The distinct and unique aroma of miso determines its quality. Many reactions occur among the components of miso, primarily the Maillard reaction, a non-enzymatic reaction of an amino group with a reducing sugar. The volatile compounds produced from this reaction give miso its characteristic flavor and aroma. Depending on the microorganism in combination with the variety of soybean or cereal used, many classes of flavor compounds are produced that give rise to the different types of miso. Fermentation products such as furanone compounds, including 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone (HEMF) and 4-hydroxy-2,5 dimethyl-3(2H)-furanone (HDMF) are novel flavor compounds of miso. HEMF is especially known for its sweet aroma and is very important for the sensory evaluation of the aroma of rice miso.