Polysaccharides from prickly pear peel and nopals of Opuntia ficus-indica:: extraction, characterization and polyelectrolyte behaviour

The extraction at natural pH of carbohydrate polymers from Opuntia ficus-indica and their physicochemical characterization are reported. Polysaccharides extracted from nepal gulp, nepal peel and pear peel, are denoted NPU, NPE and PPE respectively. For comparison, extraction of the peer of fruit has also been carried out using the conventional acid process generally employed for pectin (PPI). The sugar composition indicates that all the polysaccharides obtained contain anionic moieties, galacturonic acid residues, from 10% for NPU to about 50% for those extracted from peels, the composition of which is typical of pectin. The macromolecular features of these compounds have been characterized by size-exclusion chromatography coupled with multi-angle laser light-scattering detection (SEC-MALLS) and by capillary viscometry, leading to information on conformation and size dimension of macromolecules in dilute aqueous solutions. Our findings show that the polysaccharides from nopals have a random coil structure, whereas those from fruit peel exist as a compact sphere which is assumed to be due to a branched structure. The polyelectrolyte character related to the presence of fixed charges on the chain has been investigated as a function of pH and ionic strength. The conformational change from a compact form in acidic medium to an expanded one takes place in the pH range 4-6. A strong tendency to form aggregates in acidic medium is evidenced by SEC-MALLS. The differences observed in the solution behaviour of the studied polysaccharides are discussed in terms of intra- and inter-macromolecular interactions. (C) 2001 Society of Chemical Industry.