Multi-element determination in acid-digested soy protein formulations by inductively coupled plasma-optical emission spectrometry

The concentrations of major (Ca, K, Mg, Na and P) and trace elements (Al, Cu and Fe) in soy protein formulations sold in Bahia (Brazil) were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES). Liquid and powdered soy protein formulation samples, both whole and light, were digested using a conventional heating program on a hot-plate. The powdered samples were prepared according to the label instructions for human consumption. A 5.0-ml aliquot of the soy protein emulsion was transferred to a borosilicate Erlenmeyer and concentrated nitric and sulfuric acid added. After a digestion time of approximately 50 min, hydrogen peroxide was added and heating continued to give a final volume of approximately 5 ml; the colorless digests were then made up to 15.0 ml with deionised water. Residual acid content was determined by acid-base titration. Good agreement between measured and certified values for all analytes in a non-fat milk powder (NIST SRM 1549) indicated that the method was suitable for major and trace elements determination in soy protein formulations.