Mangrove leaves with outstanding content of free amino acids especially GABA, makes them candidates for functional food

Herbal remedies by using mangrove plants have been recognized in several countries, and the plant materials are often consumed as a tea, which are infusions of dried plant parts steeped in boiling water. These infusions from mangrove plants are believed to be effective agents for treating and/or prevent infections, ailments, and diseases, and some mangrove plants have attracted attention as functional food resources in recent years. To reveal the beneficial properties of mangrove plants, clarification of the chemical ingredient and its characteristics of mangrove plants is required. This study focused on amino acids, which were functional ingredients (i.e., bioactive compounds) because amino acids are expected to function as ‘functional ingredients’ in addition to conventional ‘nutrients’ in recent years. To demonstrate the further possibilities of mangrove tea, we firstly evaluated the content of total amino acids in the leaf of several dominant mangrove species with commercial teas as references. Next, we clarified the composition of amino acids by using LC-MS analysis. As the results, free amino acid content (FAC) in leaf sample of Rhizophora stylosa, Bruguira gymnorrhiza, Kandelia obovata, Avicennia marina, Sonneratia alba, and Lumnitzera racemosa was 747, 1056, 946, 829, 623, and 896 mg/100 g DW, respectively. The FAC in all mangrove leaf samples were higher than those in green tea of low price, black tea, oolong tea, barley tea, and mate tea. Moreover, it was found that GABA accounts for over 10% of the total amino acid in the mangrove leaf samples. Especially in mature leaves of K. obovata, the ratio was very high levels 35%. In the case of K. obovata, the contents were higher than GABA tea. Based on this, we concluded that several mangrove plants, especially K. obovata has the potential of a functional food which is GABA-rich.