oilseed crops.
As in traditional pulping, lignin is either softened or removed, and individual cellulosic fibers are released creating pulp. While bioconversion pretreatment is based on existing pulping processes, however, traditional pulping parameters are defined by resulting paper properties and desired yields, while optimum bioconversion pretreatment is defined by the accessibility of the resulting pulp to enzymatic hydrolysis.
Once pretreated, the cellulose and hemicellulose components of wood can be hydrolyzed. The majority of the commercial hydrolysis programs use enzymes to facilitate fast, efficient, and economic bioconversion of the wood. Enzymatic hydrolysis of lignocellulose materials uses cellulase enzymes most commonly produced by fungi such as Trichoderma, Penicillium, and Aspergillus. A cocktail of cellulase enzymes is required in order to break down the cellulosic microfibril structure into its carbohydrate components in an efficient manner, unlike the bioconversion of starch, which has a simpler chemical structure. The enzymatic hydrolysis step may be completely separate from the other stages of the bioconversion process, or it may be combined with the fermentation of carbohydrate intermediates to end-products.
Separate hydrolysis and fermentation (SHF) offers the platform more flexibility, and makes it easier in theory to alter the process for different end products; however a separate process requires additional engineering and will cost more to build and operate. Simultaneous saccharification and fermentation (SSF) has been found to be highly effective in the production of specific end products, such as bioethanol.
Separation techniques are being developed to isolate the base components of cellulose, hemicellulose, and lignin in order to facilitate industrial processing of these components. Sometimes, the most effective isolation may be carried out by combining correct pretreatments with enzymatic hydrolysis.
The advantage of the bioconversion platform is that it provides a range of intermediate products, including glucose, galactose, mannose, xylose, and arabinose, which can be relatively easily processed into value-added bioproducts. The bioconversion platform also generates a quantity of lignin or lignin components; depending upon the pretreatment, lignin components may be found in the hydrolysate after enzymatic hydrolysis, or in the wash from the pretreatment stage.
Once hydrolyzed, six-carbon sugars can be fermented to ethanol using age-old yeasts and processes. Five-carbon sugars, however, are more difficult to ferment; new yeast strains are being developed that can process these sugars, but issues remain with process efficiency and the length of fermentation. Other types of fermentation, including bacterial fermentation under aerobic and anaerobic conditions, can produce a variety of other products from the sugar stream, including lactic acid.
See also: Biochemical Conversion, Biofuels – Platform, Biorefinery, Thermochemical Platform.
Biodegradation
Biodegradation (transformation of a chemical by microorganisms) is the decay or breakdown of chemicals that occurs when microorganisms use an organic substance as a source of carbon and energy. For example, sewage flows to the wastewater treatment plant where many of the organic compounds are broken down; some compounds are simply biotransformed (changed), others are completely mineralized. These biodegradation processes are essential to recycle wastes so that the elements in them can be used again. Recalcitrant materials, which are hard to break down, may enter the environment as contaminants.
Another term, biotransformation, refers to the conversion of a substance through metabolization, thereby causing an alteration to the substance by biochemical processes in an organism. Metabolism is divided into the two general categories of catabolism, which is the breaking down of more complex molecules, and anabolism, which is the building up of life molecules from simpler chemicals. The substances subjected to biotransformation may be naturally occurring or anthropogenic (made by human activities) which may consist of xenobiotic molecules that are foreign to living systems.
Biodegradation is a microbial process that occurs when all of the nutrients and physical conditions involved are suitable for growth. Temperature is an important variable; keeping a substance frozen can prevent biodegradation. Most biodegradation occurs at temperatures between 10 and 35°C (50 and 95°F), and water is essential for the biodegradation process. Bacteria and fungi, including yeasts and molds, are the microorganisms responsible for biodegradation. The biodegradation of organic matter in the aquatic and terrestrial environments is a crucial environmental process. Some organic pollutants are biocidal; for example, effective fungicides must be antimicrobial in action. Therefore, in addition to killing harmful fungi, fungicides frequently harm beneficial saprophytic fungi (fungi that decompose dead organic matter) and bacteria. Herbicides are designed for plant control, and insecticides are used to control insects.
The biodegradation process can be divided into three stages: (i) biodeterioration, (ii) biofragmentation, and (iii) assimilation. The first stage (biodeterioration) is often described as a surface-level degradation that modifies the chemical, physical, and mechanical properties of the contaminant and occurs when the material is exposed to abiotic factors in the environment and allows for further degradation by weakening the structure of the contaminant. Some abiotic factors that influence these initial changes are compression (mechanical), light, temperature, and chemicals in the environment. While biodeterioration typically occurs as the first stage of biodegradation, it can in some cases occur in be parallel (simultaneously) to biofragmentation which is the conversion of the spilled chemical to lower molecular weight fragments that are more amenable to removal from the environment (or ecosystem). Assimilation occurs when the fragment (or fragments) are assimilated into the environment (or ecosystem) without any deleterious effect to the system.
The resulting products from biofragmentation can be assimilated into microbial cells; this is the assimilation stage. Some of the products from fragmentation are easily transported within the cell by membrane carriers. However, other products of the biofragmentation stage still have to undergo biotransformation reactions to yield products that can then be transported inside the cell. Once inside the cell, the products enter catabolic pathways that either lead to the production of adenosine triphosphate (ATP) or elements of the structure of the cell.
The biodegradation process is, in general, an important process for the removal of chemical compounds (especially organic chemicals) from the environment. The versatility and activity of microbial enzymes as catalysts mean that biodegradation is much more significant than purely chemical reactions such as hydrolyses and redox reactions. Enzymatically catalyzed transformation also occurs in higher organisms, but this process is quantitatively less important than the contribution from microorganisms. Some of the most important microorganism-mediated chemical reactions in aquatic and soil environments are those involving nitrogen compounds and the cycle of such compounds throughout the Earth system. Among the biochemical transformations in the nitrogen cycle are (i) nitrogen fixation, whereby molecular nitrogen is fixed as organic nitrogen, (ii) nitrification, the process of oxidizing ammonia to nitrate, (iii) nitrate reduction in which nitrogen in nitrate ions is reduced to nitrogen in a lower oxidation state, and (iv) denitrification, the reduction of nitrate and nitrite to ammonia.
Physical-chemical and biological treatment processes are employed as for wastewater treatment. In addition, chemicals are introduced for precipitation of nutrients, followed by coagulation and filtration for removing solids remaining after biological treatment. In some cases, granular activated carbon or membrane filtration or a combination of membrane-assisted solvent extraction is used for additional purification of the groundwater streams and waste streams. This higher level of treatment is advisable because of the damage that any visual traces of chemical waste can do to the appearance of the waters. In addition, the treatment may combat the potential eutrophic effect that the nutrients phosphorus and nitrogen can have on a water system.
For the most part, anthropogenic compounds resist biodegradation much more strongly than do naturally occurring compounds.
