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 δ 13C value of glucose reflects that of its rice source. However, the influence of saccharification parameters (glucose concentration, degradation temperature, and reaction time) on glucose δ 13C values is unclear. Here, we investigated the influence of saccharification on the δ 13C value of glucose. Our experiments showed a significant difference in the δ 1³C 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 δ 13C values. These findings demonstrate the remarkable preservation of glucose δ 13C values. The δ 13C values remain aligned with the original δ 13C value of the rice, even with up to 60 % degradation during A. oryzae saccharification. This persistence of the δ 13C value throughout the process offers a potential tool for authenticating the origin of rice-fermented beverages based on the δ 13C value of their glucose component.