Soy-protein quality versus quantity

Soybeans contain growth-promoting and heart-protecting proteins. To boost these benefits, the United Soybean Board, an industry group based in Chesterfield, Mo., has been pushing growers to develop soy with even higher protein yields. A new study suggests that such an achievement might come at a serious price: a reduction in the protein’s quality.

A limitation of soy protein has always been that it contains too few sulfur-based amino acids, which are particularly important for growing children and for livestock. That’s why soy-based feed and foods are typically supplemented with an expensive sulfur-based amino acid such as methionine, explains Hari B. Krishnan of the U.S. Agricultural Research Service plant genetics unit in Columbia, Mo.

He and his colleagues measured sulfur-based amino acids in five groups of soybeans that had had their protein content boosted by increasing amounts of fertilizer. The resulting protein content of the soybeans ranged from 29 percent to 37 percent.

The higher the protein content, however, the lower the concentration of sulfur-based amino acids in the beans, the researchers report in an upcoming issue of the Journal of Agricultural and Food Chemistry. They traced the drop-off in sulfur-based amino acids to a lower content of a particular protein called Bowman-Birk protease inhibitor.

Growers now should try to breed soybeans with extra protein rich in the Bowman-Birk protease inhibitor, Krishnan says. It shouldn’t be impossible, he adds, since soy breeders have already circumvented a former problem in which increasing protein yields came at the expense of lower yields of oil, another of the bean’s most important products.