This study reports the isoflavone contents of 210 soybean cultivars grown in South Dakota and explores possible relations between isoflavone contents and agronomic characteristics. Total isoflavone contents (normalized) ranged from 1161 to 2743 μg/g. A number of agronomic characteristics were documented for each variety including maturity group, hilum color, disease resistance, seed weight, yield, maturity (in days), and plant height. Varieties in maturity group I had significantly higher total isoflavones when compared to maturity group 0. Hilum color was related to differences in genistin, daidzein, and genistein content. No differences in isoflavone content were observed based on disease resistance profiles. Genistein content was found to be negatively correlated with yield, days of maturity, and plant height. Weak but significant correlations also existed between these agronomic characteristics and other isoflavones.

Soy protein isolate (SPI) is a widely used food ingredient and is made by extracting soy flour (SF) under slightly alkaline pH, followed by precipitation, washing, and drying. Soy foods and foods containing soy protein ingredients have great potential in the prevention of cardiovascular diseases and cancers. These health benefits have been attributed to isoflavones in soy protein ingredients. However, the current processing techniques were developed many years ago without this knowledge. The objective of this study was to investigate the mass balance of different isoflavones during manufacturing of SPI and to provide basic information to assist further development efforts leading to preservation of soy isoflavones in soy protein ingredients. The study revealed that only about 26% of the total isoflavones in SF remained in SPI. The percentages of total isoflavones lost during extraction, precipitation, and washing were 19, 14, and 22%, respectively. Washing was the step where most isoflavones were lost. The isoflavone profile of the SPI was different from that of SF. The former contained much more aglucones (genistein and daidzein), while the latter had almost none. The high content of aglucones in SPI was probably due to the hydrolysis of glycosides. JAOCS 75, 337-342 (1998).