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Title: Biosolubilization of lignin by ruminal microbes
Author: Salem, A.Z.M.
Camacho, L.M.
Cardoso, D.
Montanez, O.D.
Cruz, B.
Cipriano, M.
Issue Date: 2011
Abstract: Lignin or lignen is a complex chemical compound most commonly derived from wood, and it constitutes an integral part of the secondary cell walls of plants (Lebo et al., 2001) and some algae (Martone et al., 2009). The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, (SjZapotitlánstrZapotitlánm, 1993) meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose, employing 30% of non-fossil organic carbon (Boerjan et al., 2003) and constituting from a quarter to a third of the dry mass of wood. As a biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary structure. Its most commonly noted function is the support through strengthening of wood (xylem cells) in trees (Chabannes et al., 2001). Forage lignin, as all phenolic substances; adversely influence the utilization of structural carbohydrates, that reducing rumen degradability of cell wall polysaccharides to the point of being virtually undigested by ruminant animals. Gaillard and Richards (1975) reported the presence of a soluble lignin-carbohydrate complex in the cell-free rumen fluid of a steer fed on grass. Further work (Neilson and Richards, 1978) suggested that the complex is not significantly digested after the rumen. Solubilization or degradation of lignins by rumen microorganisms under anaerobic conditions was confirmed by experiments with radiolabeled lignocellulose (Akin and Benner, 1988) and with an artificial rumen reactor (Kivaisi et al., 1990). Structures of soluble complexes have often been studied (Neilson and Richards, 1982; Conchie et al., 1988; Nordkvist et al., 1989) but little is known about the structures of solubilized or degraded lignin portions. Characterization of dissolved degradation products of lignin may provide further information on the chemical factors that affect forage digestibility (Akin and Benner, 1988). Part of the lignin of forage grass fed ruminants is solubilized in the rumen as a complex with carbohydrate (Gaillard and Richards, 1975; Neilson and Richards, 1978) by rumen bacteria and fungi (Akin and Benner, 1988; McSweeney et al., 1994). Organic solvent-soluble lignin fragments, which are likely to be degraded products of lignin or lignin-carbohydrate complexes, are also released from forage grass during its passage through the rumen and then excreted in the faeces without further significant modifications (Kondo et al., 1995). In addition, grass lignin undergoes the demethylation of syringylpropane units in the rumen (Mosoni et al., 1994) and the disappearance of bound phenolic acids (Kondo et al., 1997). Little is still known about the fate of legume lignin during ruminant digestion. Legume lignin appears to inhibit digestion of cell-wall polysaccharides less effectively than does grass lignin (Buxton and Russell, 1988). The inhibitory effect of lignin is closely related to its structure (Jung and Deetz, 1993; Besle et al., 1994) and hence considered to be affected by structural modifications which lignin suffers in the digestive tract. The analysis of the fate of forage lignin during ruminant digestion could provide further detailed information on their nutritional effect. The decrease of lignin biodegradability by forming the bonds to cell-wall carbohydrates and are toxic for the rumen fibre degrading microorganisms (Kondo et al., 1994). Phenolic monomers and dimers bound as esters (to carbohydrates) or ethers (to core lignin) may form ester-ether bridges between lignin and carbohydrates (Lam et al., 1992). The growth rate of the rumen microorganisms and their ability to degrade cellulose in vitro is most effectively inhibited by p-coumaric acid (PCA) and ferulic acid (FA) (Chesson et al., 1982). The objectives from this chapter are to scope on these findings in detail. Rather, the focus will be on specialized or recently revealed aspects of the biosolubilization by ruminal microbs. Zapotitlán2011 Nova Science Publishers, Inc.
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