Pectins from food waste: Extraction, characterization and properties of watermelon rind pectin

Abstract

Fresh and lyophilized watermelon rinds were treated with boiling ethanol for pigment removal and then used to extract pectins giving rise to fractions FW and LW, respectively. The pectins were characterized by chromatographic and spectroscopic methods. Both pectins had a galacturonic acid content higher than 65%, meeting the commercial requirement. However, FW had a higher yield (19.3%) than LW (14.2%). The protein contents were 3.9 and 3.6% for FW and LW, respectively. FW and LW had a high degree of methyl-esterification (∼60%) and low molar mass, 34,510 g/mol for FW and 40,390 g/mol for LW. The viscosity of 5% (w/w) aqueous solutions of watermelon rind pectins at 25 °C showed shear thinning behavior with a low shear Newtonian plateau of ∼30 Pa s for FW and ∼10 Pa s for LW. Mechanical spectra of FW and LW at 5% (w/w) at 25 °C showed a weak gel like behavior for both pectins. The dynamic surface tension, foaming ability and emulsifying properties of FW and LW were evaluated and compared with gum Arabic. FW and LW showed lower dynamic surface tension and higher foaming ability than gum Arabic at the same concentration. Emulsions were prepared with polysaccharides at concentrations ranging from 0.5% to 4% (w/w). The droplet size decreased for both pectins as the concentration increased. Values of surface weighted mean diameter D[3,2] and volume weighted mean diameter D[4,3] obtained for emulsions prepared with FW and LW were found to be similar to gum Arabic, which is well known for its emulsifying properties.