Browsing by Author "Gomes, Katiana Raquel da Gama"

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    Enzymatic modification of the functional properties of xylan from lignocellulose feedstocks
    (Stellenbosch : Stellenbosch University, 2012-12) Gomes, Katiana Raquel da Gama; Chimphango, Annie F. A.; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: In the past decades, sustainable alternatives to petroleum-derived products have been explored. While fossil derived products are still the main source of energy and chemicals worldwide, they are the major contributors to the increased emission of green house gasses (GHG’s), responsible for the climate change. Lignocellulose represents a more sustainable alternative since it is biodegradable, renewable and does not contribute to GHG emissions to the same extent as fossil-based resources. Hemicelluloses are the second most abundant class of polysaccharide biopolymers on earth after cellulose, based on widespread availability in nature of the lignocellulosic plant biomass in which they occur. Xylan represents between 15 and 30% of lignocellulose in hardwood plant species, between 25-35% in grasses and in lower proportions in softwood (between 7 and 12%). In pulp and paper industries, xylan is dissolved and separated from cellulose along with the lignin under harsh pulping conditions, and subsequently burned as black liquor for energy generation. This process represents an under-utilisation of hemicellulose feedstocks, due to its low specific heating value (13.6 MJ/kg) compared with lignin (27 MJ/kg). However, the xylans that can be extracted from lignocellulose, either during pulping or in dedicated processes, have limited applications due to low functionality as a biopolymer, mainly high solubility in water. The objective of this study was to investigate the enzymatic hydrolysis of xylans, extracted from different lignocellulosic feedstocks available in South Africa and to find optimum conditions for modifying chemical and functional properties of the xylans for industrial applications. The glucuronoxylan from Eucalyptus grandis was extracted using protocols adopted from Höije et al. (Höije et al., 2005) and Pinto et al. (Pinto et al., 2005). The arabinoglucuronoxylan from sugarcane bagasse was extracted following the protocol adopted from Höije et al. (Höije et al., 2005). Beechwood xylan (Sigma) was used as the model xylan. The xylan was extracted from E. grandis and sugarcane bagasse using the Höije protocol with yields of 20 and 71% and uronic acid contents of 21 and 7.05%, respectively. The molecular weight distribution showed that the major fraction had a degree of polymerization of 287 in E. grandis xylan (Höije). The xylan was extracted from E. grandis by the Pinto protocol with a yield of 89%. However, the degree of polymerization was 133 due to polysaccharide degradation. The model xylan from beechwood had the highest purity levels in terms of xylose content but the lowest degree of polymerization, corresponding to 77. Selective removal of arabinose and 4-O-methyl glucuronic acid by the enzymes α-L-arabinofuranosidase and α-D-glucuronidase, respectively, caused formation of water-insoluble xylan particles. Partially purified α-D-glucuronidase at concentrations between 416 and 462 mg/L were used in the selective enzymatic hydrolysis of 4-O-methyl glucuronic acid side-groups in xylans extracted from different lignocellulosic feedstocks. The minimum time required for selective hydrolysis of the glucuronoxylans at concentrations ranging from 1.1 to 5.0% (w/v) in study was 24 hours. Consequently, the minimum degree of substitution required for the precipitation of beechwood xylan, E. grandis xylan (Höije) and E. grandis xylan (Pinto) was 1:40, 1:6 and 1:25, respectively. The highest release of 4-O-methyl glucuronic acid was found at α-D-glucuronidase dosage of 6.4 mg/g combined with 1.87% (w/v) substrate in beechwood xylan, 3.08% (w/v) in E. grandis xylan (Hoije) and 5.03% (w/v) in E. grandis xylan (Pinto). Optimum conditions for increase in viscosity were found at an enzyme dosage of 6.4 mg/g combined with 4.98% (w/v) of substrate in beechwood xylan, 4.3% (w/v) in E. grandis xylan (Höije) and 4.5% (w/v) in E. grandis xylan (Pinto). The most significant factor for the release of 4-O-MeGlcA was the α-D-glucuronidase dosage (p-value < 0.03), whereas the substrate concentration (p < 0.006) was the most significant factor to maximise viscosity. Enzymatic hydrolysis with both α-D-glucuronidase and α-L-arabinofuranosidase for release of 4-O-methyl glucuronic acid and arabinose from sugarcane bagasse xylan resulted in precipitation. A positive synergy was verified in the release of both side-chains, which was dependent on the dosage ratio between α-L-arabinofuranosidase and α-D-glucuronidase. Optimum conditions for precipitation were found at a dosage of 6.4 mg/g of α-D-glucuronidase and 150 nkat/g of α-L-arabinofuranosidase, with 25% precipitation. On the other hand, the highest release of side-chains was verified at a dosage of 6.4 mg/g of α-D-glucuronidase and 350 nkat/g of α-L-arabinofuranosidase, with 69.5% release of arabinose and 24% release of 4-O-MeGlcA. Morphological analysis of the modified xylans indicated that enzymatic treatment improved both the gelling and plasticising properties. Removal of 4-O-methyl glucuronic acid from glucuronoxylans resulted in increased viscosity, and formation of hydrogels. Arabinose and 4-O-methyl glucuronic acid removal from arabinoglucuronoxylan also resulted in formation of insoluble xylan particles that settled with gravity. The highest viscosity rheological properties was observed with E. grandis xylan (Höije), followed by the model xylan from beechwood. Particle agglomeration was more evident on less substituted glucuronoxylans with a minimum degree of substitution of 4-O-MeGlcA of 3%. The micro particles were found to increase in size with an increase in the xylan concentration, from a minimum concentration of 4.45% (w/v), during enzymatic hydrolysis. The glucuronoxylan hydrogels and arabinoglucuronoxylan insoluble particles formed at optimum conditions had particles sizes ranging from 0.4 to 1.97 μm. It was concluded that the enzymatic modification is a mild and selective chemical process that can add functionality to beechwood, E. grandis and sugarcane bagasse xylans, by reducing their solubility in water and can be further applied as pulp additives, paper coatings, packaging films and gel encapsulation matrices.