Polysaccharide from leaf skin of Aloe barbadensis Miller: Part I. Extraction, fractionation, physicochemical properties and structural characterization

Aloe barbadensis Miller ( Aloe vera ) is widely used for healthy foods, medical and cosmetic products, while its leaf skin is usually treated as industrial waste. To have a profound understanding of the polysaccharides in the leaf skin of Aloe vera , crude polysaccharide was extracted from the leaf skin of Aloe vera and purified into three fractions, designated as ASP-4N, ASP-6N and ASP-8N, using gradient ammonium sulfate precipitation. The physicochemical properties and structural characteristics of ASP-4N were systematically investigated by methylation analysis and 1D/2D NMR spectroscopy. Results showed that decreasing contents of neutral sugar (79%–74%) were detected among the three fractions, with the concentration increasing of ammonium sulfate used for precipitation. Small amounts of protein and uronic acid were also detected by colorimetric method. Homogeneity identified by high performance gel permeation chromatography (HPGPC) indicated that the three fractions were highly purified. The relative weight average molecular weights for ASP-4N, ASP-6N and ASP-8N were 339, 130 and 67.6 kDa, successively. Monosaccharide compositions and characteristic FT-IR spectra of these fractions suggested the presence of typical acetylated glucomannan and the ratios of mannose/glucose were 19.13, 8.97 and 2.96, successively. Further structural analysis of ASP-4N suggested that it was a highly acetylated (1 → 4)-β-glucomannan. There was also some (1 → 3)-β-Man p mixed in the backbone.