Although they employ a few simple machines to wash grains and mix their miso, the basic process used at the Onozaki shop dates back to pre-industrial Japan. Cooked organic soybeans are mixed with koji (grain inoculated with Aspergillus culture), salt, and water. This mixture is placed in large cedar vats to ferment.

Gradually the enzymes supplied by koji along with microorganisms from the environment break down the complex structure of the beans and grains into readily digestible amino acids, fatty acids and simple sugars.

Omitting many of the details, the hallmark of the Onozaki traditional process is handmade koji made in their unheated koji room.

In this uniquely constructed room, heat and humidity naturally generated by the fermenting grain are carefully monitored for 48 hours until the mature koji, covered with a fluffy, white, glistening mycelium, is sweet and loaded with powerful digestive enzymes. Making koji this way is a labor intense process that requires skill, sensitivity, and stamina.

At the Onozaki shop the koji room is the domain of miso master Takamichi Onozaki, and slight variations in the process are closely guarded family secrets.

Onozaki believes that the heightened medicinal value and deep, rich flavor characteristic of traditional miso is the result of the following: high quality ingredients such as organic grains and beans, sun-dried sea salt, and pure, natural water; strong koji that effectively breaks down beans and grains; slow cooking and cooling of soybeans; long established strains of "wild" microorganisms that get into the miso; and unhurried natural aging in old, seasoned wooden vats. Mitoku Onozaki barley, red, and brown rice misos are sold unpasteurized and should be refrigerated.