Determination of Major Isoflavones in Soy Drinks by Solid-Phase Micro Extraction Coupled to Liquid Chromatography

A simple method for determination of major isoflavones (daidzein, genestein, genistin) and equol, a daidzein metabolite, has been developed using solid-phase micro extraction coupled to liquid chromatography with diode array UV detection. All the aspects influencing adsorption (fibre coating, extraction time, temperature, pH and salt addition) and desorption of the analytes (desorption and injection time and minimize carryover) were carefully investigated. The limits of detection obtained using a polydimethylsiloxane /divinylbenzene fiber coating were in the range 4 (genistein) to 38 (genistin) nM. The method proved to be useful for determination of analytes in soy-based products. Daidzein, genistein, and genistin were found in all samples (three soy milks and two blended rice/soy beverages) at concentrations ranging from 0.8?±?0.2 (daidzein) to 180.4?±?12.1 (genistin) μM, while glycitein and equol were found in three and in one of the drinks at levels between 1.7?±?0.1 and 5.0?±?0.2 μM and at 2.7?±?0.2 μM, respectively. The procedure was used to define the binding constants of isoflavones with serum albumin bovine that were found from 2.5?×?10⁴ (daidzein) to 2.9?×?10⁵ (genistein) L mol^?1. The method proposed restricts the use of organic solvents and can be used widely in most laboratories with standard equipment. In addition, it is suitable for easily studying the isoflavone interaction with the main constituents in plants, e.g., proteins and lipids, to evaluate their bioavailability and to clarify the controversial question on benefits for or damage to human health resulting from soy intake.