Bae Suk-chul, a professor at Chungbuk National University College of Medicine who has conducted cancer research for 35 years, has highlighted precautions to take when consuming doenjang (Korean fermented soybean paste), vegetable oils, and nuts. Even foods widely regarded as healthy, he explained, can develop cancer-causing risk factors depending on how they are produced and stored.
Beware of Mold Toxins in Doenjang… "Hygiene Is Crucial When Making It at Home"
Appearing recently on the YouTube channel "Doctor Friend," Professor Bae discussed "foods that can unexpectedly cause cancer" and brought up doenjang. "Doenjang contains both anti-cancer compounds and carcinogenic substances," he said. "The issue is aflatoxin, a mold toxin that can form during the manufacturing process."
Aflatoxin is a toxin produced by certain molds and is known as a carcinogen that can have devastating effects on the liver. Professor Bae illustrated the danger of aflatoxin by recalling a past case in Europe in which 100,000 turkeys at a single farm suddenly died of liver cancer. An investigation at the time found that soybean feed imported from the United States had been contaminated by mold during transport to Europe, causing the outbreak.
"It wasn't the soybeans themselves that were the problem, but the mold that developed during transport," he said. "In Korea, products made by fermenting soybeans are subject to aflatoxin testing through the Ministry of Food and Drug Safety."
Hygiene management is especially important when making doenjang at home. Mold develops during the process of boiling soybeans, kneading them into meju (fermented soybean blocks), and letting them ferment. "Among them are both good molds and bad molds," Professor Bae said. "Blue mold, black mold, and yellow mold can all appear together."
The problem is that it is difficult to judge safety with the naked eye alone. Even if some mold is not visible, the mycelium may already have spread inside. "Aflatoxin doesn't disappear even when boiled," Professor Bae stressed.
Watch Out for Old Oils and Nuts… More Air Exposure Means Greater Rancidity Risk
Beyond doenjang, old oils and nuts were also flagged as foods to watch. Professor Bae explained that liquid vegetable oils contain large amounts of unsaturated fatty acids, which can become rancid when exposed to air or repeatedly used at high temperatures.
He noted that oil rancidity can be problematic in cooking processes that involve prolonged frying, such as for fried shrimp or sweet-and-sour pork, and that such oxidized oils are bad for the entire body and can also affect the skin.
That does not mean vegetable oils themselves are bad. "There's no reason to worry about lightly coating a frying pan with oil to fry an egg," he said. "Heating it once and being done is fine. The problem is using it repeatedly multiple times."
Storage condition matters for nuts as well. Nuts contain a lot of fat and can become rancid with prolonged exposure to air. Issues can arise particularly when a can or bag is opened and then kept for several months.
"Leaving nuts for a week or a month doesn't immediately cause a problem," Professor Bae said, "but if you open a can and don't eat it all at once, keeping it for three months or several months can cause rancidity." He added, "Eating them within a few days of opening is not a major problem."
To eat nuts safely, it is best not to leave them out for long after opening. They should be portioned into amounts that can be eaten at one time and stored separately. For long-term storage, sealing them and keeping them refrigerated or frozen helps.
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