The arabinogalactan of dried mango exudate and its co-extraction during pectin recovery from mango peel

Abstract

Hot-acid extraction of the peel of fully ripe ‘Nam Dokmai’ (NDM) and ‘Maha Chanok’ (MHC) mangoes yielded pectins (PNDM, PMHC) being comparatively poor in uronic acids (38–47 g hg−1) in favor of the neutral sugars (23 g hg−1), chiefly galactose. Beyond the high-molecular-weight fraction, they displayed a characteristic, almost monodisperse fraction with a peak molecular weight of 18,200–19,100 relative to dextrans. In order to examine the hypothesized co-extraction of non-pectin polymers, the pectin PNDM and the exudate of mature-green ‘Nam Dokmai’ fruit (SAPNDM) were characterized regarding composition and molecular properties both before and after fractionation. By fractionation of PNDM on a weak anion exchanger, the high-molecular-weight pectin was largely separated from this characteristic fraction, which was composed of arabinogalactans. Likewise, the almost monodisperse molecular weight distribution of the dried fruit exudate SAPNDM with a peak molecular weight of 22,400 was shown to be caused by arabinogalactans being the main component (77.6 g hg−1) with a molar ratio of 82/10/4/4 for the major carbohydrate residues galactose, arabinose, rhamnose, and glucuronic acid. Fractionation of SAPNDM by size exclusion chromatography (SEC) and rechromatography (HPSEC) confirmed the high molecular uniformity of these arabinogalactans. The occurrence of hydroxyproline-rich proteins (≤0.2 g hg−1) in these exudate fractions may indicate small percentages of arabinogalactan-proteins. Co-extraction of residual fruit exudate may thus be considerable in pectin recovery from mango peel. Options for obtaining both pure pectins (galacturonic acid, 94 mol hmol−1) and arabinogalactans from this source were discussed.