Rinse the soybeans and cover them with water in a large glass jar or another suitable container. Keep in mind that they will double in size when they absorb the water.
Soak them overnight, or for 12- 16 hours. (Add more water, as needed, to allow them to fully hydrate.) Notice in the photo that they have doubled in size once hydrated.
Drain the soybeans and cook them by steaming them in a pressure cooker for an hour and a half, or cook them in boiling water for around 6 hours. A slow cooker is a good option in that case. I used a pressure cooker and poured water in the bottom of the pressure cooker. I then put a stainless steel steamer basket over the water and filled the basket with the soaked soybeans. I set the pressure cooker to cook for an hour and a half.
When fully cooked, you should be able to easily crush the soybeans with your fingers.
Prepare the wheat
Brown the wheat berries in a skillet for several minutes until they turn golden brown. You can also broil them in the oven, spread over a baking sheet, stirring every couple of minutes to prevent burning, if you prefer.
Grind the wheat coarsely using a food processor, grain mill, or another type of grinder. You are aiming to break each grain into several pieces. You don't need to completely pulverize the wheat berries into powder.
Preparing the koji
Mix together the ground wheat berries and the cooked, drained soybeans in a large mixing bowl. Allow the mixture to cool.
Sprinkle a koji starter over the wheat and soy mixture and thoroughly incorporate it into the mixture.
Spread the inoculated mixture over glass or stainless steel trays or bowls. (Use something with a broad base so that you can spread out the shoyu koji.) Make furrows (small valleys) in the mixture every 2 inches or so to avoid having thick layers where hot spots emerge. (Hot spots are areas where the mold gives off too much heat.)
Place a thermometer in the koji and cover the trays with either a plastic wrap or a tight weave cloth to help keep the moisture inside. Incubate the trays at around 85ªF (32ºC). (I used my oven with the oven light on to keep the temperature slightly elevated. You can also use hot water bottles or ice cold water bottles under the trays in an ice chest to raise or lower the temperature as needed.)
Check the koji temperature every few hours to make sure it is staying in the 80-95ºF (27-35ºC) range. If the temperature gets too high for too long (over 104ºF (40ºC), unwanted bacteria can grow in the koji and spoil it. Each time you check on the koji, stir it, breaking up clumps as needed, and spread it out and add the furrows back in before covering and incubating again.
After 2- 2.5 days
Continue to monitor the koji for 2 to 2 and a half days. You'll notice that a fuzzy, soft white mold will cover the koji.
It's OK for some areas to begin to turn yellow or even a light yellow-green color. The yellowish green color is the formation of new koji spores. (Discard any koji that has grown black, or dark green mold. You also want to avoid any koji with shiny or sticky spots.)
Prepare the brine solution
In a large glass container (around 2 gallons), mix together 1 gallon of filtered or spring water with 3.5 cups of sea salt. Stir until the salt has fully dissolved.
Prepare the moromi
The moromi is the mixture of the koji and brine solution. Stir the koji into the brine solution and cover the mixture with a tight fitting lid. Label it with the date.
Keep the moromi in a warm spot to continue to ferment. Stir the mixture daily for a week or so. After than, keep it in a warm spot (ideally around 77ºF (25ºC)), stirring it at least once a week for around 6 months.
After around 6 months, you'll notice that the color of the moromi will have darkened into a deep rust brown color. It may be either have separated into a liquid and solids (like mine) or be more or a homogenous thicker mixture.
Straining the moromi
After 6-12 months fermentation, you'll want to strain the moromi to obtain your homemade shoyu. The easiest way is probably to pour it into a cloth-lined strainer and pour some of the mixture into the strainer. You can then twist the cloth to press the filtered liquid through the cloth into a bottle.
After around 9 months, around 3 months ago, I strained mine and it was a light, rust-colored liquid that tasted and smelled like soy sauce, but was much lighter in color.
I was making homemade tamari at the same time (I'll share that process soon- or as soon as I'm happy with the result), and this shows the 2 bottles I strained out 3 months ago. (I left the rest in the jar to continue fermentation there with the soybean and wheat mixture.)
Obtaining a dark-colored soy sauce
One thing I had learned from my first failed attempts at making soy sauce was that one of the things that helped achieve a dark-colored soy sauce with complex flavors was to allow the sauce to ferment in the hot sun. So, I placed both bottles (of soy sauce and tamari) out in the hot sun from the end of June until the end of September. I was happy to see that both sauces had darkened with time out in the sun.
You'll notice that after 3 months outside in my sunny terrace, not only did they darken, but they also separated slightly leaving a bit of soy residue floating on top of the bottles. I strained the mixtures to obtain my sauces.
I filtered the soy sauce again through a cloth to remove any soybean residue. The resulting liquid was a wonderfully dark colored shoyu with wonderful flavor.
Calorie information is likely elevated due to the fact that the nutritional information is based on the ingredients, but you are straining out the soybeans and wheat rather than actually consuming them.