Deamidated wheat protein-dextran Maillard conjugates: effect of size and location of polysaccharide conjugated on steric stabilization of emulsions at acidic pH

A soluble isolated wheat protein fraction (sIWP) prepared from commercially deamidated wheat protein (30-35 deamidation) was incubated with dextrans D10 or D65 (MW 6400 Da or 41,000 Da) at 60 degrees C and 75 relative humidity to form Maillard type complexes. After 72 h reaction, approximately 1-2 smaller dextran D10 molecules were attached to per protein molecule whereas only about 0.5 mol of the larger dextran D65 was attached to per protein molecule. Both sIWP-dextran complexes formed a thicker interfacial layer at the surface of the polystyrene particles (23.6 nm and 21.2 nm for the sIWP-D65 and sIWP-D10 respectively) than that adsorbed protein alone (17.7 nm). Both conjugates provided enhanced steric stabilization of the emulsions at acidic pH (similar to pH 4), with the sIWP-D65 conjugate being more effective than the sIWP-D10 conjugate even though there was lower number of dextran D65 molecules conjugated to the protein. This is because that the conjugation of larger size dextran D65 was preferred at the N-terminal domain and thus provided an additional steric layer of similar to 6 nm thickness in addition to the protein layer at the oil-in-water emulsion interface which is sufficient to provide steric repulsion for the oil droplets even in the absence of electrostatic repulsion. The number of dextrans conjugated to the protein and the location of conjugation was dependent on the size of the dextran. This, in turn, influenced the ability of conjugates to form effective interfacial steric layer and to maintain emulsion stability at acidic pH (similar to pH 4).