Meet Koji, the Friendly Fungus

You may not have heard much about koji, but you’ve certainly tasted it before — it’s a key component of staple umami-boosters like miso and soy sauce. 

But what is koji?At the narrowest, it’s an edible fungus, Aspergillus oryzae.

More broadly, it’s a tenderizer, sweetener, umami-booster, and preservative. It’s a fermentation accelerator, meat cure, and pickler. It truly does and is a bit of everything, but ultimately it performs one key act: breaks down huge molecules (whether starch, proteins, or fats) into smaller ones, creating a symphony of new flavors in the process.

This might sound familiar to fermentation fans, but the way koji breaks down these molecules is quite unique. Familiar pickling bacteria or brewing yeasts gobble up bits of starch or sugar from their surroundings. Koji (like other molds) eats via extracellular digestion, releasing enzymes into its surrounding, and it’s these enzymes that are of high value in food preparation.

Koji’s enzymatic output is a function of how fungus feeds. When mold comes to your mind, it’s creeping across a damp surface, like a piece of spoiled food, the corner of a humid basement, or a matted forest floor. Throughout these environments, fungus is secreting enzymes capable of digesting just about anything organic in its path — on a molecular level, the long protein, starch, and fat molecules of other organisms. The hard work takes place outside the cell wall, then the smaller, more digestible pieces are absorbed into the koji for energy. 

While any mold produces enzymes like koji, they usually also crank out nefarious molecules, like mycotoxins, which can lead to allergies and infection. But koji is different, producing a brilliant balance of enzymes with no harmful outputs. It is a very friendly fungus.

Specifically, koji’s most important enzymes are protease and amylase, capable of breaking down proteins and starches.When koji’s protein-cutting proteases go to work, they chop up long proteins into individual amino acids. These are essential nutrients for humans, especially the 9 “essential” amino acids which we must get from our food. This is also the source of koji’s great power as a seasoning: one of the amino acids product is glutamic acid, otherwise known as glutamate. This is the major driver of the ‘umami’ flavor we associate with red meat, tomatoes, and mushrooms, and is why koji-based foods like miso or soy sauce are regarded as such savory ‘umami bombs.’

The starch-chopping amylases similarly break up gigantic starch deposits into smaller sugars, primarily glucose, maltose, and maltotriose. Amylase activity therefore makes kojified foods taste remarkably sweet, like in the traditional Japanese rice porridge ‘amazake,’ a mix of koji rice, water, and addition irce held at a warm temperature for the amylases to maximize the sugar production.

This is all a scientific description of something much more poetic: koji allows cooks to take an ingredient and reimagine its building blocks, looking anew at the ‘body’ of a bean or potato or chicken breast. Koji lets the cook switch from Monet, with thick smears of expressive paint onto the canvas, to Rothko, exploring and rewiring the hulking background itself.

Consider the soybean. Protein, starch, oil, fiber, minerals. Not at all unpleasant when cooked, but certainly a simple stable crop: inoffensively grassy and gently ‘beany.’ Now, let koji grow on its surface, and the mealy starch and proteins are transformed into a sweet bouquet above a profoundly deep, umami backbone. A dried bean with a bit of water, koji, and time taste like they’ve been meticulously hit with a crumble of parmesan, a drizzle of brown butter, and a reduction of pan-fried mushrooms.

It’s truly a transformative kitchen tool, and well worth exploring. While the idea of growing mold on your countertop is understandably daunting, there is now a range of koji producers in the US and Germany selling koji grown on rice and then dehydrated. These dehydrated grains preserve koji’s enzymes, meaning you can use them safely and easily at home for most any koji application.

This all means the potential described above is yours to unlock — with simple applications like these and pre-made koji rice, you too can perform a little alchemy with this magical little mold ✨