Multilayer emulsions as a strategy for linseed oil microencapsulation: Effect of pH and alginate concentration

Abstract

The influence of pH (4–7) and sodium alginate (SA) concentration (0.125–0.25 wt%) on the properties of linseed oil-in-water emulsions stabilized by a whey protein isolate (WPI) was investigated. Droplet size, droplet charge, creaming stability and optical microscopy measurements as well as determination of non-adsorbed biopolymers at the oil–water interface were made. At pH 6 and 7, anionic alginate did not adsorb onto the surfaces of WPI-coated droplets due to strong electrostatic repulsion between biopolymers. Remaining SA molecules in the continuous phase induce emulsion destabilization by depletion flocculation, with the formation of floc chains that, after a period of latency, promoted phase separation with high creaming indexes. Both the delay time and the cream layer thickness increased when increasing SA concentration. At pH 5, ζ-potential measurements demonstrate deposition of SA onto WPI interfacial membrane to form a bilayer around oil droplets. Besides, no droplet aggregation was observed and emulsions were stable to creaming after one-week storage. At pH 4, 0.125 wt% SA emulsions were prone to extensive droplet aggregation probably enhanced by bridging flocculation, exhibiting a gel-like microstructure of interconnected flocs, which then promoted phase separation. However, when increasing initial SA concentration in these systems, the degree of droplet aggregation decreased. These results suggest that the best conditions to produce stable emulsions as encapsulation matrices for the delivery of high polyunsaturated fatty acid oils would be 0.25 wt% SA pH 5.