Recently, a few friends have asked me, "I heard it's unsafe to make soy milk with soaked soybeans, because aflatoxin, a carcinogen, is produced during the soaking process. Is that true?" We've been drinking soy milk and eating tofu made from soaked beans since we were little!
When I heard this, I was very surprised. Aflatoxin is indeed a terrifying carcinogen; its toxicity is tens of times that of potassium cyanide. Even with微量 (trace amounts) intake, it can accumulate over time and lead to malignant tumors like liver cancer. If soaking soybeans produced aflatoxin, that would be a huge problem, because all the various tofu products we eat daily are made by first soaking the beans, then grinding them into slurry, and finally curdling them. If that were the case, does that mean we can't drink soy milk or eat tofu anymore?
However, don't panic too much first. A rational person, upon hearing such a sensational claim, should at least first judge whether the claim holds water.
First, we need to clarify a few questions: What are the growth conditions for Aspergillus flavus? In which foods does it easily proliferate? Under what circumstances does it produce aflatoxin?
Aspergillus flavus is a mold that easily grows in seed-type foods. At a temperature of 25-30°C, 80%-100% relative humidity, and a seed moisture content of 17%-18%, products like grains, nuts, and oilseeds are most susceptible to Aspergillus flavus. Fortunately, various beans, including soybeans, are not the preferred material for Aspergillus flavus to reproduce; it prefers foods like peanuts, corn, and rice.
From this, we can see that Aspergillus flavus grows when seed-type foods like grains and beans become damp. The seeds are generally in a state of low moisture, not a fully hydrated state. Everyone has this experience: watery congee or mung bean soup never gets moldy; instead, it spoils and tastes bad quickly—because when there's too much water, bacteria multiply very fast, leaving no chance for mold to grow. The culture media that molds love are mostly foods that are relatively "dry" but not dry enough, such as steamed buns, bread, pastries, and dampened seeds. That's why bread, after being left out for a long time, will grow white, green, or black mold spots but won't turn sour and stink; its state is just right for mold's appetite.
Why is that? Because molds love oxygen and have strong "terrestrial" characteristics. If food is soaked in water, there's insufficient oxygen, and they can hardly reproduce. Knowing this basic knowledge of microbiology, you will understand that molds can hardly produce toxins under waterlogged conditions, so soaking soybeans cannot produce aflatoxin.
Therefore, when buying soybeans, pay attention to their dryness, and during storage, avoid letting them get damp. If you're worried that the beans might be contaminated with Aspergillus flavus during storage, soaking them first and then discarding the soaking water is actually safer.
In fact, the majority of bean products require a soaking process during their preparation. Whether it's making bean congee, tofu, natto, or bean paste. The claim that soaking beans poses a cancer risk is purely a false statement that defies scientific common sense. There is no need to be alarmed, and certainly no need to stop drinking soy milk or eating tofu because of it.
It should be noted that when soaking beans in the summer, if it exceeds 4 hours, it's best to put them in the refrigerator. Because the soaked beans have very high moisture, various bacteria can easily grow. Bacteria won't produce aflatoxin, but they might make the beans smell bad. If unfortunately contaminated with certain pathogenic bacteria, their excessive proliferation or the production of bacterial toxins can cause gastrointestinal diseases. Although heating and sterilizing are still required when making soy milk or tofu, this risk should not be ignored!