Production of soy protein isolates with low phytic acid content by membrane technologies: Impact of the extraction and ultrafiltration/diafiltration conditions

The content of the antinutrient, phytic acid, of soy protein was analyzed during their extraction and purification by a series of ultrafiltration and diafiltration steps. The phosphorus content of the extracts was used as an indication of their phytic acid content and their ash content as an indication of their mineral content. The extraction of soy proteins was conducted by using a 23 factorial experimental design, pH (7.5 or 9), solvent (0.06 M KCl or water), and temperature (25 °C or 50 °C). The most promising extraction conditions were 0.06 M KCl/pH 9.0/25 °C for the lowest phosphorus to protein ratio (12.2 ± 0.1 mg P/g protein) and H2O/pH 9.0/50 °C for the combination of low phosphorus to protein ratio and the lowest ash content (13.9 ± 1.2 mg P/g protein, 9.6 ± 0.8% w/w ash content). After extraction, soy proteins were purified by sequential ultrafiltration (UF) with a volume concentration ratio (VCR) of 5 and diafiltration (DF) with volume diafiltration ratio (VD) of 4. Extracts were purified with no pH adjustment or with pH adjustment to 6.5 between the UF and the DF steps. The extraction conditions 0.06 M KCl/pH 9.0/25 °C and the purification conditions UF pH 9.0/DF pH 6.5 showed the lowest phosphorus to protein ratio (4.4 ± 0.3 mg P/g protein) and reduced membrane fouling when compared to extraction conditions with water.

► The lowest phosphorus to protein ratio was obtained for 0.06 M KCl/pH 9.0/25 °C extraction.
► Water soy extracts were more prone to membrane fouling than 0.06 M KCl soy extracts.
► Isolate with total phosphorus to protein ratio as low as 4.4 mg P/g protein d.b. was obtained.