A systematic study of the impact of the isoelectric precipitation process on the physical properties and protein composition of soy protein isolates

The functional properties of soy protein isolates (SPIs), which are crucial for their successful use in food applications, depend on their protein physical properties and composition. Although the production process of SPIs is well-known and established industrial practice, fundamental knowledge on how the different isolation steps and varying isolation conditions influence these properties is lacking. Here, these characteristics were systematically investigated by assessing the impacts of the various steps of a conventional isoelectric precipitation based SPI production protocol. Protein denaturation and colloidal state were evaluated with differential scanning calorimetry and dynamic light scattering combined with (ultra)centrifugation, respectively. The protein composition (on protein subunit level) was assessed via size-exclusion chromatography. Hexane defatting was found not to cause protein denaturation. Alkaline extraction at pH values between 7.0 and 9.0 resulted in no differences in protein physical properties or composition. Subsequent acid precipitation at pH 5.5 resulted in SPIs with a lower 7S/11S ratio and higher protein solubility at neutral pH than when produced at pH 4.5 and 3.5. SPIs obtained at all evaluated precipitation pH values contained a considerable amount of aggregated protein structures. Spray-drying of SPI did not result in a higher degree of protein denaturation or in a loss of protein solubility compared to freeze-drying, but a smaller amount of soluble aggregates was observed in spray-dried SPIs. Hence, alterations in the isolation procedure can result in SPIs with moderately different physical properties and protein composition, which might lead to different functional properties and thus applicabilities in certain food systems.