Black soybeans

Geomjeongkong ('black beans') are soybeans (Glycine max) with black seed coats. Koreans name soybeans by size and color, and several black types are sold. Seoritae is a large black bean with green cotyledons inside, which is why it is also called sokcheong ('green inside'); heuktae is a black bean with a yellow interior; and the small, glossy seomoktae, or jwinunikong ('rat's-eye bean'), is a separate small-seeded black bean valued as both food and medicine.

Seoritae is a traditional landrace rather than a modern cultivar. In a 2021 study of 29 Korean colored soybeans it was among the slowest to mature, taking about 148 days from sowing. Its name, 'frost bean,' is popularly explained by its late harvest after the first autumn frosts.

Soybeans as a crop are generally believed to originate in Manchuria, the northeast of China and the old territory of Goguryeo; soybeans have been excavated at the Bronze Age site of Odong in Hoeryong, North Hamgyeong, and the crop is thought to have spread from Korea to Japan.

Black beans are the usual choice for kongjaban (also kongjorim), whose name comes from jwaban, salty side dishes that 'sit beside the rice.' The late-19th-century Siuijeonseo braises boiled beans with soy sauce, scallion, ginger, oil, sesame, chili, honey and minced meat; today the beans are simmered in soy sauce and water until wrinkled, then finished with sugar or syrup and sesame.

• Protein and fiber: Like other soybeans, black soybeans are rich in protein and fiber and supply folate, iron, magnesium and potassium.
• Pigments under study: The black seed coat is rich in anthocyanins, and green-cotyledon types such as seoritae contain more lutein than yellow-fleshed beans; research on what these compounds mean for health is ongoing.
• Soy isoflavones: Soybeans contain isoflavones, plant compounds with weak estrogen-like activity. Harvard's Nutrition Source describes soy as a nutrient-dense protein that can safely be eaten several times a week.
• Considerations: Soy is a major food allergen. Kongjaban is braised in soy sauce with sugar or syrup, so it contributes sodium and added sugar.