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Delignification is the removal of the structural polymer lignin from plant tissue, so that it can be used for applications like making paper. The process primarily refers to the chemical process for the removal of pulp from wood. There can also be done mechanically.
Lignin is a mixture of phenolic compounds that is intermeshed in plant secondary walls, cross-linking the cellulose carbohydrates that can be used to form paper fibers. This complex forms a hydrophobic matrix, meaning it repels water, allowing the plant to transport water up through its system. Lignin adds a lot of mechanical strength to the cell walls, and reduces their digestibility, both by animals and by the delignification chemical process. This polymer also reduces the susceptibility of the plants to attack from insects and plant pathogens. It is one of the last compounds remaining as plants decay, and builds up in the soil as humus.
The removal of lignin from the wood has traditionally taken place by a method called the Kraft process. This name is derived from the German word for strong. The mass of fibers remaining after the lignin is removed is known as pulp. The Kraft process produces stronger pulp than the methods used previously, and removes 95% of the lignin from the wood.
This process usually involves digesting wood chips at high temperature and pressure, and in a solution of sodium hydroxide and sodium sulfide in water, which is a combination known as white liquor. It chemically dissolves the bonds that interconnect the cellulose fibers. The delignification of wood takes place in a vessel called a digester, which can withstand high pressure. There are two types of digesters — batch and continuous — with the more recently developed ones being continuous.
At this stage, the solid pulp is brown and is called brown stock. The combined liquids are known as black liquor, and contain the lignin fragments along with chemicals and by-products. The pulp is separated from the used cooking liquors by a series of washes. The fragments are collected and burned to help power the factory. The rest of the process is designed to recycle the heat and cooking chemicals.
Some by-products of the Kraft process are turpentine and tall oil, which is a resin with a variety of industrial uses. Current Kraft factories are self-contained and recover most of their chemicals, producing very little water pollution. They can, however, produce air pollution.
The brown stock produced by the Kraft process contains about 5% residual lignin, and is further delignified by a series of bleaching stages. Bleaching removes additional lignin, making the paper brighter. Sometimes, the bleaching needs are minimal. For example, if the paper is destined to be a brown paper bag, it does not have to be bright white. There is an incentive to avoid bleaching, since it decreases the mass of pulp produced, adds to the cost, and decreases the strength of the fibers.
Oxygen delignification is a newer process that removes more lignin and uses fewer chemicals. It involves treating the pulp with oxygen in a pressurized vessel at a high temperature in alkaline solution. This process is followed by a washing stage. The amount of residual lignin can be decreased to about 1.5% with this method, limiting the degree of bleaching needed to make paper from the pulp.
While this type of engineering has traditionally focused on wood pulp for paper and fiberboard, more recent efforts involved the use of biomass, large quantities of plant material, as a source of ethanol and an alternative to fossil fuels. This plant material must undergo delignification before it can be used for this purpose. Microbial systems are being engineered that combine the removal of lignin with the conversion of cellulose to ethanol. Research into biofuel production is a very active area.
There is quite a bit of biotechnological research being conducted both on the synthesis of lignin in the plants, and its degradation by microorganisms. Scientists are trying to alter the structure and content of lignin in the hope of improving its digestibility to animals, and increasing the utility of the cell walls for paper and biofuel production.
There is also great interest in developing industrial applications for the enzymes of microorganisms to degrade lignin. Some fungi are quite adept at living on wood, because they produce enzymes — such as peroxidases — that catalyze the breakdown of lignin in the presence of oxygen. Other microbial enzymes being studied for biofuels production act in the absence of oxygen. There are genetic engineering experiments underway to improve the properties of these enzymes for the industrial use of lignin degradation.