pH‐adjusted solvent extraction and reversed‐phase HPLC quantification of isoflavones from soybean (Glycine max (L.) Merr.)

Abstract

Soybeans, Glycine max (L.) Merr., are among the most important food crops worldwide. Isoflavones are major bioactive phytochemicals in soybeans, and have a variety of health benefits, including antioxidative, antiatherosclerotic, antiinflammatory, and weak estrogen‐like effects. The isoflavone content and composition of soybeans vary according to the cultivar and the extraction solvent conditions. Therefore, we investigated the effects of three different solvent pHs (1.0, 5.5, and 10.0) on the isoflavone, total phenolic, and total flavonoid contents and antioxidant capacities of eight soybean cultivars developed in Korea. Twelve isoflavones in soybeans were efficiently separated and identified on a reversed‐phase high‐performance liquid chromatography (HPLC) system. The percentage distribution of isoflavones measured by HPLC in the eight soybean cultivars at various extraction pHs decreased as follows: malonyl isoflavones (67.2% to 81.3%) > isoflavone glucosides (16.2% to 29.0%; as nonacylated form) > acetyl isoflavones (1.6% to 5.9%). The highest contents of isoflavone glucosides, malonyl derivatives, and acetyl derivatives were extracted at solvent pHs of 10.0, 1.0, and 5.5, respectively. The solvent extraction at pH 1.0 yielded a lower total isoflavone content than those at pHs 5.5 and 10.0. However, the highest total phenolic and flavonoid contents were extracted from soybeans at pH 1.0. Soybeans extracted at pH 10.0 displayed the highest antioxidant capacities in the 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid) radical assay. Taken together, these results suggest that proper solvent pH adjustment is needed to maximize the extraction of targeted forms of isoflavones from soybeans. Practical Application Soybeans contain a variety of bioactive compounds, including isoflavones, which function as antioxidants and weak phytoestrogens. Chemical and instrumental analyses can facilitate the selection of soybean cultivars with high amounts of isoflavones for soybean breeding and isoflavone‐enriched product development. Proper solvent pH adjustment allows for the efficient extraction of high amounts of targeted isoflavone subgroups (acetyl and malonyl forms) from soybeans for functional food products.

A brief history and spectroscopic analysis of soy isoflavones

“…Kudou et al (1991a) were the first to present the composition ratios of 12 soy isoflavones, of which malonyldaidzin and malonylgenistin accounted for the largest portion of 66%. This is consistent with other reports saying that malonylglucosides of isoflavones are predominantly contained in foods such as soybeans and soy-based products (Azam et al, 2020;Bustamante-Rangel et al, 2018;Cho et al, 2020).…”

“…Isoflavones, however, had the highest resolution at 25°C rather than 40°C (Baranowska and Magiera, 2011). By using the HPLC equipped with an ODS column, when the column oven was set at 25°C, 12 soy isoflavones were separated and simultaneously analyzed to quantifiable levels within 24 min (Cho et al, 2020).…”

“…3 HPLC traces of simultaneous analysis of 12 soy isoflavone standards at 254 nm (A) and UV spectra at 200-400 nm (B). HPLC analytical conditions refer to Cho et al (2020). Each isoflavone was used at a concentration of 100 lM ratio (m/z) of ions through sequential processes of the ionization, separation, and detection of molecules (Yates, 2011).…”

Comparative study of the effects of selenium nanoparticles and selenite on selenium content and nutrient quality in soybean sprouts

“…However, SeNPs showed more positive effects on the total content of isoflavones than selenite (Figure 5B). Isoflavones are biosynthesised by the phenylpropanoid pathway in plants (Sugiyama et al, 2017;Cho et al, 2020); SeNPs may enhance the activities of several catalytic enzymes in the phenylpropanoid pathway in soybean sprouts.…”

Nondestructive Prediction of Isoflavones and Oligosaccharides in Intact Soybean Seed Using Fourier Transform Near-Infrared (FT-NIR) and Fourier Transform Infrared (FT-IR) Spectroscopic Techniques

“…Modern laboratories in chemistry have approached many precise measurement methods to determine the chemical composition inside the sample, including high-performance liquid chromatography (HPLC), mass spectrometry (MS), and capillary electrophoresis (CE). These methods have proved their preciseness and reproducibility in determining the macro- and micro-chemical components of soybeans [ 14 , 15 , 16 , 17 , 18 , 19 ]. However, those methods cannot address environmental issues related to the high amount of chemical waste, as well as being highly cost- and labor-intensive.…”