Furanone Formation

Among the aroma compounds characterized in good-quality soy sauce, 4H-pyrone and 3(2H)-furanones are of special interest. 4-hydroxy-5-methyl-3(2)-furanone (HMF), as well as 4-hydroxy-2(or5)-ethyl-5(or2)-methyl-3(2)-furanone (HEMF), with a very low odor threshold, has a caramel-like odor and is organoleptically considered to be one of the most important constitutes of the characteristic soy sauce flavor. Interestingly, a very common soy sauce ingredient, licorice (the root of Glycyrrhiza species) was found to contain significant amount of maltol glycoside, licoagroside B (60).

HEMF gives a mild, soy sauce-like aroma. HMF exhibits a heavier note, a cooked beef-like aroma, and also an odor reminiscent of soy sauce. 4-Hydroxy-5-methyl-3(2)-fura-none was isolated and characterized from soy sauce for the first time by Nunomura et al. (61). Sasaki et al. (62) reported this compound to be a product of a reaction between aldopentoses and primary amine. Tonsbeek et al. (63) reported that ribose-5-phosphate and pyrrolidone carboxylic acid or taurine, or both, were natural precursors of HMF. The content of HEMF only increased during the later stages of the fermentation and its yield was highest after 60 days. Biosynthesis route of HEMF was claimed to be through the pentose-phosphate cycle by Zygosaccharomyces rouxii (64). It is known that pentoses such as arabinose, xylase, and ribose are present in soy sauce. The precursors of HMF in soy sauce can be the sugars and certain amino acids that react together in the Maillard reactions, including the Amadori rearrangement and subsequent cyclization. The quantity of HMF increased remarkably during the pasteurization of raw soy sauce, which indicates that the intermediates formed during moromi fermentation play an important role in HMF synthesis.

4-Hydroxy-2,5-dimethyl-3(2)-furanone (HDMF) and 4-hydroxy-5-methyl-3(2)-fura-none (HMMF) are derived from a Maillard reaction. During thermal treatment of carbohydrates, several a-dicarbonyls are formed (65). The condensation of the carbonyl group of the reducing sugar with the amino compound gives a glycosylamine. Subsequently, this rearranges and dehydrates, via deoxyosone, to various sugar dehydration and degradation products such as furfural and furanone derivatives. When a ketose is involved instead of an aldose sugar, then a ketosylamine is formed that undergoes a Heyns rearrangement to form a 2-amino-2-deoxyaldose (Heyns product). At temperature above 100°C, 1-amino-1-deoxy-2-ketoses undergo 2,3-enolization to give a 1-amino-2,3-enediol from which an amine is eliminated to form a methyl-2,3-dicarbonyl intermediate (66). 1,2-Enolization of the Amadori product will result, after dehydration and deamination, in the formation of a 3-deoxyosone. The 3-deoxyosone is readily converted to the corresponding furfural from a pentose, and 5-methylfurfural from a hexose. Furan derivatives are also formed by the further dehydration of the 1-methyl-2,3-dicarbonyl compounds, yielding 4-hydroxy-5-methyl-3(2H)-furanone or 4-hydroxy-2,5-dimethyl-3(2)-furanone (HDMF) from the pentose and hexose sugars, respectively (67).

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