## 2 Results and Discussion
### VOC Metabolites in Five Rice Varieties
2-Pentrylfuran, which belongs to the furanone subclass and which has a nutty odor [17], was abundant in all rice grains. Finally, 2,4-dimethylbenzaldehyde is considered to be a vital aromatic compound in wild rice cultivars, with a mild, sweet, bitter-almond odor [18]. In contrast, [PERSON] et al. [4] showed that alkanes, terpene, and alcohols were the major groups of VOCs in milled rice, which demonstrates the differences in VOC metabolites between milled rice and rice grains. Compared to milled rice, raw rice grains are alive and thus have more abundant metabolites and stronger aromas [19]. Rice bran accounts for 5-8% of the weight of the whole rice grain and has been used to extract oil in recent years [2]. Alcohols and phenols were the main volatiles in rice bran. 4-vinylguaiacol and 4-vinylphenol were reported associated with the aroma of rice bran and further contribute to the aroma of cooked rice and steam-distilled rice bran [2]. It was reported that unmilled black rice had more total volatiles than milled black rice [19]. Milling also substantially affects rice odor. As milling increases, raw rice flavor decreases, while glossiness, plumpness, and sweetness increase [20]. The abundances of five metabolites distinguished Meixiangzhan 2 from other rice varieties: 2,2-dichloroethanol, 3-methylbutanoic acid, pentanoic acid, nonanal, and 5-ethenyldihydro-5-methyl-2(3_H_)-furanone. 3-Methylbutanoic acid, a metabolite of butanone, is produced by the digestion of carbohydrates and proteins [16]; 5-ethenyldihydro-5-methyl-2(3_H_)-furanone gives off a vegetal odor [11]. The abundances of three VOC metabolites distinguished Daohuaxiang 2 from other rice varieties: 2-butanone, 2-acetyl-1-pyrroline and 1-methylcyclohextanol. Of these, 2-butanone is a common food-flavoring agent and is one of the markers that distinguish Indian rice from other rice varieties [4], and 2-acetyl-1-pyrroline (2-AP) is the main aromatic component of aromatic rice and contributes a popcorn-like aroma [21]. It is thought that 2-AP content mainly depends on varietal differences as well as the methods used for rice processing, storage, harvest, and evaluation [10]. According to the index, 1-methylcyclohextanol is an intermediate compound of flavors and fragrances that is commonly used as a food flavoring. N-butyl ether, indole, 1,2,4,5-tetramethylbenzene and 2-hexen-1-ol were the most important metabolites in Huanghuazhan rice, while Yanfeng 47 was distinguished from other rice varieties by the metabolites 3-isopropoxy-1,1,1,7,7,7-hexamethyl-3,5,5-tris(trimethylsilox)tetrasiloxane, decane,2,4,6-trimethyl, 2,6-dimethylhonane, 2-furanmethanol, 5-ethenyltetrahydro,5-trimethyl-2,2,4-trimethyl-1,3-pentenedii disobutyrate and 3-octanol. Overall, the abundances of two volatile metabolites distinguished the three aromatic rice varieties (Meixiangzhan 2, Daohuaxiang 2, and Yexiangyoulisi) from the two non-aromatic rice varieties (Huanghuazhan and Yanfeng 47): acetoin and 2-heptanone. Acetoin, also known as 3-hydroxy-2-butanone, is mainly synthesized by microorganisms and can also be produced in the cells of both mammals and plants such as rice and maize [22]. This compound, which has a pleasant yogurt odor and a creamy taste, is widely used in food flavorings, fragrances, and even biological pest controls [22]. The metabolite 2-heptanone contributes a fruity floral smell [2].