Changes in cell wall polysaccharides from ripening olive fruits

Abstract

Cell wall material was isolated as alcohol insoluble solids (AIS) from olives at three stages of ripeness. AIS contained 38.8 to 45.6% carbohydrates and 21.5 to 24.8% proteins. Glucose, xylose, arabinose and uronic acids were the major constituent sugars. AIS was sequentially extracted with hot buffer (HBSS), chelating agent (ChSS), dilute alkali(DASS), 1 m alkali (1 m ASS), 4 m alkali (4 m ASS), 4 m alkali + borate (A/BO 3 3-), 6 m alkali (6 m ASS) and water (WSS) to leave a cellulose-rich residue (RES). These extracts were characterised by their sugar composition and their molecular weight distribution. The yields, sugar composition, degree of branching and molecular weight distribution of the pectin-rich extracts (HBSS, ChSS, DASS and 6 m ASS) changed during ripening, whereas no significant changes were detected in the other extracts. The degree of branching of the pectins increased with increasing strength of the extractant; the ratio ara:uronic acids increased from 0.46 to 2.59. These extracts were further characterised by their degradability with polygalacturonase (PG) and rhamnogalacturonase (RGase) after chemical saponification. The digests were analysed by high-performance size-exclusion chromatography (HPSEC) and high-performance anion-exchange chromatography (HPAEC). The amount of oligomers released by PG from pectic material in olive fruits of corresponding extracts from various stages of ripeness decreased with increasing ripening stage. Oligomers released by RGase were formed in similar amounts from corresponding extracts from various stages of ripeness as well as from each extract. HBSS and 6 m ASS were fractionated by anion exchange chromatography. Five pools were obtained for each fractionated extract. HBSS was rich in strongly bound, uronic acid-rich polysaccharides, whereas 6 m ASS was rich in unbound, neutral sugar-rich polysaccharides.