The Art of Soy Sauce Making

Soy sauce or, soya sauce, is a staple ingredient in many global cuisines and it represents a fascinating blend of tradition, craftsmanship, and flavor. Originating in ancient China over 2,500 years ago, it has transcended cultural boundaries to become a seasoning present in kitchens worldwide. Its deep umami flavor, rich history, and adaptability makes it a vital component in the culinary arts. 

Production

Although there are some variations in making different types of soy sauce, their basic steps are the same, including treatment of raw materials, koji making, brine fermenting, pressing, and refining. In the traditional method, it usually takes months to prepare and achieve that rich umami flavor, whereas there are more modern preparations methods which take a significantly lower time than the traditional method.

1. Soaking and cooking: The soybeans are soaked in water and boiled until cooked. Wheat is roasted and crushed.

2. Koji culturing: Equal amounts of boiled soybeans and roasted wheat are mixed to form a grain mixture. A culture of Aspergillus spore is added to the grain mixture and mixed, or the mixture is allowed to gather spores from the environment itself. The cultures include:

• Aspergillus: a genus of fungus that is used for fermenting various ingredients (the cultures are called koji in Japanese). Three species are used for brewing soy sauce:

A. oryzae: Strains with high proteolytic capacity are used for brewing soy sauce.

A. sojae: This fungus also has a high proteolytic capacity.

A. tamarii: This fungus is used for brewing tamari, a variety of soy sauce.

Saccharomyces cerevisiae: the yeasts in the culture convert some of the sugars to ethanol which can undergo secondary reactions to make other flavor compounds

Other microbes contained in the culture:

• Bacillus spp. (genus): This organism is likely to grow in soy sauce ingredients, and to generate odors and ammonia.

• Lactobacillus species: This organism makes a lactic acid that increases the acidity in the feed.

In traditional koji-making, the inoculated mixture is put into small wooden trays and kept for 3 or 4 days in a koji-making room. During mold growth, the temperature and moisture are controlled by manual stirring. In modern koji-making, however, the cultured mixture is put into a shallow, perforated vat and kept in a koji room where forced air is circulated and temperature and humidity are controlled (as in the case with an automatic koji-making system). After about 3– 4 days, when the mixture turns green-yellow as a result of sporulation of the inoculated mold, it becomes mature koji.

In the early stage of koji making, temperatures as high as 30–35°C are preferable for mycelium growth and prevention of Bacillus as a contaminant. In the later stage, just before spore formation or after the second cooling, a lower temperature (20–25°C) is necessary to allow maximum enzyme production. Alternatively, koji may be prepared at a constant low temperature of 23–25°C for relatively longer times (66 h). In any cases, when temperature rises to >35°C due to active mold growth, it is advisable to cool the koji material twice either by hand mixing or a mechanical device.

3. Brewing: The cultured grain mixture is mixed into a specific amount of salt brine for wet fermentation or with coarse salt for dry fermentation and left to brew. Over time, the Aspergillus mold on the soy and wheat break down the grain proteins into free amino acid and protein fragments, and starches into simple sugars. This amino-glycosidic reaction gives soy sauce its dark brown color. Lactic acid bacteria ferments the sugars into lactic acid and yeast makes ethanol, which through aging and secondary fermentation makes numerous flavor compounds typical of soy sauce.

To go in depth, mature koji is mixed with an equal amount or more (up to 120% by volume) of a salt solution to form the liquid mash known as moromi in Japan. The final salt concentration of the mash should be 17–19%. Lower salt concentration allows growth of undesirable putrefactive bacteria during subsequent fermentation and aging; however, higher salt concentration (>23%) may retard the growth of desirable halophilic bacteria and osmophilic yeasts. In the home, the mash is put in an earthen crock and fermented at ambient temperatures. In this case, a period of 10 –12 mo may be necessary to complete the brine fermentation stage. On an industrial level, however, the mash is kept in large wooden containers or concrete vats with aeration devices. The temperature of their surroundings can be mechanically controlled. Thus, fermentation time can be shortened.

Temperature is also an important factor during brine fermentation. In general, the higher the temperature, the shorter the fermentation time; however, fermenting at lower temperatures gives better products because the rate of enzyme inactivation is slow.

1. Pressing: The fully fermented grain slurry is placed into cloth-lined containers and pressed to separate the solids from the liquid soy sauce. The isolated solids are used as fertilizer or fed to animals while the liquid soy sauce is processed further.

2. Pasteurization: The raw soy sauce is heated to eliminate any active yeasts and molds remaining in the soy sauce and can be filtered to remove any fine particulates.

3. Storage: The soy sauce can be aged or directly bottled and sold.

Chemical Soy Sauce

Traditionally, soy sauce is made by fermentation as described above; however, soy sauce can also be made by acid hydrolysis. The resulting product is known as chemical soy sauce, or protein chemical hydrolysate. In this process, defatted soy products or other proteinous materials are first hydrolyzed by heating with 18% HCl for 8 –12 hours. After hydrolysis, the hydrolysate is neutralized with sodium carbonate and filtered to remove the insoluble materials. The resulting product (chemical soy sauce) is a clear dark-brown liquid. However, chemical soy sauce does not possess the flavor and odor of fermented shoyu. Therefore, to improve its quality, chemical soy sauce is often blended with fermented shoyu to become a semi-chemical product before being sold.

Chemical Composition 

The chemical composition of soy sauce is affected by the proportions of raw materials, fermentation methodologies, fermenting molds and strains, and post-fermentation treatments. Although the formation mechanism of chemical composition in soy sauce is complex, it has been widely accepted that free amino acids, water-soluble peptides and Maillard reaction products in soy sauce are considered as essential chemical composition and to provide core sensory effects. The primary fermentation of lactic-acid-fermenting halophiles lowers the pH of the moromi, and this directly results in the acidic pH range (4.4–5.4) of soy sauce products. The secondary fermentation conducted by heterofermentative microbes provides soy sauce with a wide range of flavor and odorant compounds by breaking down macronutrients. Soy proteins and grain proteins are hydrolyzed into short peptide chains and free amino acids, which adds umami to the product. Based on the result of free amino acid analysis, the most abundant amino acids in Chinese soy sauce product are glutamic acid, aspartic acid, alanine and leucine.

Starch is hydrolyzed into simple sugars which contribute to the sweet flavor in soy sauce. Legume fats may also be decomposed into short chain fatty acids, and the interactions among lipids and other macronutrients also result in a richer flavor in the final product. Non-enzymatic browning also contributes significantly to the development of the properties of soy sauce. The hydrolysis of proteins and large carbohydrates also provides free amino acids and simple sugars as reagents for the Maillard reaction.

Sensory Profile

The taste of soy sauce is predominated by saltiness, followed by moderate umami, sweetness, and finally slight bitterness, which is hard to perceive due to the masking effect of other tastes. The overall flavor of soy sauce is a result of the balance and interaction among different taste components. The saltiness is largely attributed to the presence of NaCl (common salt) in brine. The sugars hydrolyzed from starch add sweetness into soy sauce. Umami is largely caused by the presence of free amino acids, mainly glutamine and aspartic acid. Sodium from the brine and disodium ribonucleotides from the soy also add to the umami. Other amino acids cause additional basic flavors, with sweet coming from Ala, Gly, Ser, and Thr; bitter coming from Arg, His, Ile, Leu, Met, Phe, Trp, Tyr, and Val; and no taste from Cys, Lys, and Pro. 

Despite a large variety of volatile and odorant compounds that have been identified in soy sauce, the food product per se does not present a strong aroma. Alcohols, acids, esters, aldehydes, ketones, phenols, heterocyclic compounds, alkynes and benzenes have been identified in Chinese soy sauces. An explanation for this observation is that the aroma of soy sauce does not depend largely on the aroma-active compounds. The subtle aroma is a result of a "critical balance" achieved among all volatile and odorant compounds, whose respective concentrations are relatively low.

Conclusion

Soy sauce exemplifies the harmony between tradition and innovation, uniting people through its rich flavors and cultural significance. Whether drizzled over sushi, mixed into a marinade, or used as a base for soups, soy sauce continues to evolve while honoring its ancient roots. Its universal appeal ensures that this time-honored condiment remains a cherished part of culinary traditions worldwide.