Method development and optimization for isoflavone quantification in dry soy extract containing products

Isoflavones are secondary metabolites belonging to flavonoids found in a variety of plants, especially soy (Glycine max L.). Due to the estrogen-like activity of these compounds several soy products are on the market and applied to relieve menopausal symptoms. The aim of our study was to develop a simple method for the determination of isoflavones in soy extract containing products. The main isoflavones of soybean are genistin, glycitin, daidzin and their respective acetyl, malonyl and aglycone forms. Quantitative measurement of each individual isoflavone is difficult because some of the reference compounds are not widely commercially available [1]. Keeping in mind that the biological effects of soy isoflavones depend upon aglycone form, hydrolysis of glycosides is a practical method for the quantitative determination of total isoflavones and assessment of biological value of extracts [2]. The objective of our investigation was to establish an acid hydrolysis method for the quantitative HPLC determination of total isoflavones including daidzein, genistein and glycitein. To optimize the extraction and hydrolysis of isoflavones, the effect of HCl concentration (1.5–6 N), hydrolysis time (25–210min) and temperature (30–100°C) on total isoflavone aglycone content was studied using an RP-HPLC-DAD method. By mathematical fitting and optimization methods optimum hydrolysis conditions for maximizing the quantification of isoflavones were determined (t=94min, c_HCl=5.09 N, T=80°C). The experimentally verified model has a good coefficient of R² (0,9928) and the recovery of isoflavones was 97.81–102.76%. The developed method allows a reliable and maximum determination of isoflavones in dry soy extract containing products.

References: 1. Rostagno, M.A. et al. (2009)J Chrom A 1216: 2–29.

2. Chiang, W.D. et al. (2001) Food Chemistry 72: 499–503.