Lettucenin Sesquiterpenes Contribute Significantly to the Browning of Lettuce

Wound-induced changes in the composition of secondary plant compounds cause the browning of processed lettuce. Cut tissues near the lettuce butt end clearly exhibit increased formation of yellow brown pigments. This browning reaction is typically been attributed to the oxidation of polyphenols by the enzyme polyphenol oxidase (PPO). However, in our previous study on Iceberg lettuce, we showed that, besides the enzymatic polyphenol browning, other reactions must be involved in the formation of colored structures. With the present study for the first time, we isolated yellow sesquiterpenes by multilayer countercurrent chromatography (MLCCC), followed by preparative high-performance liquid chromatography (HPLC). Further analyses by nuclear magnetic resonance (NMR) and mass spectrometry (MS) techniques identified lettucenin A and three novel derivatives. We call these compounds lettucenins A1, B, and B1. Color-dilution analyses revealed these lettucenins as key chromophores in the browning of Iceberg lettuce. A time formation curve showed the accumulation of lettucenins A and B within 40 h after cutting. Thereafter, these structures were degraded to unknown colored compounds. Lettucenin A was verified in five varieties of Lactuca. In contrast to that, lettucenin A was present only at trace levels in five varieties of Cichorium. Therefore, lettucenin A might be used as a chemosystematic marker of the genus Lactuca.