Antioxidant properties of ginger leaves: An overview

Past studies on the antioxidant properties (AOP) of ginger species were confined to rhizomes. Although leaves of ginger species have been used for food flavouring and in traditional medicine, little research has been done on their AOP until recent years. This overview is on recent work done on the AOP of ginger leaves. Emphasis is on variation between species, comparison with rhizomes and flowers, altitudinal variation, effects of thermal and non-thermal drying methods, herbal teas, and commercial potentials. Of 26 ginger species, belonging to nine genera and three tribes, AOP of leaves were strongest in Etlingera followed by Alpinia and Hedychium. Eleven out of 14 species (78%) had significantly higher values in leaves than in rhizomes. Similar trends were also observed in other species of Zingiber, Boesenbergia and Elettariopsis. Leaves of highland populations of Etlingera had higher values than their lowland counterparts. Thermal drying of leaves of four species led to drastic declines in AOP but freeze drying led to significantly increase for leaves of Etlingera elatior and Alpinia zerumbet. AOP of hot-water extracts of the freeze-dried tea of A. zerumbet were found to be significantly higher than the commercial tea. A protocol to produce a standardised herbal extract of chlorogenic acid (CGA) from E. elatior leaves (40% purity) has been developed. With high CGA content of 1.6 times that of commercial extracts from honeysuckle flowers (25% purity), the standardised extract has great potential to be developed into functional foods and other health products.