Isoelectric protein precipitation from mild-acidic extracts of de-oiled sunflower (Helianthus annuus L.) press cake

Sunflower protein extraction using sodium chloride solutions allows high extraction yields at pH similar to 6 while preventing oxidation and covalent binding of phenolic compounds to the proteins, which usually occurs under alkaline conditions. Since protein solubility is enhanced at high NaCl concentrations, isoelectric precipitation had to be adapted in the present study besides developing subsequent desalting steps. Precipitation losses decreased from 62 to 31% with increasing NaCl (0.6-2.0 mol/L) and protein concentrations of the solution (6-14 mg/mL) and decreasing pH of precipitation (pH 4.5-3.0). Maximum yields were achieved at low temperature (8 A degrees C) and upon instant acid addition. After adsorptive removal of co-extracted phenolics from the protein extracts, overall protein yields were considerably higher after precipitation at pH 3.5 compared with 4.5, but only slightly higher after washing of the precipitates. The physico-chemical properties of the protein isolates did not differ significantly except for the marked protein denaturation upon precipitation at pH 3.5. From the proposed process, light-coloured protein isolates of high purity (> 98%) are obtained, which are suitable for food use.