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  • Koji can turn bitter vegetables into delicious bites. Photo: Jonas Drotner Mouritsen

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Koji works wonders in the kitchen: How to get tender meat and crunchy vegetables full of umami

Article The Japanese koji can turn bitter vegetables into delicious bites. Try, for instance, to add koji to cabbage and store it in the fridge for a few days to experience the magic.

The Japanese kitchen has a well-preserved secret which can do magic to bitter vegetables and lean meats which easily become dry:

Koji.

The taste of food prepared with koji (pronounced koh-dji) is complex, sweet, and filled with umami – the basic taste which gives the much-wanted rich, savoury taste.

A mould and a tiny miracle

The strong effect koji has on food is caused by the mould Aspergillus oryzae. Koji is made from boiled soybeans, rice, barley or other grains that are grafted with the spores of the mould.

The spores then sprout in the mass of grains and form a mycelium. This has to grow for three days in a temperature of around 30˚C at high humidity. The completed koji holds a large number of enzymes that can break down proteins and carbohydrates. This is the process that produces the tastiness. More about the enzymes later – let’s first have a taste.

Koji has been used in Asia for hundreds of years, e.g. in the production of soy sauce (shoyu), sake, miso, and the traditional crunchy tsukemono-vegetables. The koji itself tastes sweet-salty-fermented but it is especially the function of koji in several processes in the kitchen that is appreciated by chefs: 

Koji is able to make meat and poultry tastier and more tender. Koji can be used for fermenting and acidify e.g. vegetables. Not least, it can be used to spice or enhance the taste in more or less everything. 

In Denmark koji is not yet widespread. However, it has gained some attention due to its ability to bring about small enzymatic miracles on especially bitter and strong-tasting food such as cabbages which naturally have an abstruse taste with a lot of bitterness and a coarse texture. 

How to turn bitter cabbage into delicious bites

The chef Klavs Styrbæk and the gastrophysicist Ole G. Mouritsen have been experimenting with adding koji to different ingredients: fish, meat, and vegetables. 

In principle, the taste of most vegetables would benefit from being prepared with koji. However, the biggest difference happens with the bitter vegetables. Try, for instance, to see how raw broccoli, broccolini, different kinds of cabbages, radish and carrots or other root crops become significantly sweeter and more umami by addition of koji – without adding calories. 

Below is Klavs Styrbæk’s approach to preparing vegetables with koji. Try it: 

  • Rinse approx. 200 grams of vegetables and cut them into smaller pieces. Pick a size that fits the people who are meant to eat them. 
  • Put the vegetables in a bag with a spoonful of koji. 
  • Close the bag by holding on to the corners and quickly fling it around to close it with air around the vegetables.
  • Shake the bag a bit to make sure the koji covers all the vegetables.
  • Let the bag infuse in your fridge for 2-3 days (max 5 days).
  • Take out the vegetables and eat them.

According to Ole G. Mouritsen, koji-treated vegetables are well-suited as a starter/snack or as cold accompaniments to fish or light meat dishes without sauce. The koji-marinade can be used as a light sauce, e.g. with boiled potatoes. You don’t need to make Asian-inspired meals to get the koji to match the meal – koji works just fine with a Danish meatball as well.  

Enzymes release the taste 

But what actually happens in the food? Ole G. Mouritsen explains: 

The vegetables in the bag with the koji ferment. The Japanese call the result koji-zuke.

Aspergillus oryzae, the mould in koji, releases different enzymes that catalyse the breakdown of the food’s larger molecules into smaller components that give rise to taste.

Vegetables get a sweet taste because koji releases the enzyme amylase which converts the starch from the vegetables to sugar. 

The mould also releases other enzymes, proteases, which break down the food’s proteins and create tasty free amino acids, among others glutamate – and voila: The taste of umami fills your mouth. 

Lipases from the mould break down the fat in the food.   

Overall, the koji not only gives the food a better and more complex taste, it also makes the food easier to digest. 

How to start using koji 

Several kinds of koji exist. Shio-koji and ikitai shio-koji can be bought on glass or in bags in Asian supermarkets. You can put these kinds of koji directly in your meal. 

Shio-koji (shio = Japanese expression for sea salt) is a turbid and cloudy, thick fluid or paste consisting of a salty substance of malted rice, containing enzymes from koji. This ingredient is not suited for fermenting fish or meat that are meant to be roasted later, because the paste will leave a burned crust. Therefore, use ikitai shio-koji instead. This is filtered and more fluid but still contains the active enzymes from koji.

Shio-koji can be mixed with soy sauce and used for different variants of shoyuzuke, meaning food marinated in soy sauce. A mix like this, shoyu-koji, has as durability of 1 year in the fridge and can easily be used for marinating different vegetables, mushrooms and meat. 

Shoyuzuke combines the taste of salt and fermentation products from the koji with the strong taste of umami from the soy sauce.

If you want to start your own koji-culture you need Koji-kin. This is a product made of dried rice grafted with spores of Aspergillus oryzae. It is a laborious process but if you want to try, you can read more in the fact box below.

Koji-kin

The traditional way to start a koji-culture is based on koji-kin – dried rice grafted with spores from the mould Aspergillus oryzae. The grafted rice is mixed with boiled rice and the culture develops in the mixture which is kept hot for around 24 hours. The result is an amazake (sweet sake) which is drinkable as a hot drink diluted with water. Furthermore, it can be used to make koji-zuke. The texture of amazake is similar to a soft porridge. 

To be able to use amazake to make koji-zuke, salt needs to be added and the mixture has to develop through 1-3 weeks while the mould ferments the rice. During this process, the substances need to be regularly stirred. Now you have a koji starter culture which can be used to make koji-zuke by mixing it with vegetables. You can keep the starter culture in a fridge for several months.