COMPOSITION AND STRUCTURE OF CELL WALL POLYSACCHARIDES FROM SUGAR BEET GROWN UNDER MEDITERRANEAN CLIMATE AND RELATION WITH BEET PROCESSING

摘要

Cell wall polysaccharides were isolated as alcohol-insoluble solids (AIS) from Moroccan sugar beet, wet pulps, pressed pulps and diffusion juice at two stages of the harvest season (Early and Late samples). Pectins and hemicelluloses were extracted from beet slices and pulp by a sequential treatment with water, CDTA, hot dilute hydrochloric acid, cold dilute sodium hydroxide, cold 1 M and 4 M sodium hydroxide, and finally water at pH 7. Pectins were also extracted from sugar beet brei by hot water under conditions simulating the sugar factory. The highest extraction yields were obtained for the alkali and dilute acid treatments. Pectins extractable with water still remained in the pressed pulps (1.5 % of AIS). Composition of the pectins and hemicelluloses extracts was determined and the purification carried by ion exchange chromatography. Percentages of methylation of pectins were determined. Phenolic acids associated with the cell wall polysaccharides of pressed pulp were also extracted and analysed by HPLC. Marked differences in yield and composition of beet polysaccharides can be obtained in relation to the variety and the growing site of crop. The polysaccharides precipitated with alcohol from factory diffusion juice were mostly pectic in nature and the neutral sugars were composed mostly of arabinose, galactose, glucose and rhamnose. The extraction and degradation of pectins in the sugar factory depends on the processing parameters. The level of pectins degraded under hot alkaline conditions during the purification step in the factory can be determined by measuring the amount of total galacturonic acid in diffusion and thin juices. It was shown on model solutions that the presence of monomers of galacturonic acid in the factory juice increases the colour formation and affected negatively the processing performances. AIS of Moroccan sugar beet pulp was very rich in xylose, but had lower arabinose and uronic acid contents. These characteristics and the content of ferulic and diferulic acid could be used to give an added value to the pressed pulp which is still used as animal feed only.