1. Koji is not one uniform culture

Aspergillus oryzae is used in fermented foods such as soy sauce and miso, but enzyme profiles and metabolites can vary with strain, substrate, cultivation, and maturation conditions. Amylases and proteases provide useful background for understanding how koji changes starch- and protein-rich materials. That general role does not establish a fixed flavor outcome or process guarantee. The cited soy-sauce study, in particular, tested co-cultures of two A. oryzae strains selected for different enzyme profiles; its conclusions belong to that defined experimental system.

2. The soy-sauce flavor result depended on the tested strain ratio

The researchers compared a strain with high carbohydrate-degrading enzyme activity and another with high protease activity, using different mixing ratios during koji preparation. The abstract reports that the resulting soy-sauce aroma and measured umami- and kokumi-related compounds varied with the ratio. A 1:1 mixture stood out on aroma-related outcomes, while a 3:1 mixture produced stronger measured umami/kokumi compound outcomes. These are comparisons among the tested strains and product conditions. They do not show that every A. oryzae culture, miso, or other fermented food will develop the same profile, nor that one ratio is universally best.

3. The miso paper studied IMP breakdown, not mycotoxins

A separate miso study targeted acid phosphatases in an A. oryzae starter to reduce degradation of inosine monophosphate (IMP), a nucleotide associated with savory taste. It examined regulation of the relevant enzymes and the effect on IMP under specified strain and fermentation conditions. The study’s endpoint was a flavor compound, not aflatoxin or another mycotoxin. It therefore cannot be cited as evidence that koji is toxin-free or that a fermented product is safe under all conditions. Flavor chemistry and microbial safety are separate questions requiring different evidence.

4. Expert perspective: assess safety at the strain and process level

A review of industrially used fungi describes A. oryzae’s history of use while emphasizing that a species name alone does not establish the safety of every strain and production condition. It also discusses historical confusion between A. oryzae and the closely related A. flavus, underscoring the importance of correct identification and strain-level assessment. This does not mean that all A. oryzae produces aflatoxin; the review explicitly explains why some such reports resulted from misclassification. For food use, rely on correctly identified food-grade starter cultures and applicable supplier and jurisdictional controls. The two flavor studies cited here did not test mycotoxin absence and cannot guarantee the safety of a home or commercial batch. / Safety of the fungal workhorses of industrial biotechnology

Sources: Synergistic effects of co-culturing two Aspergillus oryzae strains with distinct enzyme profiles on soy sauce flavor enhancement / Reduction of the degradation activity of umami-enhancing purinic ribonucleotide supplement in miso / Safety of the fungal workhorses of industrial biotechnology