Aspergillus sojae

What it is

Aspergillus sojae is a filamentous fungus grown as a starter, or koji, on cooked soybeans, wheat or other substrates in the making of fermented soy foods. As a starter it is bought or kept as a culture and used to inoculate the substrate, where it grows as a mould and produces enzymes that break the substrate down.

It belongs to the Aspergillus section Flavi, the same group as A. flavus and A. oryzae. Its taxonomy is contested: a 2023 study describes A. sojae as a domesticated relative of the aflatoxin-producing A. parasiticus, and it is not a close sibling of A. oryzae in every framing. Older safety assessments put it at about 92 per cent DNA homology with A. parasiticus. Its strains form a single clonal clade in that study, which supports treating it as a stable species.

The supplied catalogue lists it as a soy-sauce and soybean koji mould. A. sojae is not simply another trade name for A. oryzae: commercial selections differ in protease, glutaminase, aroma and colour, according to the catalogue note.

Role in soybean fermentation

In Korean meju, doenjang and ganjang, a 2017 study of fourteen confirmed A. sojae isolates from meju reports protease activity of 2.66 to 10.64 U per gram of initial dry substrate and alpha-amylase activity of 21.53 to 106.73 U per gram. The authors describe these levels as comparable to Japanese soy-sauce koji strains. Protease and amylase break soybean proteins and starch into smaller molecules, which feed the flavour and texture of the finished paste or sauce.

The same study singles out two strains, SMF 127 and SMF 131, for high leucine aminopeptidase activity, 6.00 and 6.06 U per gram of initial dry substrate. The authors link leucine aminopeptidase to flavour development and reduced bitterness. They recommend the strains as starters for soybean fermentations, including meju, doenjang and ganjang, while framing that as a recommendation rather than a demonstrated fermentation outcome, because the abstract reports enzyme activity and not fermentation trials.

A genome study of A. sojae NBRC4239 found that about 81.7 per cent of its well-characterised proteins are highly similar to A. oryzae genes. It also found one copy of an alpha-amylase gene where A. oryzae has three. These differences are one reason why commercial A. sojae and A. oryzae koji are treated as separate starter choices.

Safety and toxin markers

Aflatoxin is the main safety concern in this group. The 2017 Korean study reports that SMF 127 and SMF 131 appear to be non-aflatoxigenic, based on an aflR termination mutation and the absence of a polyketide synthase gene. It also reports that they do not produce cyclopiazonic acid, due to deletions in maoA, dmaT and pks/nrps genes. These results come from genetic markers and not from a direct toxin test, and they describe those two strains only. They are not a safety verdict on the species or on any other strain.

A 1997 EPA risk assessment, which the 2023 literature summary also cites, treats A. sojae as a domesticated organism with a long record of safe food use, and it judges its hazard comparable to that of A. flavus and A. oryzae. A safety assessment of this kind does not establish the safety of a particular culture supplied by a particular producer, which the reader should check with the producer.

Choosing and handling the culture

The sources do not give a shelf life, storage temperature or inoculation rate for A. sojae cultures, and this page does not supply them. A reader who buys or keeps a culture should follow the producer's storage and handling instructions and the process the culture was selected for.

Where the sources differ, the difference matters for the result. A strain chosen for protease and flavour activity behaves differently from one chosen for its amylase profile, so a species name alone does not identify the culture a recipe needs.

Notes

The supplied catalogue lists A. sojae for soy sauce and protein-rich soybean fermentations in Japan, Korea and other East Asian food systems. A Korean study tests A. sojae directly in a food, while a Japanese reference compares it with koji strains rather than documenting a Japanese culture.