A galactan from onion (Allium cepa Linn.) pectic substance

The pectic substance of onion was treated with the enzyme polygalacturonase and then extracted with 70% ethanol to remove galacturonan and araban components respectively. The resulting material was methylated and fractionated to yield pure methylated galactan which on hydrolysis gave 2,3,4,6-tetra-, 2,3,6-tri-, and 2,3-di-O-methyl-D-galactose. Polysaccharide material containing 91·2% galactose, was isolated by separating the crude pectic substance on a DEAE–cellulose column followed by fractionation. Periodate oxidation studies were carried out on these two galactan-rich fractions. The structure of the repeating unit of galactan is discussed in the light of these results.