Solution properties of laccase-treated pectic polysaccharides derived from steam-dried sugar beet pulp

Abstract

Steam-drying of sugar beet pulp enables hot water extraction of pectic polysaccharides without adding acid to the extraction medium with a yield higher than the conventional preparation method but a lower molecular weight. The objective of the present study was to investigate the effect of laccase treatment on the molecular weight and solution properties of the pectic polysaccharides derived from steam-dried sugar beet pulp using hot water extraction. The weight average molecular weight and intrinsic viscosity of the pectic polysaccharides increased from 97.1 kDa to 147 kDa and from 0.383 dL/g to 0.525 dL/g due to isopropanol fractionation to remove low molecular weight fractions, respectively, and then to 661 kDa and 0.832 dL/g by the laccase treatment at the highest laccase rate examined, respectively. The intrinsic viscosity values showed a power-law relationship with the molecular weight, the exponent value of which was found to be 0.32 prior to the isopropanol fractionation or 0.38 after the isopropanol fractionation. These values were much lower than that derived theoretically for a random coil (0.5) and closer to that for a sphere (0.33). Compared to the results previously reported for various polysaccharides, the concentration dependence of the specific viscosity in the semi-dilute region was weaker, the critical concentration which is the threshold between dilute and semi-dilute solutions was higher, and the so-called coil overlap parameter was lower. The present results indicate that the pectic polysaccharides examined in this study had highly branched, compact, and spherical molecular structure.