Rice flavor chemistry

Abstract

Rice is the most important food crop consumed by man in the world. Because of its importance, rice breeders have been focused to develop rice quality. Rice flavor quality is a critical breeding objective in that the quality has a significant impact on consumer preference. Using a dynamic headspace system with Tenax trap, GC-MS, GC-olfactometry (GC-O), and multivariate analysis, the aroma chemistry of various rice flavor types (aromatic rices, colored rices, waxy rices) was analyzed and characterized. First, in black rice with a unique flavor, a total of 31 odor-active compounds were determined. Among them, 2-acetyl-1-pyrroline (2-AP), guaiacol, indole, and p-xylene largely influenced the difference the aroma in cooked black rice and white rice. The former two compounds were key aroma compounds responsible for the unique character of black rice due to lower odor threshold and unique odor description. Second, a total of 29 major odor-active compounds from six distinctly different rice flavor types (basmati, jasmine, two Korean japonica cultivars, black rice and a non-aromatic rice) when cooked, were determined based on their odor activity value. Among them, 13 odor-active compounds, including 2-AP, hexanal, (E)-2-nonenal, octanal, heptanal, nonanal, 1-octen-3-ol, (E)-2-octenal, (E,E)-2,4-nonadienal, 2-heptanone, (E,E)-2,4-decadienal, decanal, and guaiacol could separate and characterize the individual rice flavor types. Third, premium-quality, black-pigmented, and waxy rice types were discriminated based on six odor-active compounds [(E)-2-hexenal, heptanal, (E)-2-octenal, guaiacol, (E)-2-nonenal, and decanal] using canonical discriminant analysis (CDA). Two discriminant equations obtained by CDA provided a predictive model for discriminating the three types of specialty rice. Fourth, volatile compounds emanating from Ilpumbyeo (traditional white rice), Heugjinjubyeo (black pigmented), and Jeogjinjubyeo (red pigmented) of cooked rice milled to different degrees (0, 6 and 8 % by weight) were compared to ascertain their site of origin. Removal of the bran qualitatively and quantitatively affected the volatile compounds formed with certain volatiles increasing, indicating the endosperm as their primary site of origin and a second group decreasing or eliminating, indicating the rice bran as the primary site of origin. These information from four experiments may be of potential use in rice breeding programs focusing on flavor quality.