Cell wall polysaccharides of bush butter (Dacryodes edulis (G Don) HJ Lam) fruit pulp and their evolution during ripening

Cell wall material was isolated as alcohol-insoluble solids (AIS) from bush butter endocarp tissue at different stages of ripeness. AIS were then extracted with 0.05 M CDTA followed by increasing concentrations of KOH (0.05, 1 and 4 M respectively). The chemical extractions solubilised a total of 51.6-60.6% of AIS, the yields of CDTA extracts accounting for approximately 9.6-12.2% of AIS. The extracts as well as the residues were analysed for their sugar composition and protein and starch contents. CDTA extracted the bulk of uronic acid in AIS, but the uronic acid content (after dialysis) of these extracts showed a significant decrease as the fruits ripened (from 439 to 252 mg g-1 between the first and the last degree of ripeness). Analysis of the CDTA extracts by anion exchange and size exclusion chromatography showed a gradual appearance of new pectic populations at low degrees of methylation and low molecular weights, indicating that CDTA-soluble pectins are demethylated and depolymerised during ripening. The dilute alkali (0.05 M KOH) extracts were essentially composed of proteins in addition to a minor quantity of pectin. The 1 M KOH and principally 4 M KOH treatments led to the extraction of hemicelluloses, mainly xyloglucan-like and mannan-like polymers. These extracts also contained substantial amounts of protein and starch. No variation related to the degree of ripeness was visible in the sugar composition of the alkali extracts. The molecular weight distribution of the hemicelluloses did not change with the degree of ripeness. The final residues accounted for 21.4-27.3% of AIS and were mostly composed of glucose (827-908 mg g-1). All these results suggested that only CDTA-soluble pectins were involved in bush butter fruit softening. © 2001 Society of Chemical Industry.