Cold-set oat protein isolate--gellan gum binary gels with various microstructures: Fabrication, characterization, formation mechanism, and controlled release properties

Cold-set oat protein isolate (OPI) and gellan gum (GG) binary gels with various microstructures was fabricated with different amounts glucono-δ-lactone (GDL) as a carrier to deliver bioactive compounds. OPI and GG formed gel-like mixture at pH 8 mainly via hydrophobic interactions with a binding constant (Ka) of 3.85 × 104 M−1, while the mixture showed highest storage modulus (G′) with 0.1% GG (w/w, based on the dry weight of protein). Novel binary gels fabricated with 7% OPI (w/v) and 0.1% GG (w/w) (OGG) were then developed through addition of various amounts of GDL. For the gels prepared with 1.5% and 4.0% GDL (w/w, based on the dry weight of protein, pH 7.1 and 6.3, respectively), heterogeneous network formed by co-aggregated OPI and GG with a small amount of small GG particles dispersed within large aqueous pores were fabricated mainly through hydrophobic interactions. For OGG-3 (9.0% GDL, w/w) and OGG-4 (20.0% GDL, w/w), the final pH was 5.1 and 4.1 that around or lower than OPI's isoelectric point, hydrogen bonding and electrostatic attractive forces were developed between two polymers since OPI contained neat or positive charges, resulting in a compact coupled OPI-GG network structure. Heterogeneous network with small GG particles dispersed in pores allowed OGG-2 (4.0% GDL, w/w) had comparable good gel hardness to the gel formed with 9.0% GDL (w/w, OGG-3) with coupled microstructure, whereas OGG-3 showed a better ability to restrict the swelling induced release of riboflavin in SGF, and allowed sustainable release of riboflavin in SIF. Therefore, OPI-GG binary gels with various microstructures prepared by simple acidification method at ambient temperature have great potential to be used as delivery systems for heat-sensitive bioactive compounds in food and non-food applications.