One of the difficulties in home composting has been the breakdown of woody or cellulosic materials without resorting to burning it. Industrially, there is considerable effort being expended to research processes such as solid-state fermentation that use cellulases, which will generate useful materials from cellulose, most notably biofuels to replace fossil fuel and intermediates for further processing. Photosynthesis is estimated to produce about 150 billion tons of dry plant material of which half is cellulose (Person et al., 1989). Of that amount, it is known that wheat straw amounts to 150 million tonnes per year in Europe (FAO, 2004).
A key group of enzymes, the cellulases can enzymatically hydrolyse cellulose to generate cellubiose which is then converted to glucose. There are some highly useful reviews covering this subject, the enzymes themselves (Gilbert and Hazlewood, 1993; Bhat, 2000), their production by moulds and fungi using fermentation (Mandels and Weber, 1969) and the strides made to engineer new cellulose enzymes and identify the gaps of which there are many in developing the process further (Bayer et al., 2007; Wilson, 2009).
Cellulase appears to be regarded as a complex made up of hydrolases, β-glucosidases and glucanases which all work synergistically to breakdown wood effectively. This enzyme complex is produced by a range of fungi and bacteria. Cellulose is the major component of cell walls and forms rigid microfibrils which in turn are made up of many dozen linear chains all oriented in parallel. The chains are made up of 1→4 β-linked D-glucose units. The microfibrils are themselves buried in a hemicellulose and lignin matrix. The enzyme complex must work on this mix. Hemicellulose itself is composed of xyloglucans (Type I in primary cell walls) or glucuronoarabinoxlans (Type II in primary cell walls) which in turn are hydrolysed by xyloglucanases and arabinases respectively. In terms of enzyme structure, the cellulases have a common basic structure with a catalytic domain linked to a cellulose binding domain by a glycosylated Pro-Thr-Ser-rich peptide (Gilkes et al., 1991).
When composting the home-user is reliant on exploiting these cellulase producing moulds to breakdown as much of the cellulose as is feasible. Providing a semi-moist, nitrogen balanced environment is crucial hence the disposal of non-wood food waste to provide other nutrients for these fungi to live on.
References
Bayer, E.A., Lamed, R., Himmel, M.E. (2007) The potential of cellulases and cellulosomes for cellulosic waste management. Curr. Opin. Biotechnol., 18 pp. 237-245
Bhat, M.K. (2000) Cellulases and related enzymes in biotechnology. Biotechnol. Adv. 18(5) pp. 355-383
FAO. (2004) Statistical yearbook production. Food and Agriculture Organization of the United Nations, Rome.
Gilbert, H.J.; Hazlewood, G.P. (1993) Bacterial cellulases and xylanases. J. Gen. Microbiol., 139 pp. 187-194
Gilkes, N. R., Henrissat, B., Kilburn, D.G., Miller, R.C., Jr., Warren, R. A. (1991) Domains in microbial beta-1, 4-glycanases: sequence conservation, function, and enzyme families. Microbiol Rev. 55 pp. 303–315
Mandels, M., Weber, J. (1969) The Production Of Cellulases. In: Cellulases and Their Applications. ACS Adv. In Chemistry. Vol. 95 Chapt. 23 pp. 391-414
Persson, I., F. Tjerneld and B. Hahn-Hägerdahl, (1991) Fungal cellulolytic enzyme production part of: Persson, I. Production and utilization of cellulolytic enzymes in aqueous two-phase systems. Thesis University of Lund, Sweden.
Wilson, D.B. (2009) Cellulases and biofuels. Curr. Opin. Biotechnol. 20(3) pp. 295-299
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