color as much as possible and to do that, the first step is to cut them using non-metal kitchen knives or stainless steel kitchen knives. A combination of vinegar and water also works well against the oxidation process. Also, before storing your lettuce in a refrigerator, make sure that they are covered properly, preferable within airtight containers or plastics.
Even while refrigerating lettuce leaves, one should be careful with the storage unit’s temperature as keeping them in extremely cold temperatures can also result in the leaves turning red. The rules of refrigerating lettuce does not end here, there are in fact other rules that are to be kept in mind as well. Before even putting the lettuce in the fridge, wash it first with cold water and then rinse it later with a salad spinner or a paper towel. Fruits like apples and bananas produce ethylene, so they are to be stored away from the lettuce leaves. Finally, if you find portions that are already reddish or brownish, then just cut them off with a non-metal knife so that the rusting does not spread.