Aspergillus oryzae, known as koji mold, is the most important fungus in East Asian food fermentations. It is used to produce koji, a starter culture for soy sauce, miso, sake, and mirin. A. oryzae is grown on steamed rice, barley, or soybeans, where it secretes powerful hydrolytic enzymes including alpha-amylases, glucoamylases, proteases (both acid and alkaline), and lipases. These enzymes break down starches into fermentable sugars and proteins into amino acids and peptides, which are then utilized by yeast and LAB in subsequent fermentation stages. A. oryzae is generally recognized as safe despite its close genetic relationship to the aflatoxigenic A. flavus.
Tempeh is a fermented soybean product originating from Indonesia, produced by solid-state fermentation with Rhizopus microsporus var. oligosporus (formerly R. oligosporus). The fungus grows as a dense mycelial mat that binds cooked soybeans into a compact cake. During fermentation, Rhizopus produces lipases that hydrolyze soybean lipids into free fatty acids, along with phytases that reduce phytic acid content, improving mineral bioavailability. The mycelium also produces a mild nutty flavor and a firm, sliceable texture. Tempeh is a complete protein source and a significant source of vitamin B12 when the fermentation is carried out by certain bacteria in addition to the mold.
Penicillium roqueforti is used in the production of blue-veined cheeses such as Roquefort, Gorgonzola, Stilton, and Danish Blue. The mold is added to milk or curd and grows throughout the cheese matrix during ripening. Needles or wires are inserted to create oxygen channels, allowing the obligate aerobic P. roqueforti to sporulate and develop the characteristic blue-green color. The mold produces lipases that release free fatty acids and methyl ketones (particularly 2-heptanone), contributing to the pungent flavor. P. camemberti, used for Camembert and Brie, produces a white, velvety surface growth and secretes proteases that soften the cheese texture from the outside inward.
Safety considerations in mold fermentations center on mycotoxin production. While the species used in food fermentations are generally regarded as safe, careful strain selection and quality control are essential to prevent contamination by toxigenic strains. Non-toxigenic strains of A. oryzae used in koji production lack the genes for aflatoxin biosynthesis. Similarly, P. roqueforti produces the mycotoxin roquefortine C and PR toxin, but these are present at very low concentrations in cheese and are largely degraded during ripening. Mycotoxin monitoring programs in raw materials and finished products are standard practice in mold fermentation industries. Mold-fermented products often involve a succession of lactic acid bacteria followed by fungal growth. Yeast fermentation may precede mold growth in some processes. Mycotoxin production by certain molds requires careful monitoring through mycotoxin analysis.
