Sunflower (Helianthus annuus L.) protein isolates from size-fractionated and fungi fermented sunflower meal: Physico-chemical, molecular and functional properties

Justification
Sunflower meal (SM), a byproduct of sunflower oil extraction, is a rich source of proteins, phenolics, and fiber. However, its utilization in food and industrial applications is limited due to low protein extractability, undesirable phenolic compounds, and poor functional properties of sunflower protein isolates (SPIs). Developing strategies like size-fractionation and fungal fermentation can improve the physico-chemical and functional properties of SPIs, enabling better utilization of SM in value-added applications.

Objective
This study aimed to evaluate the effects of size-fractionation and fermentation of SM with Aspergillus niger and Aspergillus oryzae on the physico-chemical, structural, and functional properties of SPIs.

Methods
Sunflower meal was size-fractionated into coarser ( >0.25 mm) and finer (< 0.25 mm) fractions. SM was sterilized via autoclaving or γ-irradiation, and fermentation was carried out using A. niger and A. oryzae. Protein extraction, recovery, and purity were analyzed alongside phenolic, flavonoid, and secondary metabolite content. Structural changes in SPIs were examined using SDS-PAGE, and functional properties, including solubility, emulsifying, and foaming capacities, were assessed.

Results
Coarser SM fractions were fiber-rich, whereas finer fractions contained higher protein, phenolics, and flavonoids. Finer fractions produced SPIs with lower protein purity but higher phenolic and flavonoid content, alongside lighter color. Autoclave-sterilization reduced protein extractability, recovery, and purity, while increasing phenolics and altering SPI secondary structures. These effects were mitigated by γ-irradiation. Fermentation with A. niger improved protein extractability but did not significantly enhance recovery or purity. Both fungal strains reduced phenolic and flavonoid content, while SPI color, particle size, and ζ-potential remained unchanged. SDS-PAGE analysis showed reduced molecular weight proteins in SPIs fermented with A. niger. Fermentation significantly enhanced SPI solubility, with emulsifying and foaming properties comparable to control SPIs.

These findings demonstrate that size-fractionation and fungal fermentation effectively influence SPI extraction and characteristics, enabling improved utilization of SM in various applications.