Aspen, acidic hemicelluloses

Figure 2. Time profiles of dry biomass, P(3HB-co-3HV) yields, and PHA contents for shake-flask cultures o/Burkholderia cepacia grow/ on detoxified aspen-derived hemicellulosic hydrolysate and 0.45 % (w/v) levulinic acid.

Table 1. Physical characteristics of P(3HB-cii-3HV) copolymers produced by Burkholderia cepacia from detoxified NREL CF/aspen-based hemicellulosic hydrolysate and levulinic acid.

Analyses of the hemicelluloses of woods indicate the presence of a high proportion of D-xylose residues in association with those of a methyl ether of a hexuronic acid.64 The first evidence for the chemical structure of such wood xylans came from the isolation of xylobiose, xylotriose,65 and the aldobiouronic acid 0-(4-0-methyl-a-D-glucosyluronic)-(l —> 2)-D-xylose acid (IX)22 from partial hydrolyzates of aspen wood. The role of such oligosaccharide fragments in wood xylans became more apparent as the result of a study of a xylan from European beechwood,26 in which it was shown... 

Cao et al. [60-62] examined a fractionation option that used corn cob and aspen woodchip as the substrates. In this biomass fractionation scheme (Fig. 6), the majority of lignin, alkaline extractives, and acetate were solubilized and separated from cellulose and hemicellulose fractions by alkaline treatment. Hemicellulose was then hydrolyzed to its sugar constituents with dilute acid (0.3 M HCl). Hemicellulose carbohydrates were then fermented to ethanol by a xylose-fermenting yeast strain (Fig. 7). The cellulose fraction, after separation from lignin and hemicellulose, was used as the substrate in the SSF process for ethanol production using a thermotolerant yeast strain as the biocatalyst (Fig. 8). 

Air-dried com stover provided by the Agricultural Utilization Research Institute was milled and then screened, and only the fraction less than 2-mm sieve was used in this research. Aspen pulping wood chips (Populus tremuloides) with a pretreatment, which were the intermediates of a biorefinery process, were provided by the Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota. Aspen wood chips that passed through a 1 -in. sieve but not through a 0.5-inch sieve were used. Aspen wood chips were pretreated with dilute sulfuric acid to remove partial hemicellulose. 

The aspen wood chips were pretreated with 1 % dilute sulfuric acid at 160°C for 4 h to remove partial hemicellulose. 

The spectrum of aspen wood (Figure 7.1) shows the presence of the three fundamental wood constituents. A band for cellulose is at 895 cm (g-anomer in pyranose ring) <22). xhe band at 1740 cm" is due to uronic acid and acetyl groups in hemicellulose. The bands from 1235 to 1605 cm, specially the one at 1505 cm", are representative of lignin while those from 950-1100 cm" are due, in part, to carbohydrates (C-0 bonds in alcohols). 

Acetic acid, formic acid and ievulinic acid are the most common carboxylic acids found in the hydrolyzates. Acetic acid is not only a by-product of hydrolysis (45) but is also a well-known by-product in fermentation (46). Acetic acid is mainly formed from acetylated sugars in the hemicellulose, which are cleaved off already at mild hydrolysis conditions. Therefore, die acetic acid yield in the hydrolysis does not significantly depend on the severity of the hydrolysis process (2). Hydrolysis of hardwoods (alder, aspen and birch) at 198-234°C, 0.5 g/1 H2SO4, and 33% wood dry materials for 7 min resulted in approximately 10 g/l acetic acid, whereas the hydrolysis of softwoods (pine and spruce) produced 3 g/1 acetic acid at similar conditions (2). 

Isol. from the partial acid hydrolysates of white spruce (Picea glauca), western hemlock (Tsuga heterophylla), jute fibre, maritime pine (Pinus pinaster), aspen (Populus tremuloides) hemicelluloses. 

HV) sample produced by Burkholderia cepacia from detoxified (aspen-based) NREL CF hemicellulosic hydrolysate with 0.3 % (w/v) levulinic acid. The expanded inset displays the field locations and relative intensities of the three carbonyl diad sequences (3HV-3HV, 3HB-3HV, 3HB-3HB, left to right). 

Thermal and viscosity-based characteristics of the P(3HB-co-3HV) samples produced from detoxified aspen hemicellulosic hydrolysate and levulinic acid are presented in Table I. The hemicellulosic hydrolysate-based samples displayed melting temperatures that decreased from 157 C to 98-103 C, with an increase in the mol % 3HV fraction of the copolymers. This T profile is relatively consistent with the pseudoeutectic behavior previously reported for other isodimorphic, P(3HB-co-3HV) copolymers (32,33). The isodimorphic properties displayed by these short-chain length copolymers relate to the similar size and crystalline lattice conformations of the monomers, which permit crystallization of polymer chains within either the 3HB or 3HV lattices (33). The Tm pattern determined for W. eutropha-der wQd P(3HB-co-3HV) samples... 

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