Phenolic Profiles of Four Processed Tropical Green Leafy Vegetables Commonly Used as Food

The phenolic profiles are presented of four tropical green leafy vegetables (Ocimum gratissimum, Vernonia amygdalina, Corchorus olitorius and Manihot utilissima) commonly used as food, after application of traditional treatments, such as boiling and abrasion. The HPLC/DAD/MS technique was mainly used to carry out this study. Preliminary evaluation of the antioxidant properties of the vegetables was also performed using the DPPH in vitro test. For the first time, seasonal variations in the phenolic content of the four investigated vegetables were highlighted. Of the four plants, all showed only quantitative differences, except for Ocimum graticimum, in which cichoric acid, previously detected as one of the main constituents of this vegetable collected in November (dry season), was absent in the sample harvested in March. The phenolic constituents are chemically unmodified after a strong heating process, such as the traditional blanching (about 15 minutes) applied by Nigerian people prior to consuming these vegetables. Nevertheless, these typical preparations showed a consistent decrease in the total phenolic compounds with respect to the raw material, particularly for Corchorus olitorius (from 42.3 to 5.56 mg/g dried leaves) and Vernonia amygdalina (from 40.2 to 4.4 mg/g dried leaves). As expected, when the blanching treatment is reduced to a few minutes, as for Manihot utilissima leaves, the cooked vegetable maintained almost unaltered its original phenolic content (around 10 mg/g dried leaves). The unique exception is the blanched Ocimum gratissimum sample that showed a consistent increment of the total phenols, particularly of rosmarinic acid (frorn 6.1 to 29.8 mg/g dried leaves) with respect to the unprocessed vegetable.