Extraction and characterization of RG-I enriched pectic polysaccharides from mandarin citrus peel

Pectin is extensively used as thickener and gelling agent in the food industry. Commercial pectin is usually comprised of homogalacturonans (HG) with small amount of rhamnogalacturonan-I (RG-I). However, recent studies have shown that RG-I region enriched pectic polysaccharides possess improved bioactivities, including cancer prevention, cardiovascular disease treatment and fibrosis. Thus, in this study, the isolation of mandarin peel pectic polysaccharides enriched in RG-I was evaluated through a sequential extraction method, consisting of acid followed by alkaline hydrolysis at room temperature. The yields of polysaccharides extracted by acid (PA) and then by base (PB) were 4.2 wt% and 18.9 wt%, respectively. Moreover, the structural and rheological properties of PA and PB were also explored. Monosaccharide composition analysis showed that PA and PB consisted mainly of branched RG-I with GalA/Rha values of 8.4 and 1.7, respectively, and high content of arabinose and galactose. The results of GPC-MALLS confirmed that both PA and PB were branched and had molecular weights of 282 kDa and 743 kDa, respectively. AFM imaging directly verified the branched-chain morphology of these polysaccharides. NMR and FT-IR analyses demonstrated that they were typical pectic polysaccharides, and the esterification degree of PA was 56%, but PB was not esterified. Rheological analysis showed that the two polysaccharides had similar thickening properties when compared to commercial pectin and could be potentially used as functional food ingredients.