Water unextractable polysaccharides from three milling fractions of rye grain

Water unextractable material from bran, an intermediate milling fraction and sieved flour of rye grain were sequentially extracted at room temperature with saturated barium hydroxide, water, 4 M potassium hydroxide and water followed by extraction with 2 potassium hydroxide in a boiling water bath, giving repeatable recoveries of extracts and polysaccharide residue compositions in collected fractions. Total recoveries of polysaccharide residues in extracts and residue from the different water unextractable materials were 78–88%. Extracts in which 90–93% of the carbohydrates were arabinose and xylose residues were obtained by extraction with saturated barium hydroxide. Subsequent extraction with water yielded a fraction in which 64–68% of the carbohydrates were glucose residues. The extraction with hot alkali resulted in extracts in which 85–89% of the carbohydrates were arabinose and xylose residues. The ara/xyl ratio in the collected fractions ranged from 0.1–1.3, with the lowest ratios in fractions that precipitated after neutralisation of the 4 potassium hydroxide extract and the highest ratios in the unextractable residues. Structural characterisation with ¹H-NMR spectroscopy revealed varying substitution patterns for arabinoxylans in the different extracts and that glucose residues in the extracts essentially originated from mixed-linked β-glucan. The proportion of disubstituted xylose residues was lower in barium hydroxide extracts compared to the other main extracts. A highly branched heteroxylan was extracted with hot alkali. The polysaccharides found in the corresponding extracts for all the starting materials had generally similar structural features, but the yield differed considerably.