How We Make Soy Sauce And What Grades Are There ?

Soy sauce in a bottle with spout and a jar of soy paste.
c/o Pixabay.

Soy sauce is a traditional seasoning with a salty, almost meat-like taste and a very sharp flavour. It is a light brown to black liquid condiment often used to flavour noodles, stir-fries and rice dishes.

The process first evolved as a traditional brewing technique that was developed in China and Japan many millennia ago. It is probably the most popular sauce in Asia and one which has great global appeal. In Japan soy sauce is known as shoyu which can be seen on many product labels

Many varieties of shoyu are produced in Japan. Unlike China, they use what is called a koikuchi type of soy sauce.

The traditional and more ancient process of manufacture involves cooking soybeans with roasted wheat which are mixed with the spores of Aspergillus molds. The batch is fermented as a solid culture state for 2 days to produce a mass called koji. The koji is mixed with a salt solution or brine to produce moromi which is a mash that is then fermented further to produce the product we know and love called soy sauce. The more modern methods use chemical preparations and advanced biotechnology to achieve almost similar results.


We tend to think of soy sauce hazily as an old fermentation tradition but it began in China a staggering 2,500 years ago. It is probable that salt which was known to produce a particular sensation was too expensive and simply not available. One way to extend salt’s properties and its use was to ferment it with soy beans and fish which were certainly prevalent at that time.

In time, soy sauce became very much associated with the Chinese Buddhist tradition. It serves to this day as a strong flavouring ingredient that could enhance the flavour of bland vegetarian diets that existed at that time.

The Process Of Manufacture

There are two types of fermentation process to consider: the conventional and older tradition, and the bioreactor process which is a three-stage fermentation using immobilised enzymes and cells. The chemical soy sauce process uses the acid hydrolysate of vegetable proteins and has only been employed since the mid-1900s. The fermented soy sauce process has been refined over centuries and is now produced in an advanced state.

The Conventional Method Or Traditional Fermentation Process

The conventional method is a 2-stage fermentation process. In Japan it is known as the honjozo process.

Soybeans or defatted soy flakes are soaked, then steamed and mixed with roasted or parched wheat. A starter culture is added to produce the koji. This starter culture is composed of seed spores of the fungi Aspergillus sojae or A. oryzae, or a combination of both. It takes 2 days to produce a solid-state culture.

The koji is different depending on the region and country in which it is produced. Each strain produces its own type of flavour to  soy sauce and indeed is responsible for the great variety of flavours obtainable.

Salt solution is added to the koji to produce a moromi mash. The mash is fermented and involves a variety of enzyme hydrolysis reactions. The enzymes involved include proteinases (proteases), peptidases and amylases. In this process lactic acid and alcohol are produced over a period of 6 to 8 months. The proteases convert protein in the kojui to smaller amino acids and peptides which in tuner are converted to various flavour compounds.

If we look more closely at the fermentation we can see a number of yeasts are involved in this process. During the first stage of the moromi fermentation, a bacteria Pediococcus halophilus thrives and produces lactic acid which lowers the pH. As the pH decreases other yeasts and bacteria predominate. The yeast, Zygosaccharomyces rouxii grows producing alcohol to about 2 or 3 per cent. Yeasts such as Candida versatilis (aka Torulopsis versatilis) and Candida etchellsii also create a number of flavours including phenolics like 4-ethylguaiacol and 4- ethylphenol. These are two of the characteristic aromas in soy sauce.

The soy sauce is allowed to age. Filtration of the whole batch follows. The soy sauce is pasteurised and then packed in bottles to produce a soy sauce.

The Bioreactor Method

Defatted soybean is steam cooked and combined with roasted wheat. A culture broth from a continuously produced collection of koji mould is added to start fermentation. The whole batch is allowed to hydrolyse at 40 to 50 centigrade for 3 days. This creates the equivalent moromi mash. The moromi mash is filtered. Salt is added to the  raw liquid or filtrate. The whole mixture is passed through a bioreactor of immobilized glutaminase, then immobilised lactic acid bacteria and Pediococcus halophilus, and immobilised yeasts such as Zygosachharomyces rouxii and Candida versatilis. Once the fermentation is complete the whole batch is pasteurized and bottled as soy sauce.

The glutaminase reactor produces glutamic acid from the amino acids glutamine in the moromi ferment. In the second part of the reaction, the P. halophilus performs the lactic acid fermentation and then in the third and final part of fermentation, the Z. rouxii produces the alcohol.

The bioreactor method has been significantly developed in Japan. It is a quicker process than the conventional method for a start and this has approach has been protected in intellectual property for a number of years (Noda et al., 1982; 1983). The immobilisation process using whole cells has been investigated by Osaki et al., 1985

In Japan, the colour and flavour of shoyu depends on how long the mash is aged and the degree of pasteurisation. This colour attribute is very important in Japanese dishes. In recent years soy sauce has become lighter in colour moving from the dark brown of dark soy sauce to a lighter variant used in more general cooking.

The Componentry In Soy Sauce

The main flavour enhancer is monosodium glutamate (MSG) which is associated with the fifth flavour sensation, umami. Umami is a category of taste found with meat broths and fermented foods. In Japan, exceptional sensory work coupled to analysis of the chemical compounds has led to unparalleled understanding of what makes a soy sauce. Aishima (1983) described the use of multivariate analysis coupled to gas chromatography of soy sauce volatiles to obtain solid understanding of the flavour profile. Since then a number of other techniques have been used such as solid phase microextraction coupled to gas chromatography and olfactometry using sample dilution analysis (Baek and Kim, 2004).

The aroma and flavour compounds are  4-ethylguaiacol and 4- ethylphenol (Yokotsuka et al., 1980). In Japanese soy sauce or shoyu, about 20 Japanese researchers have identified at least 130 flavour compounds relevant to the flavour.

Amino acids provide many of the volatiles suing koji fermentation. They contribute to nearly 50% of the total volatiles. These compounds are mainly aldehydes, fusel alcohols and acids. The compounds are:-

  • Aldehydes: 2-methyl-propanal, 2-methyl-butanal, 3-methyl-butanal, benzaldehyde, benzeneacetaldehyde,
  • Fusel alcohols: 2-methyl-propanol, 2-methyl-butanol, 3-methyl-butanol,
  • Acids: 2-methylbutanoic acids, 3-methylbutanoic acid,
  • 3-(methylthio)-propanal.

Branched short-chain aldehydes (2-methyl-propanal, 2-methyl-butanal and 3-methyl-butanal) and their corresponding alcohols (2-methyl-propanol, 2-methyl-butanol and 3-methyl-butanol) and acids (2-methylbutanoic acids and 3-methylbutanoic acid) were mainly produced from branched-chain amino acids via the Ehrlich pathway through the action of various fungal enzymes during fermentation (Chung et al., 2005).

Soy sauce is almost 20 per cent by weight of salt which makes it unsuitable for anyone with kidney issues. However, soy sauce is used to reduce the total salt content in food because it contains sufficient flavour to impart saltiness to a food. It is possible to reduce added salt by between 33 and 50% in foods when soy sauce replaces added salt during food preparation. If people are already familiar with soy sauce then the addition of the condiment to foods might help with its employment in foods.

Grades of Soy Sauce

When you look on the supermarket shelves there are different grades of soy sauce. Japan produces five different types:-

  • Koikuchi-shoyu (regular soy sauce)
  • Usukuchi-shoyu (light coloured soy sauce)
  • Tamari-shoyu
  • Saisikomi-shoyu
  • Shiro-shoyu.

Each type is validated by a Japanese Agricultural Standard (JAS).   The types are classified in three grades: special, upper and standard. Sensory assessment and evaluation, total nitrogen and alcohol content, soluble solids, salt and colour are all measures used in the classification.

The special grade is only given to soy sauce made by fermentation. No chemical or enzymic hydrolysates are permitted for this type of soy sauce. In Japan since 1986, nearly 75% are produced by pure fermentations. The remainder are 22% using a part chemical process outlined above and the remainder are mixed products using purely chemical soy sauce.

Koikuchi (濃口, “thick taste”): The soy sauce originated in the Kanto region and is now popular throughout Japan. It is regarded now as the classic Japanese soy sauce. It is prepared from equal quantities of soybean and wheat. The unpasteurized form is called kijōyu (生醤油) or namashōyu(生しょうゆ).

We can buy Koikuchi which is a special grade of sauce that is prepared by the traditional fermentation process. About 81% of the annual sauce production in Japan is koikuchi. It is prepared from equal parts of soybeans and wheat with no other additions. It is described as having an exceptional aroma with a complex and sophisticate flavour and a deep red-brown colour.

There is a unique soy sauce called in-yu produced from black beans, wheat and rice which is fermented by the traditional method. I have not seen this particular product in the UK. Soy sauce is also graded on its colour intensity which is measured spectrophotometrically at 555nm. The colour intensity of light soy sauce is less than 3 according to this method whilst a standard colour is higher than 19.

Usukuchi (薄口, “thin taste”) soy sauce is prepared from soybeans only or with a small amount of wheat added. In the mixing of these ingredients, the addition of steamed rice or saccharified rice is an option designed to reduce the colour producing reactions that occur in the soy sauce.

You may see Tamari or supernatant juice soy sauce which is also only made from soybeans.

Soy Sauce Paste And Low-Sodium Types

Soy sauce paste is one that has a thickener added to it to achieve a viscosity value of 250 cps at 25ºC. These pastes include ‘in-yu and ‘pot-bottom’ sauce.

Low-sodium soy sauce – this type of soy sauce has been created for those of us seeking to reduce our overall sodium content. Even though salt is an important part of the manufacturing process it also serves as an antimicrobial agent. Most low sodium soy sauces are produces with acid-hydrolyzed vegetable proteins. These do not need either bacterial or fungal cultures and thus less salt. In some cases, the salt is removed by electrodialysis which is a relatively new process.

Storage Of Soy Sauce

Soy sauce in in unopened bottles is shelf-stable but is ideally kept in a cool, dark place to reduce any changes. On opening, soy sauce is best kept refrigerated to retain flavour and stability. Fortunately the high salt content in soy helps preserve this condiment and prevents pathogens from growing.


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