Corn gluten meal (CGM) is a protein-rich by-product of corn starch production that is primarily used as animal feed. In this study, we aimed to enhance the value of CGM while retaining its high-protein characteristics through solid-state fermentation (SSF) with Aspergillus oryzae 6002 to produce CGM koji. During Aspergillus-mediated SSF, the total amino acid content increased markedly, reaching 247.76 ± 0.60 μmol/g dry koji at 72 h. Notably, substantial increases in the free Asp and Glu were observed, although these amino acid residues are scarcely present in zeins, the major storage proteins of CGM. Additionally, γ-aminobutyric acid (GABA) accumulated to 18.38 ± 0.13 μmol/g dry koji at 48 h. These results indicate that SSF promotes protein biotransformation and metabolic conversion, leading to the generation of functional compounds, particularly GABA, whose level was comparable to that reported for GABA-rich plant-based products, as well as taste-active compounds such as umami-related Glu and Asp. Seventy-eight dipeptides accumulated during fermentation, with the total dipeptide content peaking at 49.3 μmol Ala-Ala equivalent/g dry koji at 24 h. Several of these dipeptides have been reported to possess bioactive properties. Although protein hydrolysis occurred during SSF, approximately 65 % of the initial protein content remained, indicating that CGM koji retained its protein-rich characteristics. Collectively, these findings demonstrate that the SSF of CGM with A. oryzae 6002 produces a novel material that retains a high protein content and accumulates functional amino acids and dipeptides, highlighting its potential as a value-added functional material.
Conventional transfer-function methods for normal-incidence absorption coefficient measurement rely on the two-microphone technique and are limited to frequencies below the cut-on frequency of the (1, 0) mode, as only plane waves can propagate normally within this range. Existing multi-microphone approaches for square cross-sectional impedance tubes extend this frequency limit but remain sensitive to higher-order acoustic modes, particularly for practical specimens exhibiting slight asymmetry or nonuniformity. We develop a robust high-frequency measurement method for square impedance tubes that accurately extracts the normally propagating (0, 0) mode while suppressing spurious higher-order modes. The method positions four microphones at the intersections of the (2, 2)-mode nodal lines and sums their signals, thereby canceling or suppressing all modes with cut-on frequencies below the (4, 0) mode, except for the (0, 0) mode. In parallel, four-point sources are positioned at the same nodal intersections to avoid exciting these higher-order modes. Numerical simulations using the Johnson-Champoux-Allard porous model and experiments conducted with a 160 mm & times; 160 mm tube demonstrate that the method enables accurate absorption measurements up to the cut-on frequency of the (4, 0) mode (similar to 4331 Hz), which is four times higher than the upper limit of the conventional two-microphone method (similar to 1082 Hz). The method remains robust even when the specimen surface is slightly tilted or exhibits minor nonuniformities. These results confirm that the proposed eight-microphone/four-point-source technique substantially extends the measurable frequency range of square impedance tubes, enabling more accurate characterization of porous sound-absorbing materials susceptible to frame vibrations and structured acoustic materials such as meta-surfaces, periodic absorbers, and other designs requiring large cross-sectional geometry.
