Extraction and characterization of okra Abelmoschus esculentus mucilage

Abstract

This research aimed to optimize okra mucilage precipitation conditions and investigate the mucilage functional and rheological properties. Okra mucilage was extracted using distilled water. The crude mucilage was then precipitated using 20, 40, 60, 80, and 95% ethanol at 1:4 (the crude extract to ethanol) ratio. The results showed that the polymer from crude extract was not precipitated in 20% and 40% ethanol. Hence, only 60%, 80%, and 95% ethanol precipitated polymers were further determined for proximate compositions and yield. The percentage of ethanol used during precipitation of the crude extract did not significantly affect the proximate composition (p>0.05), but affected the extraction yield (p≤0.05). The concentration of 95% ethanol gave significantly higher precipitating yield than 80% and 60% ethanol did, respectively (p≤0.05). Therefore, the concentration 95% ethanol was used as the precipitating solvent for okra mucilage in this study. The surface tension and rheological properties of okra mucilage solutions as well as that of 0.2-1.0% w/w xanthan gum solution and guar gum solution were determined. The results showed that okra mucilage acted as a weak surfactant, as evidenced by a decrease in surface tension with an increase in concentration of okra mucilage. The results on flow test and dynamic rheological test showed that the flow behavior of okra mucilage was more similar to that of xanthan gum than guar gum. At low concentration (0.4% w/w), okra mucilage exhibited a viscous flow behavior. At higher concentration (0.6-1.0% w/w), G' was higher than G'' over the tested frequency range, indicating a weak network formation in the okra mucilage solution. Decreasing the pH of the okra mucilage solution caused the crossover frequency to shift to a higher value which suggested that lowering pH value lessened polymer entanglements. At every concentration, the complex viscosity of okra mucilage at neutral pH was higher than that at acidic pH. In addition, increasing concentration and pH of okra mucilage, and decreasing temperature caused an increase in shear thinning behavior of the gum solution. Microstructure, rheological properties and stability of oil-in-water emulsion stabilized by okra mucilage at various concentrations (0.2%-1.0% w/w) was investigated. The size of oil droplets varied from 2 to 20 μm. The okra mucilage stabilized emulsion exhibited a viscous flow at low frequencies and then exhibited an elastic, gel-like behavior at higher frequencies. An increase in concentration not only caused an increase in moduli and complex viscosity, but also caused a shift of the G'/G'' crossover frequency from a higher value to a lower value. This indicated the more polymer-polymer entanglement. The emulsion stabilized by 1.0% w/w okra mucilage could be stabilized for up to 13 days. An addition of okra mucilage in a 50% reduced fat salad dressing led to an increase in moduli and complex viscosity, comparing to the 50% reduced fat recipe that did not contain any stabilizer. An addition of okra mucilage slightly changed the color of salad dressing. It was stable over the tested period of 30 days.