Changes in Contents of Simple Phenols and Volatile Compounds in Leaves of Black Rice Varieties Following Growth Stages

Abstract

In this study, we aimed to explore bioactive volatile compounds in leaves of black rice which emphasized on phenolic compounds. Leaves of four black rice varieties including IR1552 and three Thai black glutinous rice varieties, Kam Huai Hong Khrai (KHHK), Kam Chiang Mai (KCM), and Kam Muang Nan (KMN) were extracted by acidic and basic aqueous solution. Gas chromatography–mass spectrometry analysis of the volatile components in these black rice extracts revealed twelve simple phenols among fifty-five volatile compounds identified. 2-Methoxy-4-vinylphenol was detected as the most abundant volatile, especially, in the booting stage for KHHK followed by the tillering stage of the same variety. Its highest content was approximately 10-folds greater than that of vanillin at the booting stage. To the best of our knowledge, seven simple phenols were detected in black rice leaves for the first time, which included 4-vinylphenol, 2,6-dimethoxyphenol, 4-hydroxybenzaldehyde, 2-methoxy-4-(1-propenyl)phenol, 2,4-bis(1,1-dimethylethyl)phenol, 4-hydroxy-3,5- dimethoxy-benzaldehyde and 4-((1E)-3-hydroxy-1-propenyl)-2-methoxyphenol. 3-Hexen-1-ol was detected at the highest amount and was only identified in the dough-grain stage for KHHK, KCM and KMN. Two major aldehydes (2E)-2-hexenal and nonanal were detected in high quantities in KCM black rice variety in the dough-grain and mature-grain stage. Among the twelve fatty acids identified, (Z,Z,Z)-9,12,15-octadecatrienoic acid was detected at the highest amount, on average, followed by hexadecadienoic acid and octadecanoic acid. Therefore, it could be suggested that the leaves of black rice are rich in phenolic compounds and could be used to promote natural sources of food ingredients that are of great interest to the food industry.