This is normal. The substance that causes foam is saponin contained in soybean milk, which will produce soap-like foam. As long as it is heated to 100 degree, saponin will be destroyed. However, we should pay attention to the phenomenon of false boiling, that is, when the soybean milk is cooked to about 90 degrees, the liquid level will rise as if it were boiling, but it is actually less than 100 degrees. It is necessary to continue to cook to 100 degree with low fire, otherwise saponin cannot be destroyed and may cause poisoning. Moreover, generally, water will also produce foam after rotating and vibrating, and air and organic matter will dissolve in water, which can improve the "surface tension" of water. Therefore, when bubbles are formed, their "toughness" is enhanced, and they can bear greater external force than fresh water before, so these bubbles are not easy to disappear.
You usually pay attention to put the beans evenly in the net cover. It can heat water, shorten heating time and reduce certain foam.
Rinse the beans with water after soaking, which can also reduce the foam.