Composition of wood pectins and the enzymatic control of pectic polysaccharides in mechanical paper pulps

Abstract

Pectin is a complex class of polysaccharides that occur in the plant cell-walls. It is concentrated in the middle lamella and cell corners with a gradual decrease from the primary cell-wall to the plasma membrane. The pectic polysaccharides consist of a diversity of monosaccharides that make them heterogeneous in composition, structure and molecular weight. Current knowledge on the structure of these pectic polysaccharides is based on intact pectins extracted from soft plant tissue. Intact pectin can, however, not be extracted from woody material by the same methods. Very little is, therefore, known about the occurrence of the pectic polysaccharides in wood and it was speculated that between 10 to 40 mg/g of the total dry weight of wood consists of pectin. The occurrence and composition of pectin in wood are of importance in the pulp and paper industry, due to its influence in pulping and papermaking processes. The current work was, therefore, the first attempt to quantify and elucidate the possible domain structure of pectin from woody tissue. Wood samples were hydrolysed and the pectic monosaccharides (D-galacturonic acid, D-galactose, L-arabinose and L-rhamnose) quantified using high performance liquid chromatography. Through the addition of all pectic monosaccharides, it was determined that eucalyptus wood contained between 15.2 and 25.8 mg/g pectin. It was shown that Eucalyptus macarthurii contained significantly more pectin than E. grandis and E. nitens. The wood tissue type also influenced the occurrence of total pectin and the cambium contained higher concentrations of pectin than sapwood and heartwood. The molar concentration of the pectic monosaccharides was expressed as a relative amount to the total pectin in wood and it was determined that D-galacturonic acid occurred in the highest concentrations in eucalyptus pectin, followed by D-galactose, L-arabinose and L-rhamnose. Models of the pectic domains were constructed on the basis of these results and homogalacturonan (HG) was the predominant pectic domain, contributing up to seven times more to the total backbone component than rhamnogalacturonan-I (RG-I). The macromolecular composition of the pectin in different eucalyptus species proved to be very diverse. Future studies should include a bigger pool of species to get a more comprehensive understanding of the occurrence and structure of pectin in the Eucalyptus genus and also other hardwoods. The tissue type of the wood also had a significant influence on the macromolecular composition of the eucalyptus pectin, confirming that a shift occurred in composition of pectin as the wood tissue aged. Information on the occurrence and composition of pectin in eucalyptus will improve understanding of the influence of these polymers on pulping and papermaking processes and this knowledge can, therefore, be applied to develop biotechnological approaches to improve these processes. The pectins in wood are released as polygalacturonic acid (PGA) when mechanical pulps are bleached with peroxide under alkaline conditions and contribute to approximately 50% of the anionic substances in the pulp and water. It was demonstrated that pectinase can efficiently depolymerise the PGA into monomeric galacturonic acid and consequently reduce the cationic demand (CD) of mechanical pulp. High-brightness softwood pulp from a chemi-thermo mechanical pulp mill was identified as the pulp source that contributed proportionally the highest amount of CD to the stock going onto a paper machine. A pectinase (Nalco 74303) was applied during a short mill trial to reduce the CD across the storage tower for high-brightness pulp. The enzyme treatment reduced the CD to levels comparable to that achieved with the usual alum treatment at the mill. The enzyme treatments did not have a negative effect on any of the strength properties of the pulp and future trials should be conducted to determine the impact of the pectinase on the wet-end processes over an extended period.