Soy is high in a substance called called phytic acid which binds to minerals in your digestive tract and keeps your body from absorbing those minerals. In many foods, the phytic acid content is reduced by soaking (in the case of beans, soaked porridge, and bread during rise time). But the phytic acid in soy is recalcitrant. You can soak and soak and still have phytic acid.
A 1985 study by Sutardi and Buckle tested the level of phytic acid in soy after different stages of preparation. I provide many of those stages in the figure above. Keep in mind that the activities I list in the figure are successive: the researchers boil the beans, pour off the water, soak them again, dehull them, steam them, drain them, and cool them. The phytic acid levels change very little with all of this effort.
It is only when they ferment the beans in the form of tempeh that the phytate levels reduce to about 45% of the levels of the soaked soybean. Fried tempeh is an improvement still, but if the tempeh is stored for two weeks at 5ºC and then fried, the researchers reached the optimal (but not perfect) reduction of the phytic acid. A 2003 study also found that the phytic acid level decreased by only 31% by fermenting soybeans (Egounlety and Aworth 2003).
Keep these results in mind as you shop for soymilk and tofu. Soybeans in soymilk are soaked, strained, and cooked. Tofu has an additional step – a coagulant is added. Both of these products retain nearly 100% of the phytates according to the results of the research I present. Eat tempeh for a soy fix, but eat it sparingly if you do not prepare it yourself and do not know that traditional preparation methods were used.
When you look at your tub of tofu and see that one of those 12 ounce tubs has 100 milligrams or so of magnesium, keep in mind that you would only absorb about 10% of that magnesium. You would likely triple that absorption in a fermented soy product. If you rely on legumes for iron and zinc, keep in mind that these minerals are also bound by phytic acid. Purchase the Phytic Acid White Paper for much more information on the world of grains, legumes, nuts, and seeds. In the meantime, enjoy some miso soup as the weather cools here in North America.