Unstained Soybeans for Perfect Tofu

By Linda McElreath
February 4, 2002

Organically grown, large-seeded, high-protein soybeans intended for the high-quality tofu market will not pass muster if they are stained. In 1999, more than half of these specialty soybeans grown in Iowa did not make the grade. By comparison, in 1988 just 5 percent of these soybeans from Iowa were rejected for making silky tofu because of staining.

Now Agricultural Research Service soil scientist Doug Karlen and colleagues at the ARS National Soil Tilth Laboratory in Ames, Iowa--working in cooperation with scientists at Iowa State University and with the Heartland Organic Cooperative, a group of organic farmers--are looking for the causes of soybean staining and how to avoid them.

To find out what caused the staining, the team conducted an experimental study on five farms in several counties across Iowa during the 2000 crop year. The researchers speculated there were six potential causes for the purple-colored stains. These included four different fungal groups--Fusarium, Phomopsis,Alternaria and Cercospora--as well as bean pod mottle virus and soybean mosaic virus.

Since organic growers cannot use insecticides on their crops, the researchers tested several natural products on the soybeans to control vectors and minimize staining.

They determined that feeding by the soybean leaf beetle was the primary culprit. Due to warmer than average temperatures in 1997, 1998 and 1999, especially in the southwestern area of the state, the overwintering beetle population had increased rapidly. This resulted in much greater first- and second-generation insect pressure during the growing season, thus causing the increase in soybean staining. In the northeastern portion of the state, where winter temperatures were colder, there were lower beetle populations and little to moderate staining of the soybeans.

You can read more about this organic farming research in the February issue of Agricultural Research magazine, available on the World Wide Web.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.