Lignin digests

Source Number Cell Wall Lignin Digestibility SiO,... 

Fiber Fescue grass [13] (animal feed) Suppression of an intrinsic enzyme (cafFeic acid O-methyltransferase) involved with the biosynthesis of lignin (decrease in lignin and increase of lignin digestibility)... 

Determination of Lignin Content. Lignin content in plants (wood) is determined by direct or indirect methods (21). The direct method includes measurement of acid-insoluble (ie, Klason) lignin after digesting wood with 72% sulfuric acid to solubilize carbohydrates (22). The Klason lignin contents of representative lignifted materials are shown in Table 2. 

Chemical pulps are produced in a digester where the wood is cooked in pressurized vessels using heat and chemicals to break the intercellular stmcture of the wood and extractives. The objective is to remove the lignin from the fibers without degrading the carbohydrate content of the wood. 

Chemical charge, Hquor composition, time of heatup, and time at temperature of reaction are all functions of the wood species or species mix being digested and the intended use of the pulp. A typical set of conditions for southern pine chips in the production of bleachable-grade pulp for fine papers is active alkaH, 18% sulfidity, 25% and Hquor-to-wood ratio, 4 1. Time of heatup is 90 min to 170°C time at temperature of reaction, 90 min at 170°C. Hardwoods require less vigorous conditions primarily because of the lower initial lignin content. 

Digester Control For control purposes at constant sulfidity and alkah charge, the deligniftcation rate is treated as a homogeneous reaction, which is first order with respect to the lignin, % remaining in the wood, —dL jdt = kL. The influence of time, t, and temperature, T (Kelvin), has been incorporated into one term, called the JT-factor (33). 

The normal limit for lignin removal in the digester is set both by pulp strength requirements, usually indicated by pulp viscosity, and by yield requirements. As noted earlier, direct control of hydroxide and hydrosulftde concentrations can maximize lignin removal while maintaining pulp strength. 

Tall oil rosin is a by-product of paper manufacturing. Raw wood chips are digested under heat and pressure with a mixture of sodium hydroxide and sodium sulfide. Soluble sodium salts of lignin, rosin, and fatty acids are formed, which are removed from the wood pulp as a dark solution. The soaps of the rosin and fatty acids float to the top of the mixture, where they are skimmed off and treated with sulfuric acid to free the rosin and fatty acids. This mixture, known as cmde tall oil (CTO), is refined further to remove color and odor bodies fractional distillation separates the tall oil rosin acids from the fatty acids (see Tall oil). 

Historically, dietary fiber referred to iasoluble plant cell wall material, primarily polysaccharides, not digested by the endogenous enzymes of the human digestive tract. This definition has been extended to iaclude other nondigestible polysaccharides, from plants and other sources, that are iacorporated iato processed foods. Cellulose [9004-34-6] (qv) is fibrous however, lignin [9005-53-2] (qv) and many other polysaccharides ia food do not have fiberlike stmctures (see also Carbohydrates). 

Kraft pulping is a common process in the paper industry. Figure 8.4 shows a simplifled flowsheet of the process. In this process, wood chips are reacted (cooked) with white liquor in a digester. White liquor (which contains primarily NaOH, NaiS, Na2C03 and water) is employed to dissolve lignin from the wood chips. The cooked pulp and liquor are passed to a blow tank where the pulp is separated from the spent liquor weak black liquor which is fed to a recovery system for... 

Consid - the Kraft pulping process shown in Fig. 8.8 (Dunn and El-Halwa, 1993). The first step in the process is digestion in which wood chips, containing primarily lignin, cellulose, and hemicellulose, are cooked" in white liquor (NaOH, Na2S, Na2C03 and... 

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