Characterization of cell wall polysaccharides of purple pitaya (Hylocereus sp.) pericarp

Abstract

While the fruit pulp of purple pitaya (Hylocereus sp.) is highly valued, its pericarp is unsuitable for consumption. Cell wall polysaccharides of fruit pericarp were analyzed to identify potential valorization of this by-product. The alcohol insoluble residue (AIR) was extracted yielding 6.16±0.09% on FW of AIR, which was subsequently fractionated into water-, oxalate-, acid- and alkali-soluble pectins, hemicellulose and cellulose. Water and oxalate-soluble pectin were the major pectin fractions. Anhydrouronic acid (AUA) contents of the AIR and of the fractions were measured using the m-hydroxydiphenyl (MHDP) assay, resulting in 24.1% of the AIR. AUA contents were higher in the water (WSP) and oxalate (OXP) soluble pectin fractions, reaching 46.62 and 56.16% of their total, respectively. Moreover, the degree of esterification (DE) of the AIR and its fractions was determined spectrophotometrically, where the highest DE values were found in the WSP and acid (HSP) soluble fractions. Fractions were analyzed for their neutral sugar composition by gas chromatography after derivatization. Pectin fractions were mainly composed of arabinose, galactose and rhamnose, while the hemicellulose fraction mainly consisted of glucose and xylose. The thickening properties of the AIR were evaluated after determining relative viscosities of AIR solutions and comparing the data with various mathematical models. With increasing shear rates a power law decay of shear viscosity was observed for AIR solutions indicating structure viscous shear-thinning behavior. Flow curves of 1% solutions of commercial thickeners showed high similarity to a 5% solution of pitaya AIR, thus indicating the requirement of further purification of the pitaya AIR.