Abstract:

In this study we have attempted to understand the nature of "charge interactions" between two neg. charged biopolymers (whey protein isolate, WPI and gum Arabic, GA) and, consequently, why their mixt. exhibits better interfacial activity. Surface tension ($\gamma$0) measurements indicated that at ca. 1 wt.% of the biopolymer mixt. (3:1 wt. ratio) the air/water surface is satd. At 5 wt.% the $\gamma$0 of the mixt. is lower than the calcd. co-operative value. The $\zeta$-potential measurements revealed that the isoelec. point of the WPI:GA 3:1 wt. ratio mixt. is 3.8. The $\zeta$-potential values up to pH 6 are below those calcd. Similarly, the elec. conductivities of the mixt. are lower than those calcd. All these measurements indicate: (1) partial charge neutralization in spite of the fact that both biopolymers are neg. or (2) partial charge-charge interactions between the two biopolymers. The thermal heating behavior of the frozen water in the aq. mixt. studied by DSC (heating cycle of the frozen sample) clearly indicates that the two biopolymers are interacting. We calcd. the enthalpy, the free energy and the chem. potential of the interactions. We found that the interactions of the biopolymers are rather weak. They are likely derived from some local pos. charged domains (pH 7) on the protein that neutralize some of the neg. charged GA. These interactions form weak charge adducts. These charge adducts are sufficient to improve its adsorption into the oil-water interface and enhance the emulsion stability. [on SciFinder(R)]