Changes in Pectic and Hemicellulosic Polymers of Green Beans (<i>Phaseolus vulgaris</i>L.) during Industrial Processing

Abstract

Pectic and hemicellulosic material was extracted from the alcohol-insoluble residues of two fresh and industrially processed green bean varieties. The carbohydrate compositions and relative molecular masses of these fractions were determined. Fresh beans of cv. Odessa contained more pectin and hemicellulose but less cellulose than cv. Masai. The major changes occurred during sterilization and concerned the pectic polymers. In fresh beans the major part of the pectins was most likely covalently linked to other cell wall polymers since they were extractable only after mild saponification of the cell wall. Sterilization resulted in degradation and solubilization of pectic polymers from the cell wall and middle lamella. The galacturonic acid backbone was partially degraded. Side chains, however, were not removed from the pectic polymers, resulting in an accumulation of branched pectins of low molecular mass in the buffer-soluble fraction. The amount of cyclohexane- trans -12,-diaminetetraacetate-soluble material showed a small increase after blanching, but the amount after sterilization was the same as compared with the fresh material. These results confirm the hypothesis that pectins are solubilized by degradation of the galacturonic acid backbone during industrial processing. The changes in pectic polymers during processing are summarized in a scheme. No significant changes were observed in the hemicellulose and cellulose fractions. Keywords: Cell walls; pectin; hemicellulose; processing; Phaseolus vulgaris; texture; dietary fiber