Vol. XIV · Fermentation Plate 7 of 11koji, Aspergillus oryzae, soy sauce, miso, doubanjiang, 豆瓣酱, 酱油, 味噌, moromi, salt, amino acids, umami, fermentationMF

Soy Sauce, Miso and Doubanjiang.

Soy sauce is about 17 % salt. That salt keeps everything but a few tolerant microbes out for the months the koji's enzymes need.

The long ferments of East Asia begin with koji: steamed soybeans and roasted wheat (for soy sauce), rice or barley (for miso and sake), or broad beans (for doubanjiang) inoculated with the mould Aspergillus oryzae and kept warm and humid for two or three days. The mould grows through them and fills them with enzymes — proteases that cut proteins into peptides and amino acids, glutamate above all (XI.5), and amylases that cut starch into sugars.

The koji is then mixed with salt — as a brine (moromi) for soy sauce, as a paste for miso and 豆瓣酱 — and left for months or years. The salt, 12–18 %, together with the amino acids and sugars the enzymes release, takes the water activity down to about 0.8 (III.9), which no foodborne pathogen tolerates; only salt-loving lactic bacteria and yeasts grow, slowly adding acid, alcohol and aroma, while the enzymes keep working and the Maillard reaction (VIII.1) slowly darkens the mash. Sichuan 郫县豆瓣 is aged in open jars in the sun, stirred by day and covered by night, for one to three years. The workshop ages a soy-sauce mash.

Below: the share of the protein broken down to amino acids against months of ageing at the temperature you choose (a teaching first-order rate with Arrhenius temperature dependence), and the water activity of a mash at the salt you choose (exact for sodium chloride).

What you will learn
Workshop
Teaching model: protein breakdown approaches 80 % as a first-order process, rate 0.25 per month at 25 °C, activation energy 50 kJ/mol. Water activity from the salt alone (NaCl, Robinson and Stokes), the salt taken as a share of salt plus water; the dissolved amino acids and sugars of a mature mash lower it further.

Salt buys time; enzymes spend it

The salt alone in a 17 % soy-sauce brine takes the water activity to about 0.87, and the amino acids and sugars that build up take it lower, near 0.8 — too low for foodborne pathogens and most spoilers; a 10 % brine would sit at 0.94, where many could grow. In this teaching model six months at 30 °C break down about 70 % of the protein and at 15 °C only 42 %: warm summers speed the ageing, which is why traditional breweries started their mash in spring, and why 豆瓣酱 jars stand in the sun.

Each product is a different recipe on the same plan. Japanese soy sauce uses about equal soybeans and wheat; Chinese 生抽 (light) is the first, thinner draw, 老抽 (dark) is aged longer and often thickened with molasses. Miso keeps the solids and ranges from sweet white (less salt, short ageing, more rice koji) to dark red (more salt, years of ageing). 郫县豆瓣 adds fresh chillies to fermented broad beans. Use them as seasoning that brings salt, umami and colour at once (XI.3, XI.5); add soy sauce late in long cooking to keep its aroma.

At the stove

Taste side by side: a light soy sauce, a dark soy sauce, a white miso, a red miso and a spoon of 豆瓣酱. Note the salt, the savouriness and the depth of colour, and match each to its age.

Soy sauce descends from Chinese 酱 (jiang), fermented pastes of meat, fish and later soybeans recorded over two thousand years ago; Buddhist monks are said to have carried the soybean versions to Japan, where they became miso and shoyu.

Terms

Koji
Grain or beans grown with Aspergillus oryzae to fill them with enzymes.
Moromi
The salted mash of koji and brine that ages into soy sauce.
Protease
An enzyme that cuts proteins into peptides and amino acids.
豆瓣酱
Fermented broad-bean paste, with chilli in Sichuan.
Koji mould fills beans and grain with enzymes; 12–18 % salt keeps pathogens out while months of ageing turn protein into umami.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.