Comparative spectroscopic characterization of different pectins and their sources

The results of atomic absorption spectrometry (AAS) analyses and Fourier transform infrared (FTIR) spectroscopic studies of several pectins obtained from pumpkin and sugar beet, as well as of their vegetable sources, are compared and discussed. Special emphasis is put on the state of carboxylic groups of the polymer backbone and the mineral composition of both the sources and the resulting pectins, including the content of alkaline (Na, K) and alkaline-earth metals (Mg, Ca), as well as traces of heavy metals (V, Fe, Cu, Pb). The pectins were obtained from dried pumpkin pulp by extraction with dilute hydrochloric add or using a biotechnological process involving the multi-enzyme cell-free culture supernatant from the bacterium Xanthomonas campestris; commercial sugar beet pectin extracted by the standard method of acid treatment was obtained from a sugar beet processing plant in Krasnodar (Russia). For comparison, a sample of commercial acid-extracted citrus pectin (Copenhagen, Denmark) was also studied. The results obtained show that potassium seems to occur as a relatively free constituent, whereas a more specific interaction between sodium ions and pectic substances may be assumed depending on the origin of the pectin and obviously on its properties. Much higher amounts of Mg and, especially, Ca found in pumpkin biopectin as compared to all of the three pumpkin, sugar beet and citrus acid-extracted products correlate with a relatively well exhibited capability of pectins to bind these two cations, which is noticeably suppressed in acidic media. The increased content of Ca (and, probably, Mg) may in principle contribute to poorer gelling properties of pumpkin pectin and, in general, of biopectins as compared to the corresponding acid extracts. The results on the mineral fraction of the samples are compared considering the FTIR spectroscopic data for the pectins studied as well as for their sources featuring, in particular, the state of carboxylic groups responsible for metal binding. It has also been found that lead and copper essentially accumulate in pectins upon extraction, whereas iron does not, being relatively more weakly bound by pectic substances (which may, however, depend on its oxidation state) than other heavy metals; the accumulation process is slightly (for Fe and Pb) or not at all (for Cu) suppressed during acid extraction. Comparing the content of vanadium in the pectins and their sources, it may be concluded that this element occurring in plant tissue obviously in different chemical forms may be partly transferred to pectin during its extraction in a proportion similar to that in which it is bound to pectic substances in the plant cell wall, thus indicating its strong binding not affected by acid treatment. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.