Rhizopus microsporus

What the mould is

Rhizopus microsporus is a filamentous mould in the Rhizopus group. It grows as a white, cottony mat over moist food, forming the dense mycelium that binds soybeans into a cake in tempeh. Its name in the literature is shared by a complex of closely related strains, so a single name on a label does not establish which strain a producer uses.

Species identity in the group is established by molecular sequencing, not by appearance. Rhizopus strains from tempeh are typically identified by sequencing the internal transcribed spacer region of ribosomal DNA, together with growth and morphology checks.

Tempeh and the Indonesian strains

Tempeh is made by fermenting cooked soybeans with a Rhizopus culture, and the culture determines the fungal growth that holds the cake together. A 2015 study sampled fresh tempeh from 26 locations across Indonesia and identified 36 Rhizopus strains by sequencing, physiology and morphology.

Most of the strains belonged to the R. microsporus complex. A single strain belonged to R. delemar. The authors found that morphological variation within R. microsporus was not enough to separate subspecies under their approach, so the group is treated as one complex in that study.

Taxonomy and naming

The name of the species has been applied to strains used in fermented foods, and the sources differ on how the closely related fermentation fungus R. oligosporus should be classed. The study read for this page did not identify R. oligosporus in its sample, so this page does not claim that the two are the same thing.

Naming matters for a producer, because the same food can be made with strains named differently, and a strain collection record is the place to check which organism a culture actually is.

Safety notes

Rhizopus species are not all the same in their metabolic output. The sources the search returned mention toxic metabolites in some R. microsporus strains, but those claims were not verified in the study read here, so this page does not make a safety verdict for the species. Tempeh safety depends on the whole process and the culture, and the producer's source should be consulted for any specific food.

Notes

Oncom and other Rhizopus-led foods are not covered by the source read for this page, and they are separate pages on this site. The scope of this page is the species as identified in Indonesian tempeh.