Rheological and physico-chemical characteristics of Okra (Abelmoschus esculentus) seed protein-stabilized oil-in-water emulsion as affected by heat and/or ionic treatment(s)

Abstract

Oil-in water emulsions (O/W) (at 25 %w/w, pH 3) stabilized by Okra seed protein (5%w/w) were prepared and treated with heat (35, 55, 75 oC), cold (4, 6, 8oC) and NaCl (1, 2, 3, and 4%) conditions alone or combinations, and the stability of the emulsions to the treatments were assessed by measuring the gravitational separation, droplet characteristics, microscopic and rheological properties, before and after storage. Treatment with 2% NaCl alone did not affect the stability of the emulsion, but ≥ 3% NaCl the emulsion to breakdown into serum and a paste-like translucent top layer. Temperature treatment up to 35 ºC in the presence of 1 or 2% NaCl also had little effect on mean droplet stability but at ≥ 3% NaCl and/or ≥ 55 oC heating, polarized light microscopy showed significant changes in the droplet characteristics, increased droplets sizes and aggregation. With further NaCl and/or heat treatment, the emulsion droplets exhibited distinctive, polydisperse size distribution characteristics, with consequent aggregation followed by oiling-off. Chilling did not cause significant effects on the emulsion stability. There was no flow at 1- 2% NaCl but a viscous flow of 13 Pa s was observed after a yield stress (Ϭ0). At 3-4% NaCl and shear rate (ý) of 11 s-1, viscosity increased to 21 Pa s depicting Bingham plastic; an indication of appreciably large droplet sizes and flocculation. With further shearing, the emulsion assumed Newtonian fluid with a straight passing through the origin when all droplets might have collapsed. The emulsion prepared at 1- 2% NaCl had significantly larger Ϭ0 than observed at the ≥3% as the thixotropic property was completely eliminated after the initial shear. The study showed that Okra seed isolate (≥ 5%) can be employed as surface-active ingredient in o/w emulsions, however, handling of o/w emulsions at temperatures much above ambient temperatures could cause emulsion breakdown. The design of processing, handling and packaging facilities for food emulsions greatly impacts on the thermodynamic stability of the emulsion systems. Keywords: emulsion, lipid oxidation, physicochemical properties, droplet size, rheology