Volume 8, Part 3, August 1994 CSIC, Estacion Experimental Del Zaidin, Prof Albareda 1, Granada 18008, Spain PLANT LIGNOCELLULOSE AND DECOMPOSITION BY FUNGI: FROM NATURE TO .INDUSTRIAL USE N. VASSILEV, M.T. BACA & M. VASSILEVA • ment procedures are a steam explosion process (a combination of high pressure and high tempera- ture) and the use of inorganic acids (sulphuric and phosphoric acid) as well as caustic soda (NaOH), ethylenediamine and n-butylamine. A serious disadvantage in the use of most lignocellulosic materials as a substrate for single cell protein is this need for costly pretreatment. However, some fungi, particularly Trichoderma reesei and T. uiride, are useful. The reason is that these fungi produce actively the enzyme cellulase and many research laboratories are attempting to improve the cellulolytic activity of Trichoderma by manipulation of culture conditions. Another method is to modify the fungi genetically - the use of some conventional mutagenic agents has enabled the selection of mutants with higher cellulase, f)-glucosidase and xylanase activity (Wood et ai., 1984l. Apart from agricultural crop residues, waste sludges from pulp and paper mills represent a possible commercial source of lignocellulosic material, cellulose and part of the lignin being used for the production ofpaper and board, but the unused waste sludges representing an important source of hemicellulose. The major hemicellulosic sugar of agricultural crop wastes is xylose and we can thus understand efforts to use pentoses for conversion into fuels. Certain fungi are able to convert xylose to ethanol, importantly those in the genus Fusarium and especially F. oxysporum (Aditya Joshi et ai., 19901. Another interesting approach in using pentoses is connected with organic acid production. In Finland researchers at the University of Technology have produced itaconic acid by immobilized Aspergillus terreus using xylose as a carbon substrate (Kautola, 1990). In our laboratory we are attempting to select organic acid producing fungal cultures, able to grow and decompose various lignocellulosic materials during composting. Degradation of the lignin part of lignocellulose As they grow, huge quantities of cellulose and wood are produced by plants on earth each year. Some of these materials might sometimes pose a significant environmental problem, but, on the other hand, lignocellulose is a potentially valu- able source of energy. Lignocellulose is composed of holocellulose bound to lignin. In fact holocellu- lose consists of cellulose (polymer of glucose) and hemicellulose (a heterogeneous polymer of hex- oses and pentoses). Lignin is another polymer based on three phenolic acids. It is important to know that different plant materials contain vari- ous ratios of cellulose, hemicellulose and lignin. The commercial problem is how to use this abundant natural material. Breakdown by fungi may be the answer. From a biotechnological point of view, one of the main products derived from cellulose and hemicel- lulose is glucose and also other sugars. However, it seems that the better economic prospect might be to use these sugars as substrates for production of other bioproducts (Lynch, 1987). Wood (1985) has suggested that the fermentation oflignocellu- lose could produce commercially valuable mater- ials such as feed and beverages. In fact, lignocellu- losic materials could be involved in commercially viable processes for the production of many different kinds of chemicals useful to man (single cell protein, carbohydrates, organic acids and biofuelsl. It should be noted that without pretreatment of some kind, lignocellulosic materials are poorly hydrolysed by fungal enzymes. The most import- ant structural--characteristics that enable ligno- cellulose to resist enzymatic hydrolysis are the degree of crystallinity of the cellulose, the space between microfibrils and the cellulose molecules, and the amount of lignin surrounding the cellu- lose fibres. For these reasons different physical and chemical methods of pretreatment have been developed in order to modify the structural features of lignocellulose. The commonest treat-