In the UK the term is "soya" is commonly used whereas in the US, "soy" is more commonly used. They mean the same thing.  

According to the Japanese, the five basic tastes are enmi (salty), kanmi (sweet), sanmi (sour), nigami (bitter), and umami (savory). The latter is not a taste in western organoleptic description. The utility of soya sauce in gastronomy is that it imparts umami unto foods to which it has been added. While soya sauce is a relatively inexpensive culinary ingredient and easily obtainable, it is fun and satisfying to brew one's own "boutique" soya sauce. This is a report on my first attempt at brewing it.

It is relatively easy to make your own soya sauce. It is produced via fermentation which is  the biochemical action of microorganisms on a substrate(s). All soya sauce fermentation has two separate stages known in Japanese as the koji (solid) and moromi salt stages. Only a few ingredients are required for soya sauce production. The magic of microbial metabolism does the rest.

The ingredients which I used:

Australian sea salt

First (Koji) Stage

Solid Substrate  fermentation

"Solid" refers to the fact that the substrate is relatively dry. The substrate is exclusively soya beans if the traditional Korean method is followed, but plus roasted wheat berries if the Chinese or Japanese method is followed. I followed the Chinese/Japanese method.

Method

• Note: The usual proportion of soya beans to wheat berries is 1:1.
  However, I used 2 parts of soya beans to one part wheat berries for this brew.

• Soak about 1 kg of soya beans overnight in clean fresh water.
  Change the water as necessary through the hours. The beans imbibe water during the soaking and become easier to cook.

• Drain the beans and place in a pot with sufficient water to cover them.  Boil until they are soft (will squash when pressed).
  This can take several hours.
  Alternatively, you may wish for a speedier cook via a pressure cooker.
  At a setting of 15 pounds per square inch (121 degrees centigrade), the beans are soft after 10 minutes.
  Do not over cook until the beans turn mushy.

Pressure cooked soya beans

• Drain the beans and allow them to cool to room temperature.

• Dry roast/toast 500 g of wheat berries until they are brown and crispy.

Roasted/toasted wheat berries prior to grinding

The roasting imparts a darker color to the resultant sauce and a different flavor profile to soya sauce which is made from only beans. Indeed, after toasting, the berries give off a nice aroma.

Grind the berries into a rough meal.

• Mix the cooked beans and roasted wheat meal together.
  The ideal preparation is moist without free water evident.
  Moisture is required for the fermentation process but too much water will create anaerobic conditions which is not desirable in this stage.

• Add 200 g of koji starter inoculum to the bean/meal mix and incorporate well.
  Koji starter comprises cooked soya beans upon which the fungus Aspergillus oryzae or Aspergillus sojae is allowed to grow.
  It serves as inoculum to get your freshly prepared substrate populated with the desired fungus.
  I ground the koji starter prior to adding. This allows for better distribution of the fungal inoculum.

Beans, ground wheat and koji starter on Day 0
 

• Cover the substrate tray with a moistened cloth and set it aside to incubate in a quiet corner of your kitchen.

• Check the tray daily and turn the mixture over.

The koji at Day 3

• During koji fermentation, the fungus breaks down bean and berry carbohydrates and proteins into complex smaller molecules (short chain saccharides, sugars and amino acids).
  This fungal colonization and breakdown of the beans and wheat substrate is accompanied by the evolution of heat from metabolism (known as "heat of fermentation"). During the first day of my koji preparation, I measured a temperature of 45 degrees centigrade in my fermenting bean/wheat. I used a fan to circulate air to dissipate heat from the tray. Overheating the koji may cause the fermentation to cease (the microbes die).
  A good koji fermentation is when the fungus converts as much of the initial substrate as possible to the desired end-products.

• The koji stage is complete when the substrate has a good coating of light colored fungus. This may be accompanied by the observation that the koji no longer heats up.
  The completion of this stage can take up to seven days.
  In my case, by the end of Day 5, the koji remained at room temperature.
  Note:

  • You should stop the koji stage before the batch begins to color in a major way. Fungal mycelium is usually white but as the fungus sporulates, it takes on the color of the spores of the fungus (yellow; light green).

  • You should retain some of this koji for use as inoculum in further production batches.
    The portions kept for use as inoculum can be allowed to sporulate as spores are good propagules and will remain viable for longer than vegetative mycelium.
    The koji can be tightly sealed in a Ziplock bag and stored kept in the refrigerator.
    Under the right conditions (dry and cold), the fungal mycelium/spores in the koji will keep indefinitely.

• I added an extra step of mashing the koji (quite soft in my case) to create a semi-paste with the rationale that mashed rather than whole beans or large chunks would be more amenable to degradation by the enzymes produced by the koji microorganisms. Mashing the koji will also make the substrate easier to be fermented by microorganisms in the moromi stage. The mashed up koji was left for several hours to hydrolyse before moving it on to the moromi stage.

  
  
  Mashed up koji at the end of Day 5

The second (moromi) stage