Carboxymethylation of hemicellulose isolated from poplar (Populus grandidentata) and its potential in water-soluble oxygen barrier films

Abstract

Hemicelluloses, the second most abundant class of biopolymers, have emerged as an immense renewable resource of biopolymers that are recognized as currently being underutilized. Carboxymethyl carbohydrates are valuable products for thickeners, viscosity control, food additive and others. Hemicellulose isolated from poplar was converted to carboxymethyl hemicellulose using sodium chloroacetate (SCA) and sodium hydroxide (NaOH). The significance of the effects of carboxymethylation conditions in an 80% ethanol/water medium on the degree of substitution (DS) and reaction efficiency (RE) was evaluated using a definitive screening design model. There was strong evidence that the dosage of SCA and NaOH, and temperature have a significant effect on DS. A significant interaction between SCA and NaOH was also statistically confirmed. The RE resulting from the ethanol/water medium was lower than 50%. A higher RE of 72% with a DS 1.08 was achieved in a 90% tert-butyl alcohol/water medium, which was conducted at 85 °C with 120 min, 1.5 mol/mol anhydroxylose unit (AXU) of NaOH and 1.5 mol/mol AXU of SCA. The potential of using hemicellulose and carboxymethyl hemicellulose as materials to produce water-soluble films was evaluated. Due to the increased water solubility, the processing of carboxymethyl hemicellulose in water and the production of films were more facile than with unmodified hemicellulose. All of the films demonstrated significant oxygen barrier characteristics, with low oxygen permeability ranging from 0.28 to 0.55 cm3 µm/(m2 d kPa). The films did not display effective water vapor barrier characteristics, with high water vapor permeability ranging from 4.8 to 6.5 g mm/(m2 d kPa). Carboxymethylation of HC decreased the tensile strength, increased the elongation at break, and decreased the water vapor and oxygen barrier properties of the produced films. Both hemicellulose and carboxymethyl hemicellulose films could offer a bio-based and biodegradable alternative to existing synthetic oxygen barrier materials. The hemicellulose and carboxymethyl hemicellulose materials might be exploited in water-soluble films, washable fruit coating or used in composites or multilayer films in conjunction with hydrophobic layers.