Analysis of nitrogen source assimilation in industrial strains of Aspergillus oryzae.

Nitrogen source assimilation is important for the biological functions of fungi, and its pathway has been deeply studied. Aspergillus oryzae mutants defective in nitrogen source assimilation are known to grow poorly on Czapek-Dox (CD) medium. In this study, we found an industrial strain of A. oryzae that grew very poorly on a CD medium containing sodium nitrate as a nitrogen source. We used media with various nitrogen components to examine the steps affecting the nitrogen source assimilation pathway of this strain. The strain grew well on the CD medium supplied with nitrite salt or ammonium salt, suggesting that the strain was defective in nitrate assimilation step. To ascertain the gene causing the defect of nitrate assimilation, a gene expression vector harboring either niaD or crnA of A. oryzae RIB40 was introduced into the industrial strain. The industrial strain containing the crnA vector recovered its growth. This is the first report that a mutation of crnA causes poor growth on CD medium in an industrial strain of A. oryzae, and crnA can be used as a transformation marker for crnA deficient strains.

Preparation of egg-koji for developing a novel food.

While chicken eggs contain many nutrients necessary for humans and there are various cooking methods, the nutritional components are used as they are, and there are no traditional foods that utilize microorganisms. Koji-mold, containing Aspergillus oryzae, A. sojae, and A. luchuensis, which has been used in various fermented foods since ancient times, grows on raw grain materials such as rice and barley to become koji. This can give flavors not found in the raw materials that can decompose and convert the nutritional components of the raw materials. Here, we succeeded for the first time in developing egg-koji that uses only eggs and koji-mold by selecting and combining cooked egg powder (CEP) and A. oryzae AO101 as the most suitable combination. To suppress the explosive growth of harmful bacteria, we improved the sterilization method, watering method, and amount of water. In addition, it was found that egg-koji has a characteristic enzyme activity balance, in which amylase is extremely low and protease at pH 6 was high compared to grain koji, such as rice and barley. Egg-koji might produce enzymes suitable for taking in nutrients when growing into CEP and would be expected to give a flavor that could not be achieved by cooking or additives.