Simple metabolite extraction method for metabolic profiling of the solid-state fermentation of Aspergillus oryzae

Solid-state culture of microorganisms is an important style of culture both in the traditional food industry and in the modern fermentation industry. We propose here a simple method for metabolite extraction from the solid-state fermentation of a filamentous fungus, Aspergillus oryzae, which is known as rice-koji. To evaluate the efficiency of metabolite extraction, liquid chromatography–mass spectrometry (LC/MS) was used for simultaneous detection of a wide range of metabolites including amino acids, organic acids and vitamins, which are of interest in rice-koji making. Among eleven different metabolite extraction methods tested, we found that extraction using acetonitrile/water (50:50), coupled with boiling at 70 °C for 5 min, was advantageous in terms of both extraction yield and metabolism quenching. We consider that our method provides a technical basis for metabolome analysis of a solid-state fermentation.