Influence of pH and biopolymer ratio on whey protein-pectin interactions in aqueous solutions and in O/W emulsions

Combining total carbon (TC) and total nitrogen (TN) measurements, the solubility of both protein and pectin in mixtures of 0.5% whey protein isolate and non-amidated high methoxyl sugar beet pectin was determined at pH 4.0, as well as pH 5.5. The experimental results indicated that insoluble complexes with a protein to pectin ratio of 3.84 +/- 0.88 were formed at pH 4.0 at protein to pectin ratios ranging from 5 to 0.5. At pH 5.5, the protein solubility was independent from the amount of pectin added, indicating that at these pH conditions, located above the whey protein's iso-electric point, either soluble complex formation or cosolubility occurred. Light microscopic analysis of the corresponding emulsions revealed that the large droplet sizes at a protein-pectin ratio of 5 at pH 4.0 were due to the low amount of soluble protein left due to complexation with pectin. Comparing the residual dissolved protein and pectin concentrations in the absence and presence of oil droplets indicated that not only the protein, but also the pectin was accumulated at the O/W interface both below and above the protein's iso-electric point. Electrophoretic mobility measurements clearly indicated that pectin adsorption to the whey proteins induced a charge reversal at pH 4.0 at higher pectin concentrations, giving rise to smaller droplet sizes. At pH 5.5, on the other hand, the electrophoretic mobility of the whey protein stabilised emulsion droplets became gradually more negative upon pectin addition, which not only resulted in a smaller droplet size, but also in a significant increase of the creaming stability. From the experimental results, it follows that pectin has an emulsion stabilising effect on protein stabilised emulsions both below and above the protein's iso-electric point, provided that electrostatic protein precipitation is prevented. (c) 2006 Elsevier B.V. All rights reserved.