Alkaline conjugation of caseinate and propylene glycol alginate to prepare biopolymer complexes stabilizing oil-in-water emulsion gels

Abstract The current study describes an approach for creating covalently bonded complexes of sodium caseinate (NaCas) and propylene glycol alginate (PGA) without using a cross-linking agent. The principle is based on a transacylation reaction between the esterified carboxylic groups of PGA and amino groups of NaCas during pH shift from alkaline (pH 12.0) to neutral condition (pH 7.0). The increased molecule weight at a higher amount of PGA observed in SDS-PAGE indicated covalent bonding between NaCas and PGA. The hydrodynamic diameter of NaCas-PGA binary complexes increased with an increase in PGA concentration while no significant changes of zeta-potential were observed. Compared to NaCas alone, the complexes with NaCas:PGA mass ratio of 2:1 exhibited the improved thermal stability and was capable of forming gel emulsions with 80% (v/v) oil without macroscopic phase separation. The much greater storage modulus than loss modulus indicated the gel-like structure of the emulsions, which agreed with jammed oil droplets observed in microscopy. The studied NaCas-PGA complexes may be significant to improving the quality of oil-in-water emulsion products such as mayonnaises.