Aspergillus oryzae

What the fungus is

Aspergillus oryzae is a filamentous mould that grows as a white to yellow-green mat on moist grain or soybeans. Its spores are usually yellow-green, and in the koji trade the name refers to a cultured preparation rather than to the species alone. The species reproduces asexually.

Taxonomists regard A. oryzae and A. sojae as domesticated forms of A. flavus. The two share about 99.5 per cent of their genome sequence, which is why a species name on a koji label does not establish which strain, or how safely it has been bred, a producer uses.

Koji and what it does

Koji is made by growing the mould on steamed rice, barley or soybeans. The fungus secretes enzymes into the substrate, and those enzymes do the work that the finished food depends on. Amylases such as alpha-amylase and glucoamylase break starch into sugars. Proteases break soybean and grain protein into peptides and amino acids, and leucine aminopeptidase releases much of the glutamic acid that gives soy sauce its savoury depth. Lipases break down fats in soybeans during miso making.

Koji therefore works as an enzyme source rather than as a fermenter on its own. In sake, shoyu, miso and vinegar production, yeasts and lactic acid bacteria carry out the fermentation that follows, using the sugars and amino acids the mould has released.

Genome and enzyme range

The genome carries a large set of proteinase genes, and the species produces a wide range of other enzymes, including feruloyl esterase, beta-galactosidase and xylanases. Feruloyl esterase contributes to the flavour of sake. The enzyme profile differs between strains and between growing conditions, so a koji is judged by what it does in the product, not by the name alone.

Safety and mycotoxins

The species is not associated with aflatoxins, unlike its close relative A. flavus. The review reports that tested koji strains were negative for aflatoxins and that disabled genes in the aflatoxin cluster are a likely reason. Other Aspergillus species do produce mycotoxins, so producers check finished products against health-authority limits.

The FDA lists A. oryzae as GRAS, and the WHO considers it safe for food use. Invasive infection in healthy people has not been reported. Rare allergic reactions and infections have occurred mainly in people who were immunocompromised or prone to allergy and who inhaled large numbers of spores. Handling koji in a dusty or poorly ventilated room is therefore a real exposure consideration, and this page does not give a safe handling protocol beyond what the review reports.

Notes

The sources do not give a dated first use of this species, and the strain-level history of commercial koji cultures is not established here. Culture collections list named strains for each product, and the species name alone does not identify which one a koji was made from.