Carbon stable isotopes of glucose during the degradation of rice by the koji fungus Aspergillus oryzae

Abstract

Glucose, a key component of traditional Japanese fermented foods, is derived from rice starch via saccharification by hydrolytic enzymes produced by Aspergillus oryzae. The δ13C value of glucose reflects that of its rice source. However, the influence of saccharification parameters (glucose concentration, degradation temperature, and reaction time) on glucose δ13C values is unclear. Here, we investigated the influence of saccharification on the δ13C value of glucose. Our experiments showed a significant difference in the δ1³C value of glucose (−27.0 ± 0.1 ‰) obtained from saccharification compared to the ingredient rice (−27.1 ± 0.1 ‰) and remaining solid residue (−27.1 ± 0.1 ‰); however, it did not differ significantly from those of rice koji (−27.0 ± 0.1 ‰) and steamed rice (−27.1 ± 0.1 ‰), despite all values being within 0.1 ‰. Notably, glucose concentration, degradation temperature, and reaction time did not significantly affect glucose δ13C values. These findings demonstrate the remarkable preservation of glucose δ13C values. The δ13C values remain aligned with the original δ13C value of the rice, even with up to 60 % degradation during A. oryzae saccharification. This persistence of the δ13C value throughout the process offers a potential tool for authenticating the origin of rice-fermented beverages based on the δ13C value of their glucose component.