What is koji processing in coffee?

Koji processing is an experimental method in which spores of the mould Aspergillus oryzae - the same organism used to produce sake, miso, soy sauce, and other fermented foods - are applied to coffee cherry or parchment and allowed to grow. The koji mould produces enzymes during its growth that significantly alter the composition of the coffee.

The key enzymatic activities are protease (which breaks down proteins into amino acids, increasing the perception of body and umami-like sweetness) and amylase (which breaks down starches into fermentable sugars, making more substrate available for subsequent microbial fermentation). In other words, koji acts as a pre-ferment: it transforms the composition of the coffee before conventional fermentation even begins, making the subsequent fermentation richer and more substrate-available.

Koji must grow in an oxygen-rich environment and at controlled humidity and temperature - requirements that make it more technically demanding than standard fermentation methods. After koji growth, the coffee typically undergoes a secondary fermentation using the koji's enzymatic output as substrate. The resulting cup is often described as having exceptional body, umami depth, and complex sweetness that's distinct from both conventional naturals and anaerobics. For buyers, koji lots are at the frontier of experimental processing - rare, premium, and increasingly appearing in competition contexts and high-end specialty offers.