[Chemical profiles of methylpyrazines contained in commercially available natto]

Structures and amounts of methylpyrazines contained in commercial natto, a fermented soybean food in Japan, were determined using HPLC equipped with an acid-resistant reversed phase column, Capcell Pak C18 ACR (Shiseido). Mobile phase solvent mixtures consisted of acidic phosphate buffer solution (pH 2.0) containing 2% acetonitrile gave satisfactory results with baseline separation of the authentic specimens, such as naked pyrazine, monomethylpyrazine, 2,3-, 2,5-, and 2,6-dimethylpyrazine, trimethylpyrazine, and tetramethylpyrazine. We used the mobile-phase solvent with a flow rate of 1 ml/min at 15.0 degrees C. Before HPLC, commercial natto samples were treated with water to prepare diluted suspensions of surface mucous materials. The suspensions were treated on Sep-Pac C18 Cartridges (Waters) with phosphate buffer solutions containing 2-7% acetonitrile. The extracts were then injected into the analytical column to obtain chromatograms that were used to determine the structures and amounts of methylpyrazines. The results showed that a commercialiy packed natto contains a considerable amount of 2,5-dimethylpyrazine instead of the tetrametyl- and trimethylpyrazines in the traditional products. This may be a result of recent efforts of natto makers whose interests have been focused on new methods for preparing odorless products.