Pinking of lettuce

The mid-ribs of whole and processed (i.e., fresh-cut) lettuce can develop a pink discoloration that reduces consumer acceptance. Both biotic and abiotic stresses stimulate the phenylpropanoid pathway in lettuce to produce a number of o-diphenol compounds. Caffeic acid, an intermediate in this pathway, usually reacts with quinic or tartaric acid to form the o-diphenols chlorogenic and isochlorogenic acids, or caffeoltartaric and di-caffeoltartaric acids, respectively. The concentration of these higher molecular weight o-diphenols is usually low in unstressed lettuce, but they can accumulates to levels that can produce tissue discoloration after the tissue has been stressed (e.g., wounded). The constitutive enzyme polyphenol oxidase (PPO) converts o-diphenols into reactive o-quinones. Diversion of caffeic acid from the phenylpropanoid pathway by reaction with PPO produces caffeic acid o-quinone that is pinkish in color, while PPO activity on the accumulated higher molecular weight o-diphenols (e.g., chlorogenic acid) produces o-quinones that are brownish in color. Some of these higher molecular weight o-diphenols (e.g., chlorogenic acid) can act as antioxidants and reduce the formation of pink o-quinone from the action of PPO on caffeic acid. The interplay among the phenylpropanoid pathway, the accumulation and sequestering of the phenolic compounds produced, the activity of PPO, the mixing and reaction of the o-diphenols with PPO, and the reaction of the o-quinones produced can account for the variability in pinking in lettuce.