Purifying sugar beet pectins from non-pectic components by means of metal precipitation

Abstract

Metal precipitation is an effective method for the purification of pectins. To study the selectivity of copper ions toward binding pectic or non-pectic compounds, two fractions, namely copper-precipitated pectins (CPP) and copper-unprecipitated pectins (CUP) were fractionated from the mother sugar beet pectins by means of copper precipitation. Comparison on chemical feature revealed the isolated fractions were structurally different in that CUP contained markedly higher neutral sugars (NS) and protein contents, but lower galacturonic acid and acetyl ester group contents than CPP. However, no appreciable difference was observed in terms of degree of methylation (DM). Elution profiles on high performance size exclusion chromatograph showed that CPP exhibited a monomodal-like molecular weight distribution (MWD), while CUP demonstrated a multimodal-like MWD pattern composed of three relatively broad peaks, and a narrow peak that was associated with a high intensity of UV signal at 278 nm. Moreover, AFM analysis showed that CPP displayed branched fibrous structures, while CUP showed distinguishable granular-like shapes. Results obtained in this work indicate the employed copper ions selectively bind the anionic regions among pectin chains, thereby separating pectic saccharides from non-uronide compounds, i.e. low molecular weight carbohydrates and free proteins. The poor affinity of copper ions toward CUP might be determined by the absence of typical homogalacturonan (HG) regions.