Samuel Wells Williams, the great Sinologist, wrote in 1848 that the best soy he tasted in China was “made by boiling beans soft, adding an equal quantity of wheat or barley, and leaving the mass to ferment; a portion of salt and three times as much water are afterwards put in, and the whole compound left for two or three months”.
The soy beans are soaked and boiled, mixed with the fungus Aspergillius oryzae, and left in brine until the starches in the soy are released, yielding a black liquid.
The next stage is the pressing. As with oil, the first press is considered to give the best, and least overbearing, flavour. The light soy produced is thinner and used for dipping and finishing. It's the stuff you get with sushi. Traditionally there was a thicker second press, which is sometimes still seen, though hydraulic presses have largely obviated it these days, scraping everything that's left after the first press into a common pot of darker saltiness.
To be both useful and delectable is a rare combination – but soy sauce manages it. It is seasoning, sure, but it is not just salt to be dispensed, it also has a rich umami hit, that platinum-coated characteristic of the best Asian foods. And with a bottle knocking about in the back of the fridge, your dinners – and your desert island – will never be boring.