Comparative Study on Aroma-Active Compounds in Thai, Black and White Glutinous Rice Varieties

Abstract

The aroma compounds of cooked, black and white glutinous rice (Kheow Ngu) that are commonly found on the Thai market were investigated. Volatile compounds were extracted from the steamed rice samples using diethyl ether and identified by gas chromatography-mass spectrometry (GCMS). Twelve volatile compounds were identified in black glutinous rice and 14 compounds in white glutinous rice. In black glutinous rice, benzaldehyde was found in the highest concentration (1,203.7 ng/kg) but 4-vinyl-2-methoxyphenol (sweet, spicy and smoky odor) had the highest odor-active value (OAV = 133). However, the results from gas chromatography-olfactometry (GCO) indicated that there were four aroma-active compounds in black glutinous rice. The most prominent odorants were (E,E)-nona-2,4-dienal (sweet, fatty) and 2-acetyl-1-pyrroline (pandan) that had a flavor dilution (FD) factor of 3. Two other compounds that had an FD factor of 1 were 2,3-butanediol (sweet, balsamic) and 1-octen-3-ol that had straw and earthy-aroma characteristics. In white glutinous rice, the compound that had the highest OAV was butanoic acid (sweaty, rancid, OAV = 1,036). From GCO data, there were seven aroma active compounds that had sweet, cooked-flour and fatty-aroma characteristics in the white
glutinous rice. These compounds were tentatively identified as nonanal, ethyl octanoate, (E,E)-nona-2,4-dienal, germacrene-D, methyl dodecanoate and two were unknown.