Aspergillus luchuensis

What the mould is

Aspergillus luchuensis is a filamentous mould that is grown as black koji. Its spores are dark, which gives the koji its name. It is the species used in Japanese shochu and awamori, the distilled spirits of the Ryukyu islands, and it sits in the same group of Aspergillus species as the industrial citric acid fungus Aspergillus niger.

A closely related white-koji strain, Aspergillus luchuensis mut. kawachii, is a mutant that lost its dark spore colour. The two are treated as separate in the sources reviewed here: the white strain is named as a distinct organism in the shochu literature, and the naming is not settled by species name alone.

Starch and citric acid in shochu

In shochu and awamori, the koji mould does two jobs. It secretes amylolytic enzymes that break starch into sugars for the yeast, and it produces citric acid. Citric acid lowers the pH of the mash, called moromi, to about 3 to 3.5. At that acidity contaminating microbes struggle to grow, so the mash is protected while the yeast ferments it.

This is the main reason black and white koji are used in spirits rather than in soy sauce or miso, where the koji mould is chosen for protease and other enzymes. Citric acid production by A. kawachii is described as distinctive among koji moulds, and the sources note that A. oryzae does not make it in the same way.

Process control and genetics

The sources describe a temperature programme in shochu koji making, rising gradually to about 40 C and then lowered to about 30 C, because the lower temperature is needed for citric acid production. Regulation of citric acid output involves the LaeA regulator and the citrate exporter cexA in the white-koji mutant, and mitochondrial citrate transporters contribute to extracellular citric acid.

These controls are specific to the strain and to the process. A species name does not establish which strain a producer uses, or whether its koji will reach the same acidity.

Notes

The sources read for this page are a preprint on citrate transport in the white-koji mutant and a related title. The black-koji citric acid data in the text read was thin, and a 2021 review of black and white koji for shochu and awamori is listed as a route to fill that gap but was not read. Safety and handling of this mould in koji making are not covered beyond the species-level caveats of the wider Aspergillus group.