Bar Tartine's extensive larder and fermentation program is the envy of restaurants across the country. What the mysterious fermented powers of koji do to a seemingly simple beef tartare on toast is truly astonishing.
'koji' on Serious Eats
One of the fundamental differences between sake and beer is this: sake rice does not contain the kinds of enzymes that barley does, so an additional ingredient is needed to help convert the rice's starch into sugar. Koji-kin is the mold that makes fermenting sake possible.