How is Soy Sauce Made?

At the heart of many an Asian dish is the deeply flavoured, ebony brown liquid called soy sauce. If you’ve only ever thought of it as an alternative to salt, then you’re missing out on an incredible seasoning that can introduce complex and savoury depth to your marinades, sauces or soups. But how is soy sauce made?

What is soy sauce?

Soy sauce is actually a generic term for a type of condiment with a startling variety of textures, tastes and appearances that reflect the culinary cultures of their origins. You could go from country to country in Asia, only to find that soy sauce in one place tastes completely different to the soy sauce in another.

Aerial view of workers stirring soy sauce in the traditional jars at a drying yard on July 21, 2024 in Rugao, Nantong City, Jiangsu Province of China. (Photo by Xu Hui/VCG via Getty Images)

The origins of soy sauce

That being said, the most well-known soy sauces come from Japan or China, where the tradition of brewing has gone on for hundreds of years – sometimes over generations of a family.¹

Europeans have taken an interest in soy sauce from as far back as the 17^th century when Dutch traders exported the condiment from Japan to their various outposts. It seems that soy sauce was known in Europe even before soybeans, as the name for soy comes from the Japanese word for soy sauce, shoyu.² Since then, soy sauce has grown in popularity as Asian and fusion cuisines spread to other shores.

What is soy sauce made of and how is it made? 

Traditional soy sauce brewing

Traditional soy sauce brewing involves four main ingredients: soybeans, wheat, salt and water. Fermentation is an important part of the process and the longer the soy sauce ferments, the deeper the resulting flavour becomes. Brewers can take months or even years to produce a batch of high-quality soy sauce.³

At the start of the process, soybeans are steamed, and wheat is roasted and then crushed before both are mashed together. This mixture is then inoculated with a specific strain of mould, usually Aspergillus oryzae or Aspergillus sojae, to enable fermentation. At this point, it forms a mash called a koji.⁴

The koji is spread out on wooden trays and left to ferment for 2-3 days, and it is during this period of fermentation that starches are broken down into simple sugars, proteins into amino acids, and oils into fatty acids.⁵ One of these simpler molecules is glutamic acid, which is the amino acid found in Parmesan cheese and mushrooms that give the soy sauce its umami flavour. The koji also has to be mixed frequently to release the heat produced during fermentation, as the surrounding temperature can greatly affect the entire batch of koji. In some traditional breweries, it was necessary to have a system of air vents, heaters and water boilers to control the overall humidity and temperature of the room while the koji fermented.⁶

The next stage is brine fermentation. Here salt and water are added to what was the koji, to make a mixture called moromi. A starter culture is also mixed into the moromi to introduce strains of lactic acid bacterium and yeast that will kickstart the second stage of fermentation.⁸

Long-established breweries will have cultivated their own strains of lactic acid bacterium and yeast that give the soy sauce they produce its own unique fragrance and flavour. This is why, like wine and cheese, every bottle of soy sauce differs according to where, when and how it was made.⁹

After months or years of fermenting, the final moromi becomes a thick gooey texture with the pungent, yeasty aroma typical of fermented foods. It is then time to refine the soy sauce by pressing layers of moromi in cloth-lined containers to extract the raw soy sauce from the solid mash. The raw soy sauce is then pasteurised by heating it to a high temperature before it is bottled and ready for consumption.

The soy and the wheat are colonised by the koji mould, that will break down the proteins in the beans, giving the soy sauce its typical umami flavour.

Modern soy sauce brewing

While traditional brewing of soy sauce has been perfected over generations, it is in steady decline amongst manufacturers as newer methods for quickly producing soy sauce have been invented to meet the high global demand for it. One of these modern methods is acid hydrolysis, which does not involve any fermentation and thus cuts down the production time for soy sauce from months to merely days.

With this method, defatted soy meal is boiled under high pressure with hydrochloric acid to rapidly break down the soy proteins, then the mixture has to be neutralised with sodium hydroxide before it is filtered and refined. Because acid-hydrolysed soy sauce has not undergone fermentation, it does not develop the aromatic compounds like esters, alcohols and carbonyl compounds that give soy sauce much of its zest and perfume; so instead, additives like salt, caramel, and corn syrup are added to improve the appearance and taste of the soy sauce.¹⁰ It is this cheaper and long-lasting type of soy sauce that you usually find in small packets alongside pre-packaged foods.¹¹ Manufacturers also sometimes combine acid hydrolysis and fermentation methods to produce an affordable sauce with some of the authentic flavours of long-brewed soy sauce.

Soy sauce is one of the oldest condiments in the world, and the process of brewing it is steeped in tradition as flavourful as the seasoning itself. It’s a staple, an essential component to whole cuisines – and it’s most definitely not just a replacement for salt.