Chemical composition and antioxidant capacity of lettuce: Comparative study of regular-sized (Romaine) and baby-sized (Little Gem and Mini Romaine) types

The aim of this study was to assess the differences in sugars, organic acids and main antioxidant compounds, as well as nitrate concentration, between different lettuce cultivars belonging to three main types: one regular-sized type (Romaine) and two baby-sized types (Little Gem and Mini Romaine), usually consumed as whole-head and fresh-cut lettuces. Overall, in the studied cultivars, chlorogenic acid and caffeic derivates were the major compounds among the free and bound phenolics, respectively. As regards folates, only 5-methyl tetrahydrofolate (5-MTHF) was detected in the monoglutamic form, whereas the hydrolysis of polyglutamatic forms released further 5-MTHF and tetrahydrofolate (THF). The major carotenoid found was β-carotene followed by lutein, lactucaxanthin, violaxanthin and neoxanthin. Romaine type showed the highest content of total sugars, phenolic compounds, vitamin C and folates. Mini Romaine showed the highest content of organic acids, carotenoids and chlorophylls. Finally, Little Gem presented the highest nitrate content, which can be considered a negative characteristic of this lettuce type. The differences found in colour and metabolite and nitrate concentrations could be attributed to the lettuce head structure and size that determine the penetration of the sunlight and therefore the synthesis of light-dependent metabolites and the nitrogen assimilation.