Novel and diverse fine structures in LiCl–DMSO extracted apple hemicelluloses

• LiCl–DMSO extracted most apple hemicelluloses in their acetylated form.
• LiCl–DMSO- and 4 M KOH-soluble xyloglucan differ by their molecular weight.
• Galactoglucomannan (GgM) differs on the content and distribution of glucose and mannose.
• Galactosylation of GgM occurs on mannose blocks in glucose rich chains.
• Apple hemicellulose contains acetylated glucuronoarabinoxylan.

Hemicelluloses are key polysaccharides in the regulation of the mechanical properties of plant cell walls during organ development and in fruit texture. Their diverse compositions and structures are partially known, in particular with regard to their function in cell walls. To that end, apple hemicelluloses were sequentially extracted by DMSO doped by LiCl followed by potassium hydroxide. The weakly bounded hemicelluloses in the LiCl–DMSO soluble extract were fractionated by ion exchange (AEC) and size exclusion (SEC) chromatographies. The structure of all the extracts and fractions was established by enzymatic fingerprinting using β-glucanase, β-mannanase and β-xylanase. Molecular weight of the fraction was established by HPSEC. MS as well as HPAEC analyses of the enzyme digests revealed the remarkable diversity of apple hemicelluloses. Different xyloglucan (XyG), galactoglucomannan (GgM) and glucuronoarabinoxylan were isolated along the extraction and fractionation process. All LiCl–DMSO soluble fractions were acetyl-esterified. Besides, the LiCl–DMSO soluble XyG differed from the 4 M KOH extracted one essentially on the basis of its molecular weight. At least two populations differing in their content and distribution of glucose and mannose composed GgM. Moreover, galactose ramifications occurred on mannose blocks in the glucose rich fraction. These results open the way for future studies on the complex structure–function relationship of hemicelluloses in plant cell walls.