Aroma chemistry of wild rice (Zizania palustris) and African rice species (Oryza sativa, Oryza glaberrima, and interspecific hybrids)
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Wild rice (Zizania palustris L.) is an aquatic cereal grain that is gaining in popularity among consumers due to its unique nutty, roasted flavor. Rice production in West Africa, in contrast, is comprised of species of Asian (Oryza sativa L.) and African (O. glaberrima Steud.) origin. The aroma chemistry of wild rice and selected cultivars of African O. sativa ssp. japonica and indica, O. glaberrima, and interspecific hybrids was analyzed and characterized using gas chromatography-mass spectrometry, gas chromatography-olfactometry, and descriptive sensory analysis. Seventy-three volatile compounds were identified and quantified in cooked wild rice, 58 of which had not been previously reported. Thirty-four odor-active compounds were identified, with the dominant nutty and roasted aroma. Descriptive sensory panelists described wild rice as having high intensities of ‘nutty’, ‘smoky’, ‘hay-like’, ‘earthy’, and ‘green’ aroma attributes. The fermentation and parching steps in wild rice processing are believed to play an important role in creating the unique nutty, roasted aroma based on the fact that a number of pyrazines conferring these aromas, typically formed via thermal reactions, were present in the processed grain prior to cooking. Of forty-two volatiles identified across seven representative rice cultivars grown in West Africa, 3,5,5-trimethyl-2-cyclopenten-1-one, styrene, eucalyptol, linalool, myrtenal, and L--terpineol had not been previously reported in rice. Thirty-three odor-active compounds were characterized. 4-Ethylphenol (‘musty’) and (E,E)-2,4-heptadienal (‘fruity’) were unique to O. glaberrima and pyridine (‘cheese’), styrene (‘pungent’), eucalyptol (‘clove’, ‘sweet’), and myrtenal (‘spicy’) were described only in the interspecific hybrid. Descriptive sensory analysis indicated ‘cooked grain’, ‘barny’, and ‘earthy’ attributes were statistically different among African O. sativa ssp. japonica and indica, O. glaberrima, and the interspecific hybrid. While based on a very limited selection of germplasm, the aroma chemistry data suggests that the general flavor of African rice is distinct from typical Asian rice and that there appears to be sufficient variation in flavor within the African germplasm to allow separating it into distinct flavor types. The aroma chemistry of wild rice and the selected African rice information provides a foundation upon which product chemistry and consumer preference can be integrated to ascertain the aroma traits conferring superior flavor.