Softwoods pretreatment

The Purdue concepts have been applied to several different agricultural products, such as corn stalks, alfalfa, orchard grass, tall fescue, and sugarcane bagasse. No experiments have been reported on either hardwoods or softwoods. The processes have been explored in two major modes. In the first, the entire agricultural residue is treated with solvent in the second, a dilute acid pretreatment to remove hemicellulose precedes solvent treatment. The first process is especially desirable for making furfural or fermentation products from hemicellulose as a separate activity. Then, the hemicellulose-free raw material can be converted to substantially pure glucose. 

Palonen, H., Tjemeld, F., Zacchi, G., and Tenkanen, M., 2004, Adsorption of Trichoderma reesei CBH I and EG II and their catalytic domains on steam pretreated softwood and isolated lignin, J. Biotechnol. 107 65-72. 

The maximal recovery of hemicellulose (83.8%) was obtained following the mildest pretreatment at 190°C for 2 min using 0.5% H2S04. The harsher the conditions, the lower the hemicellulose recovery was reached. The maximum cellulose recovery was 89.7%, following steam pretreatment at 200°C, but the recovery was sufficient, about 75%, even at 210°C. Cellulose and hemicellulose exhibited maximum recovery at different conditions, which was also found in other studies (24,25). However, in the case of softwood, the hemicellulose solubilization needed harsher pretreatment conditions than corn stover, and the maximum mannose yield from wood was obtained at between 200 and 210°C (26). 

A typical wood-to-ethanol bioconversion process consists of at least three major steps pretreatment, hydrolysis, and fermentation. The pretreatment stage has been shown to be the key step to providing a substrate susceptible to the subsequent hydrolysis. Steam explosion is one of the most intensively studied pretreatment methods for bioconversion of softwood materials (6-10). 

A 72-h hydrolysis profile of a 10% acetic acid-pretreated softwood substrate (Fig. 1) represents a typical enzymatic cellulose hydrolysis course with the majority of the cellulose (up to 70%) broken down within the first 24 h. However, the conversion of the remaining cellulose ( 30%) was incomplete, even after another 2 d of incubation. The decrease in the hydrolysis rate in the latter phase is likely owing to accumulation of end products. To demonstrate that the end products played a major inhibitory role, we removed the produced sugar from the hydrolysate through ultrafiltration. Fresh buffer was then added to the retained protein and the residual substrate to attain the initial volume, and the hydrolysis was continued under the same condition. As shown in Fig. 1, significant increases in the hydrolysis rate were observed after the sugar removal at both 24 h and 48 h of incubation, with complete hydrolysis attained after 48 h and 60 h of incubation respectively. 

Avicel, and acetic acid-pretreated softwood—after a 30-min incubation with respective enzymes. 

Fig. 7. Inhibitory effects of supplemented hemicellulose-derived sugars on hydrolysis of 10% acetic acid-pretreated softwood.

Fig. 9. Total sugar concentrations during combined hydrolysis (10% acetic acid-pretreated softwood substrate with prehydrolysate).

The NREL Pichia strain NPw9 was equivalent to the D5A yeast in ethanol production however, it did not utilize xylose under the anaerobic conditions used in the screening experiments (data not shown). However, the Pichia strain NPw9 has been shown to ferment xylose in toxic pretreated softwood hydrolysates under microaerophilic conditions (23). 

Soderstrom, J., Pilcher, L., Galbe, M., and Zacchi, G. 2003. Two-step steam pretreatment of softwood by dilute H2S04 impregnation for ethanol production, Biomass Bioenergy, 24,475 186. 

Schell, D., Nguyen, Q., Tucker, M., and Boynton, B., Pretreatment of softwood by acid-catalyzed steam explosion followed by alkali extraction. Appl Biochem Biotechnol 1998, 70-2, 17- 24. 

Preparation of Hot-Melt and Self-Bonded Boards. Reaction conditions for benzylation are dependent on wood species and specimen size. For Akamatsu particles, pretreatments and reaction conditions are as follows wood is first oven-dried at 105°C for 24 h and then immersed in 40% sodium hydroxide solution for 1-2 h at room temperature. These particles are then squeezed to remove excess sodium hydroxide and reacted with benzyl chloride at 120°C for 1-2 h. Benzylated particles are then washed in water and any unreacted reagent is removed by squeezing. Particles are then washed further in a mixture of water-methanol (1 2 v/v) and air-dried for 48 h at 20 C. In general, benzylation of softwood species is more difficult than for hardwoods, and the reactivities of wood from fast-growing trees, i.e., willow or monocotyledons,... 

A peroxide stage, P stage, is occasionally used as a polishing bleach for chemical pulps, using the conditions discussed. Peroxide has also been tested for initial bleaching of softwood kraft pulps after an acid pretreatment. A significant reduction in waste loads is obtained but at high cost [28]. 

Chemically pretreated chips, for example with sodium sulphite in the well established CTMP process, require more energy to produce pulps of comparable freeness than corresponding TMP pulps (Table 13.3). On the other hand most softwoods CTMP pulps generally require less energy to reach a given level of strength (for example tensile index) than corresponding TMP pulps. The reasons for such differences include ... 

Leary and Giampaolo [70] found that unbleached softwood TMP behaved differently upon alkali treatment than peroxide-bleached TMP. Unbleached softwood TMP was bleached with peroxide (4% charge) to an ISO brightness of 78% however, alkali-darkened pulp could only be bleached to 68% with the same peroxide charge. Borohydride reduction of the pulp before the alkali treatment inhibited the darkening and almost fully restored its bleachability. The absorption maximum for alkali darkening of unbleached pulp appeared at 420 nm, but was shifted to 360 nm if the pulp was pretreated with borohydride. 

Pew (26) found digestibility of hardwoods by cellulases improved markedly by pretreatment with aqueous NaOH the effect was considerably more pronounced with hardwoods than with softwoods. Stranks (35) reported that the in vitro rate of digestion of hardwoods by rumen-inhabiting bacteria, measured by succinic acid production, was markedly increased by pretreatment with 1-5% NaOH, whereas softwoods were unaffected by the treatment. The effect of NaOH in improving the digestibility of straw has long been known (5). 

---