Unstable compounds with intense sweet odor were isolated from shoyu and identified as 4-hydroxy-2-ethyl-5-methyl-3 (2 H) -furanone and 4-hydroxy-5-ethyl-2-methyl-3 (2 H) -furanone (tautomer ratio, about 3:2) on the basis of spectrometrical investigation. This may be the rare case of isolation of the tautomeric compounds from a natural product. They were concluded to be major and important flavor components in shoyu.
