Korean soup soy sauce
A pale, very salty Korean soy sauce made from fermented soybean blocks, used to season soups and vegetable side dishes.
Korean soup soy sauce, called guk-ganjang in Korean, is traditionally a product of soybean paste making: dried, fermented soybean blocks mature in brine, and the liquid drawn off becomes the sauce. It is paler than ordinary soy sauce, considerably saltier and less sweet, with a deep, slightly earthy fermented note. Because it barely colours food, it is the choice for clear broths and blanched vegetables that should keep their colour. Its high salt content means it is used more sparingly than regular soy sauce and adjusted by taste. It is available in Asian grocery shops with a Korean range or by mail order; the label usually marks it as being for soup.
Substitute:
The closest match is Japanese light soy sauce (usukuchi), which is also pale and salty but milder and lacks the earthy fermented note of the Korean sauce. Ordinary soy sauce is only a stopgap: it darkens broth and vegetables and tastes sweeter, so use less of it and make up the difference with a little salt.