Soak your chosen beans in plenty of cold water overnight. Drain the beans and put them in a combi-oven or steamer on full steam for approximately 1 hour or until the beans are soft to the touch.
Remove the beans from the steamer and place them on a sheet of clean, damp muslin (cheesecloth) in a single layer. Allow the beans to cool to approximately 35°C (95°F), checking with a temperature probe. Working quickly and wearing a face mask so as not to inhale any koji spores, mix the spores and rice flour together and then use a fine sieve to sift the mixture evenly over the still-warm beans. Mix the beans thoroughly, ensuring they are evenly coated with the spores and flour mixture. Bundle the beans into the centre of the muslin and fold over the cloth to form a snug wrapping around the beans. Wet a clean tea towel (dish towel) and wring it out well. Wrap the bean parcel in the damp tea towel then wrap a clean dry tea towel around the outside.
Put the parcel into an incubator set at 35°C (95°F) for 24 hours. Alternatively, you can put the parcel into a styrofoam container with a tight-fitting lid. Put a water bottle filled with 35°C (95°F) water next to the beans. Secure the lid and place the box in a warm part of the kitchen. If using the water bottle method, you will need to keep a check on the temperature and replace the water bottle 3–4 times over the 24-hour period.
After 24 hours, remove the beans from the incubator or styrofoam container and open up the package. You should start to see a whitish, fragrant sweet-smelling mould developing on the beans. If so, you are on the right track. If not, discard the beans and start the recipe again.
Spread the beans out in a single layer onto a tray lined with clean damp muslin. Cover with another layer of clean damp muslin and return the beans to a 35°C (95°F) incubator or styrofoam box with a water bottle for a further 24 hours. After this period, the beans should be covered in a fine whitish mould and the smell should be sweet, fragrant and slightly yeasty, like freshly baked bread. The smell and colour is the best indicator that you have successfully grown the correct mould. If the smell is not sweet and the mould has other coloured hues in it I would recommend discarding the batch and starting again.
The white koji mould, if incubated for longer than 48 hours, can start to form greenish–yellow patches that indicate sporulation of the mould. Although it's not necessarily dangerous for use in this particular recipe, it would be preferable to start the batch again. If the mould is white and smelling sweet, proceed to the next step.
Dry out the koji-inoculated beans in a dehydrator or low oven set at 80°C (175°F/Gas ¼–½) for approximately 1 hour or until the beans start to shrivel a little. Make a brine by combining the filtered water and salt, then mix until dissolved. Place the beans into a cryovac bag and pour in the cold brine. Vacuum seal the bag on medium pressure. Put the bagged beans in the pantry and leave them at room temperature—approximately 23°C (73°F)—for 1 week. Transfer the beans to a cool dark place, such as a cellar, at about 15°C (59°F). If this is not possible, place in the refrigerator. The beans will have enough flavour to use after a 3-week waiting period; however, as long as they are kept in the sealed bag with the brine in the refrigerator they can be stored for up to 6 months.
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