A complex, organic material in wood that binds fibers of cellulose together. One of the purposes of the pulping process is to separate the cellulose fibers from the lignin and other non-fibrous materials such as hemicelluloses. It is the presence of lignin in paper pulp that reduces paper permanence and contributes to the yellowing of paper over time. Wood consists of approximately 45:60% cellulose, 15:35% lignin, and 15:25% hemicelluloses, depending on the tree. Chemical pulping, which dissolves these extraneous materials, is the only pulping method that can almost completely remove lignin and hemicelluloses. Mechanical pulping methods make no real attempt at lignin removal, being a method that generates a high pulp yield, due in part to a high lignin content. (See also Pulping.)

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