SOP for analysis of cell wall polysaccharides from yam and plantain

Cell walls in plants are mainly composed of polysaccharides. Among them, pectin, cellulose and hemicellulose are composed of various sugars residues (acidic: galacturonic acid in pectins; or neutral: rhamnose, galactose, arabinose, xylose, glucose in cellulose, hemicellulose and pectin side chains). This fine composition impacts the structure but also the cooking behavior and mechanical properties of cell walls. This method seeks to determine the cell wall composition (neutral sugars composition and galacturonic acid content) of destarched AIS (Alcohol Insoluble Solid) extracted from raw and cooked yam and banana. It is applicable for AIS extracted from every fruit or vegetable product and containing a low amount of residual starch. For RTB products a destarching step is mandatory before analysis. First the samples are hydrolyzed in order to obtain sugar residues. Pectin and hemicellulose are degraded into their individual neutral and acidic sugars by a simple acid hydrolysis (hot concentrated sulphuric acid). In contrast, cellulose has to be solubilized under harsher conditions. For this reason, a double hydrolysis is performed. Afterwards, the free neutral sugars are derivatized into volatile alditol acetates before their separation in Gas Chromatography with a Flamme Ionization Detector (GC-FID). They are then quantified thanks to an internal standard of inositol. Whereas free galacturonic acid, transformed to its furfuric derivatives, is quantified by colorimetry after reaction with 3-phenylphenol (MHDP) which produces a fuschia pink complex with a maximum absorption at 520 nm. This SOP could be applied in all labs working on cell wall and pectins.