Mechanistic insights into enhancement of sunflower protein isolates through a combined fractionation and fungal bioprocessing of sunflower meal

Sunflower meal (SM) is an affordable, balanced, and hypoallergenic plant protein source; however, its low protein extractability and phenolic compounds limit functionality, digestibility, color and flavor. Therefore, removing phenolics and modifying proteins are essential for food applications. This study evaluated the impact of size-fractionation and fermentation of SM ( Aspergillus niger and Aspergillus oryzae) on the physico-chemical, molecular, and functional properties of sunflower protein isolates (SPI). Coarser SMs (>0.25 mm) were fiber-rich, whereas finer SMs (<0.25 mm) contained higher protein, phenolics, and flavonoids. SPIs from finer SMs exhibited lower protein-purity but higher phenolics, flavonoids, and lighter color. Autoclaving reduced protein-extractability, and purity, but increased phenolics, flavonoids, chlorogenic and caffeic acids and altered SPI's molecular and secondary structure, whereas γ-irradiation mitigated these effects. Fermentation with Aspergillus niger enhanced protein extractability but did not improve protein recovery or purity. Both fungi reduced phenolics, flavonoids and chlorogenic acid and protein molecular weight. SPIs from fermented SM significantly enhanced SPI solubility (up to 22.8 % and 151.2 % in autoclaved and γ-irradiated SM, respectively) and surface-hydrophobicity (up to 133.8 % in γ-irradiated SM), although color, particle-size, ζ-potential, foaming, and emulsifying properties were unchanged. These findings demonstrate that size-fractionation and fermentation significantly influence protein extraction and SPI characteristics. • Size fractionation separated sunflower meal into protein- and fiber-rich fractions. • Protein isolates (SPIs) from protein-rich fractions had more phenolics and flavonoids. • Fiber-rich fractions yielded SPIs with higher protein purity and lower phenolics. • γ-Irradiation sterilization preserved SPI quality better than autoclave sterilization. • Fermentation reduced phenolics/flavonoids and enhanced SPI extractability, solubility, and surface hydrophobicity.