Without Koji, sake would not be with us today in its current form. Rice is a grain just like barley but since it’s polished removing the germ (the reproductive part that germinates to grow into a plant) it is unable to go through a malting process that beer does so a different approach is required. Koji is actually the term for the rice that has been inoculated with Koji-kin (a mould, microrganism or fungus usually in a powder form). As well as saccharifying rice, other grains and potatoes to produce alcoholic beverages, the magic mould is used to ferment soybeans to make soy sauce and miso (fermented bean paste).
Around 20% of the steamed rice will be allocated for the Koji rice. Koji-mould is cultured in steamed rice to produce this Koji rice which takes around 48 hours in a room with wooden panels called a koji-muro not too dissimilar to a sauna where the temperature is around 32 degrees. The process of making koji has several stages and is quite intense, not too dissimilar to looking after a newborn baby with a need for it to be check every few hours monitoring the temperature and growth (ok, that last part doesn’t apply to a baby). The mould on the rice grain needs to have even growth so requires the clumps to be broken up usually by hand however some brewers are automating the process but where’s the fun in that. There’s nothing better than digging your hands into some koji rice, it borders therapy. Once there is sufficient growth, this furry looking creature is added to the starter with rest of the steamed rice, water and yeast. The Koji supplies the shubo (fermentation starter) and moromi (main mash) with enzymes to convert the starch in rice to sugar so that the yeast can convert this to alcohol.
There are three varieties of Koji mould with yellow Koji-kin, scientifically known as Aspergillus oryzaebeing, being the default option for sake brewers. There are brewers out there such as Niida Honke from Fukushima prefecture with their Odayaka range who are experimenting and breaking the mould or is it mold (pun intended) by using a different strain. They use the Shochu (the Japanese distilled beverage made from sweet potato (imo), barley (mugi), and rice (kome) that has an alcohol content of around 25%-30%) variety (Shiro-koji or white koji-kin) which brings out more citric acidity comparative to wine as oppose to the standard sake lactic tones. You have to admire those experimenting with new ways in an industry that is steeped in tradition and history that can sometime hold you back.