George T. Tsao

Process considerations in the enzymatic hydrolysis of biomass

Abstract The processes by which cellulases hydrolyse cellulose are a function of substrate reactivity as well as enzyme activity. The two must be considered together if an accurate description of biomass saccharification is to be developed. To accomplish this, cellulolytic systems must first be modelled using realistic, but well-defined, substrates so that optimum cellulolysis conditions can be formulated. This, together with reduction of enzyme cost, total utilization of biomass, and an efficient pretreatment are key elements for the economical conversion of biomass to sugars and fermentation products.

Pretreatment for cellulose hydrolysis by carbon dioxide explosion

Cellulosic materials were treated with supercritical carbon dioxide to increase the reactivity of cellulose, thereby to enhance the rate and the extent of cellulose hydrolysis. In this pretreatment process, the cellulosic materials such as Avicel, recycled paper mix, sugarcane bagasse and the repulping waste of recycled paper are placed in a reactor under pressurized carbon dioxide at 35 °C for a controlled time period. Upon an explosive release of the carbon dioxide pressure, the disruption of the cellulosic structure increases the accessible surface area of the cellulosic substrate to enzymatic hydrolysis. Results indicate that supercritical carbon dioxide is effective for pretreatment of cellulose. An increase in pressure facilitates the faster penetration of carbon dioxide molecules into the crystalline structures, thus more glucose is produced from cellulosic materials after the explosion as compared to those without the pretreatment. This explosion pretreatment enhances the rate of cellulosic material hydrolysis as well as increases glucose yield by as much as 50%. Results from the simultaneous saccharification and fermentation tests also show the increase in the available carbon source from the cellulosic materials for fermentation to produce ethanol. As an alternative method, this supercritical carbon dioxide explosion has a possibility to reduce expense compared with ammonia explosion, and since it is operated at the low temperature, it will not cause degradation of sugars such as those treated with steam explosion due to the high‐temperature involved.

Ethanol Production from Renewable Resources

Vast amounts of renewable biomass are available for conversion to liquid fuel, ethanol. In order to convert biomass to ethanol, the efficient utilization of both cellulose-derived and hemicellulose-derived carbohydrates is essential. Six-carbon sugars are readily utilized for this purpose. Pentoses, on the other hand, are more difficult to convert. Several metabolic factors limit the efficient utilization of pentoses (xylose and arabinose). Recent developments in the improvement of microbial cultures provide the versatility of conversion of both hexoses and pentoses to ethanol more efficiently. In addition, novel bioprocess technologies offer a promising prospective for the efficient conversion of biomass and recovery of ethanol.

Production of multifunctional organic acids from renewable resources.

Recently, the microbial production of multifunctional organic acid has received interest due to their increased use in the food industry and their potential as raw materials for the manufacture of biodegradable polymers. Certain species of microorganisms produce significant quantities of organic acids in high yields under specific cultivation conditions from biomass-derived carbohydrates. The accumulation of some acids, such as fumaric, malic and succinic acid, are believed to involve CO2-fixation which gives high yields of products. The application of special fermentation techniques and the methods for downstream processing of products are described. Techniques such as simultaneous fermentation and product recovery and downstream processing are likely to occupy an important role in the reduction of production costs. Finally, some aspects of process design and current industrial production processes are discussed.

Mycelial Pellet Formation by Rhizopus oryzae ATCC 20344

Factors in a cultivation medium affecting fungal growth morphology and fumaric acid production by Rhizopus oryzae ATCC 20344 were investigated. These factors included the initial pH value and trace metals such as zinc, magnesium, iron, and manganese in the cultivation medium. It was found that a significant change in the growth morphology of R. oryzae ATCC 20344 occurs when the initial pH value is varied. A lower initial pH value in the cultivation medium was inhibitory to fungal growth, and fast growth in the cultivation medium at a higher initial pH value promoted the formation of large pellets or filamentous forms. Trace metals in the cultivation media also had significant effects on pellet formation and fumaric acid fermentation.

Cellulose to Sugars: New Path Gives Quantitative Yield

Cellulosic residues that had been treated with a small amount of chemical solvent under room conditions were quantitatively saccharified on enzyme hydrolysis. This treatment can be used to obtain simple sugars for the production of alcohol and other chemicals.

Pretreatment of sugar cane bagasse hemicellulose hydrolysate for xylitol production by yeast.

Sugar cane bagasse hemicellulose hydrolyzate was prepared by dilute sulfuric acid (3% w/v) hydrolysis with a high-solid, low-liquid ratio followed by leaching. The hydrolyzate contains 11% (w/v) of fermentable sugars with xylose as the major component, which comprises up to 75% of the total reducing sugars. The neutralized hydrolyzate exhibited strong inhibition toward cell growth and ethanol production by yeasts. The inhibitory effect of hydrolyzate can be alleviated by treating hydrolyzate either with ion-exchange resins or with acidified activated charcoal.

Supercritical carbon dioxide explosion as a pretreatment for cellulose hydrolysis

SummaryCellulosic material Avicel was treated with supercritical carbon dioxide to increase the reactivity of cellulose, thereby to enhance the rate and the extent of cellulose hydrolysis. Upon an explosive release of the carbon dioxide pressure, the disruption of the cellulosic structure increases the accessible surface area of the cellulosic substrate to enzymatic hydrolysis. This explosion pretreatment enhances the rate of the Avicel hydrolysis as well as increases glucose yield by as much as 50%.

Theory and practise of rapid liquid chromatography at moderate pressures using water as eluent

Abstract The rapid separation of oligo- and monosaccharides on long, narrow columns packed with easily compressed, 4% cross-linked, cation-exchange resin adds a new dimension to the chramatography of carbohydrates on gel type supporst: speedy analysis at low pressure. The discovery of a pressure-critical diameter effect in the packing of Aminex 50W-X4 (Ca 2+ ) makes it possible to pack 60-cm long columns capable of separating malto-and cellodextrins in 12–15 min using water as the sole eluent. The surprising phenomena of a 50-fold increase in pressuer due to an increase in column diameter from 6 to 8 mm is reported and reasons for this effect are explained. Equally noteworthy is the stability of the 6-mm columns. One column described in this report has been in continous operation for over 2600 h. The application of low- pressure liquid chromatography to enzyme kinetics as well as to separation of oligo- and monosaccharides is also discussed.

Quantitative production of xylitol from D-xylose by a high-xylitol producing yeast mutant Candida tropicalis HXP2

SummaryXylitol was produced as a metabolic by-product by a number of yeasts when grown on medium containing D-xylose as carbon and energy sources. Among the yeast strains tested, a mutant strain of Candida tropicalis (HXP2) was found to produce xylitol from D-xylose with a high yield (>90%). Ethanol was also produced by HXP2 when D-glucose, D-fructose, or sucrose were used as substrates. The high-xylitol-producing yeast mutant is a good organism for the production of xylitol from biomass that contains D-xylose.