1. Why Is Miso Thick and Soy Sauce Liquid? How Water Makes the Difference
Although miso and soy sauce share a similar framework, the amount of water used during production is a major factor that separates them. Miso is made by mixing steamed soybeans with koji and salt, then letting it ferment and mature. Depending on whether the koji is made from rice, barley, or soybeans, the final product is classified as rice miso, barley miso, or bean miso [1]. Soy sauce, on the other hand, is made by mixing steamed soybeans with roasted, crushed wheat inoculated with koji mold, and then adding salt water to this mixture [2].
The difference in water content is significant. In one test brewing example from the Iwate Prefectural Industrial Technology Center, miso was made using 5 kg of soybeans, 4 kg of rice, 2 kg of salt, and 1 liter of starter water. The resulting miso had a moisture content of about 46%. In contrast, the soy sauce test used 1 kg of soybeans and 850 g of roasted wheat, mixed with 3.3 liters of 20% salt water [5]. The total solid ingredients weighed 1.85 kg. If we approximate 1 liter of water as 1 kg, the salt water added was about 1.8 times the weight of the solid ingredients. For the miso, the water was only about one-ninth of the solid ingredients' weight.
These numbers come from a single test, not standard commercial values. However, they clearly show that the difference in moisture content relates to whether the result is a thick paste or a pourable liquid. This leads to the next question: does the difference in water also change how the flavors develop?
2. How Koji Mold Enzymes Work Differently in Solid and Liquid Fermentation
Koji mold produces various enzymes, such as protease, which breaks down protein, and amylase, which breaks down starch. These enzymes are responsible for breaking down the raw ingredients during brewing [4]. In soy sauce production, the protein in soybeans is broken down into amino acids, which create umami, while the starch in wheat turns into sugar, adding sweetness and depth. In the honjozo (standard brewing) method, the mixture is aged for about 6 to 8 months [3].
In miso, the same enzymes are at work, but the conditions are different. Because miso ferments in a low-moisture, solid state, the breakdown is incomplete. This means miso contains more intermediate compounds, such as peptides that have not yet fully become amino acids, compared to soy sauce [4]. This "incompleteness" is not a flaw; it is actually a key element that creates the distinct character of miso.
Another major difference is that miso does not have a pressing step to separate the liquid. Managing the fermentation of miso is also considered easier, which is why it has often been made at home [4]. In fact, miso did not have long-standing national standards in Japan. The Japanese Agricultural Standards (JAS) for miso were only established in March 2022. How did these two condiments develop from their shared roots?
3. Hishio: The Ancient Salt-Preserved Food Behind Both Miso and Soy Sauce
The roots of both miso and soy sauce are said to be "hishio," a preserved food made by mixing salt with ingredients. It is believed that the method was brought from China along with Buddhism. From hishio, a precursor called "misho" was born, which eventually evolved into miso [4].
Old records in Japan date back to the Taiho Code of 701, which indicates that hishio made from soybeans was being produced. There is also a theory that miso was introduced from Goguryeo (an ancient Korean kingdom) in the 5th year of Emperor Tenji (666), but this remains just a theory [1].
Initially, miso seems to have been a condiment eaten in small amounts directly at the table. Later, its uses expanded as it began to be used in miso soup [4]. Up to this point, we are looking at the history before the two became "brothers" in the modern sense. So, when and where was the liquid soy sauce born?
4. Did Soy Sauce Begin as the Liquid at the Bottom of a Kinzanji Miso Barrel?
During the Kamakura period, a Zen monk named Kakushin brought the method for making Kinzanji miso from Song Dynasty China back to Japan and taught it to Yuasa in Kii Province (now Wakayama). A theory suggests that the delicious liquid that accumulated at the bottom of the miso barrels began to be used as a condiment. There is a theory that this liquid is the beginning of today's soy sauce [6]. Materials from the Ministry of Agriculture, Forestry and Fisheries also introduce the idea that Kakushin brought Kinzanji miso, which became the prototype for tamari soy sauce [1].
However, brewing scholar Hiroshi Tate acknowledges this theory but writes that he considers the liquid collected from Kinzanji miso and tamari soy sauce to be different things. Tate states that the liquid collected from miso barrels is said to have become independent during the Muromachi period and evolved into the liquid condiment known as tamari soy sauce [4].
Thus, views differ depending on the source. There is no evidence here to definitively prove one side as fact. However, it is certain that the idea of a close relationship between "liquid collected from miso barrels" and "soy sauce" has been discussed for a long time. This brings us to the next question: when did liquid soy sauce begin to take its current form?
5. How Dark Soy Sauce (Koikuchi) Developed in Edo-Period Kanto and Spread Across Japan
Liquid soy sauce later evolved into "koikuchi" (dark soy sauce) in the Kanto region during the Edo period. Because of its pleasant aroma and taste, it spread throughout Japan [1]. Other regions also developed their own unique types of soy sauce. It is said that "usukuchi" (light soy sauce) originated in Tatsuno, Hyogo Prefecture; "saishikomi" (twice-brewed soy sauce) in Yanai, Yamaguchi Prefecture; and "shiro soyu" (white soy sauce) in Aisai, Aichi Prefecture [1].
In 2022, the production ratio by type was 84.9% for koikuchi, 11.4% for usukuchi, 2.1% for tamari, 0.9% for saishikomi, and 0.7% for shiro soyu [4]. It makes sense that when people say "soy sauce" in general, they usually mean koikuchi. Tamari soy sauce has a production ratio of about 2.1% and is a dark-colored soy sauce mainly produced in the Chubu region. It has viscosity, strong umami, and a unique aroma [7]. If you are interested in the story of the liquid from miso barrels, remember the character for "tamari" in its name.
6. Compare the Ingredient Labels of Miso and Soy Sauce in Your Kitchen
You can explore this topic at home by lining up the labels of miso and soy sauce in your kitchen and comparing the "Ingredients" list. Miso labels often specify the type of koji used, such as rice miso or barley miso. Soy sauce labels often specify the type, such as koikuchi or usukuchi. If there is a column for "Salt Equivalent," comparing the numbers for the same amount can show the differences between the two in numerical form.
If you want to read more, the Ministry of Agriculture, Forestry and Fisheries' "Nippon Traditional Food Encyclopedia" provides a concise summary of the basics of miso and soy sauce [1]. The work by brewing scholar Hiroshi Tate, "The Power of Fermentation," discusses the two different views on the liquid from barrels in his own words [4]. Reading them side by side helps clarify where facts end and theories begin.