STRUCTURE AND PROPERTIES OF APPLE AND SUGAR-BEET PECTINS EXTRACTED BY CHELATING-AGENTS

Pectins were extracted from apple or sugar-beet cell walls (alcohol-insoluble solids) at pH 4.5 and 20-degrees-C and at pH 6.5 and 80-degrees-C in acetate and phosphate buffer solutions, respectively, and in the same buffers containing cyclohexanediaminetetraacetic acid or ethylenediaminetetraacetic acid. The yields were very different between the two pH and temperature conditions, but variations with the nature of the extractant were small at pH 4.5 and 20-degrees-C, and no differences were observed at pH 6.5 and 80-degrees-C. The pectins extracted at pH 6.5 and 80-degrees-C were richer in neutral sugars and acetic acid. Degradation of the pectins extracted at high pH and temperature was shown by the decrease of the intrinsic viscosity and by the shift of the uronic acid peak toward the total volume in gel-filtration chromatography. These pectins contained a high molecular weight population rich in neutral sugars and a low molecular weight acid population, in contrast to the pectins extracted at low temperature and pH, for which neutral and acidic sugars had similar elution profiles. At pH 6.5 and 80-degrees-C, rather extensive degradation of the pectins occurred. Calcium-binding experiments (potentiometry with specific electrode and conductimetry) indicated a statistical distribution of free carboxyl groups. These results do not support the hypothesis that chelating-agent extractable pectins are held in the cell wall by calcium cross-links involving the formation of ''egg-boxes''.