Hydrolysis and application of pectins from passion fruit peel obtained by subcritical water treatment

Abstract

Passion fruit (Passiflora edulis) peel is an underutilized by-product from juice industry. This research investigated the potential of subcritical water for extraction of pectins from the peel and application of the obtained pectins. The effect of ethanol addition on subcritical water extraction of pectin was firstly investigated using a batch-type reactor. The maximum pectin yield of ca.12% was obtained from the subcritical water treatment at 140 °C with 10% ethanol. In addition, the ethanol addition also resulted in an increasing of antioxidant activity and total phenolic content in the obtained pectin. However, this extraction technique gave pectins with low molecular weight. To investigate the degradation process of pectin in subcritical water, the degradation of pectin in a continuous flow-type reactor was kinetically evaluated. Reduction of molecular size and formation of reducing end of pectin in subcritical water at 80–160 °C were monitored. The change in degree of polymerization could be modeled by the Emsley equation, whereas the change in reducing end could be modeled by zeroth-order kinetics. The Arrhenius equation was also used to describe the temperature dependency of the reaction rate constants. The activation energies for pectin degradation and reducing end formation were 62.8 and 86.9 kJ/mol, respectively. Subcritical water-hydrolyzed pectins were evaluated for viscosity both in distilled water and buffer (0.05 M of sodium phosphate containing 0.4 M NaCl, pH 7.0) systems, molecular weight, as well as moisture sorption isotherm. The results demonstrated that under more severe hydrolysis conditions the hydrolyzed pectin had lower viscosity. Pectins dissolved in distilled water showed overestimated intrinsic viscosities and molecular weights which caused from electrostatic repulsion effect. Moisture adsorption isotherms of the hydrolyzed pectins could be described well with GAB model. At aw ≤ 0.75, the reduction of molecular weight did not affect the water adsorption characteristics of pectin; however, at aw > 0.75, low molecular weight pectins showed higher equilibrium moisture content than high molecular weight pectins. Finally, the application of subcritical water-hydrolyzed pectins (20, 77, and 146 kDa) was demonstrated by conjugation with whey protein isolate using a dry-heating method. SDS-PAGE analysis indicated the formation of conjugates after the reaction. The obtained conjugates were studied for their emulsifying properties in an oil-in-water emulsion with an aqueous phase pH at 5. The smallest emulsion droplet sizes and most stable emulsions were obtained using the conjugates at 6 h of dry-heating. In addition, the conjugates from 146 kDa pectin gave significantly higher emulsion stability than those from 20 kDa pectin. The result indicated that the conjugation could improve the emulsifying properties of whey protein isolate at the pH around its isoelectric point.

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