Formulation and application of alginate-based and soy protein isolate-stabilized emulsion gels
University College Cork
Alginate gels filled with emulsion droplets (i.e., alginate-based emulsion gels) have received increased interest in recent years, and soy protein isolate (SPI), used as a emulsifier, has been widely investigated in the food industry. However, the effect of SPI on properties of alginate-based emulsion gels has rarely been reported. This study investigated three kinds of alginate-based emulsion gels containing SPI-coated droplets, according to their morphological properties (i.e., the diameter/length of gels): bulk emulsion gels (> 1 cm), emulsion macro-gel beads (1-10 mm) and emulsion micro-gel particles (0.2-1,000 µm). For preparation of emulsion gels, the first step is to prepare emulsions containing alginate in the continuous phase, and the gelation of the continuous phase is then triggered by introducing Ca2+. The addition sequence of oppositely charged dispersions may affect the structure and stability of emulsions and thus the mechanical and structural properties of bulk emulsion gels. Stable emulsions containing unflocculated alginate/SPI-coated droplets were produced by adding low levels of SPI-stabilized emulsions into alginate solutions at pH 3.0, and bulk emulsion gels prepared from the above emulsions with GDL and CaCO3 had less flocculated droplets, higher L* values, and stronger mechanical properties than those prepared by adding alginate solutions into SPI-stabilized emulsions. In terms of emulsion macro-gel beads, their Young's modulus kept increasing during the gelling process by external gelation (i.e., dropping emulsions into CaCl2 solutions), before reaching a plateau accompanied by syneresis (i.e., water loss), shrinkage, and structural tightening. SPI adsorbed at the surfaces of emulsion droplets could prevent coalescence of droplets during gelation. However, the presence of SPI decreased Young's modulus of emulsion gel beads, compared to emulsion gel beads without proteins. In addition, the presence of SPI accelerated changes in Young's modulus and shrinkage during simulated gastric digestion and delayed the release of lycopene from emulsion gel beads during intestinal digestion. The pH of emulsions can also influence the gelation mechanism and properties of emulsion gel beads. Emulsion gel beads at pH 3.0 showed lower mechanical strength, higher storage stability of encapsulated lycopene, and faster release of lycopene during in-vitro digestion than those at pH 7.0 or 5.0. In terms of emulsion micro-gel particles, external/internal O/W/O emulsion-gelation methods were compared to prepare emulsion micro-gels. External gelation could produce emulsion micro-gels with small size (< 100 µm), while emulsion micro-gels prepared by internal gelation had bigger size and a more narrow size distribution. The suspensions of emulsion micro-gels prepared by external gelation had higher ϕrcp, ϕj, G', and G'' values than those prepared by internal gelation. Emulsion micro-gel particles prepared by external method collapsed more rapidly than those prepared by internal method during intestinal digestion. Moreover, SPI-stabilized emulsions turned into gel-like emulsions at high levels of externally-induced micro-gel particles (> 6.0%) in the presence of alginate (> 0.1%). Viscosity and creaming stability of emulsions and storage modulus (G') of gel-like emulsions increased with increasing contents of micro-gel particles in emulsions. Findings in this study are important for structuring emulsions and emulsion gels with naturally occurring polymers to achieve improved emulsion stability and controlled release of encapsulated compounds.
Alginate , Emulsion gel , Soy protein isolate , Delivery system , Structure
Lin, D. 2021. Formulation and application of alginate-based and soy protein isolate-stabilized emulsion gels. PhD Thesis, University College Cork.