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Ligno-cellulosic residues: a suggestion for freeing cellulose
Robert Stanton
Botany Department, Faculty of Science, Malaya
University, Kuala Lumpur, Malaysia
Much research effort has been expended on increasing the availability of the cellulose component in ligno-cellulose complexes such as wood and hard fibre.
As a recurrent natural resource, ligno-cellulose is the preferred product from the point of view of the biomass agronomist/forester, and it is likely to remain the recurrent source of carbon which is in greatest supply. Its components, lignin, cellulose, and other compounds, especially as found in rain-forest timbers, are useful raw materials for industry, but cellulose probably has the greatest utility as a molecular structure. Furthermore, when freed from lignin, it can be used as an energy source for ruminants.
It is suggested, therefore, that future emphasis should be placed on freeing the cellulose by degrading the lignin and using lignin by-products. The technology for doing this became available to the big-technologist as a consequence of the findings of Kuhlwein1 in which he reported that the yeasts he had found inhabiting the deliberately degraded wood known as "palo podrido" were taxonomically related to the resting yeasts. That is, these yeasts are able to use ring compounds and are characterized in taxonomic keys of biochemical properties by their growth on arbutin, quercetin, and similar compounds that function in nature as mycostatic agents. These compounds are in the breakdown products of lignin in the form of monomers, of which the skeletal structure is a phenyl propane unit.
Kuhlwein did not report his findings in terms of their significance as a fermentation system, in which metabolite repression is circumvented by the removal of the products of metabolism, namely, the lignin breakdown products.
Making "palo podrido" is apparently a slow process, taking up to a year to reduce the wood of the cool, evergreen rainforest species in highland Chile to a jelly-like material that is fed to the cattle. This process might possibly be accelerated, but undoubtedly the existing organisms- white-rot fungus and yeasts-would not dominate when cultured on other timbers at higher temperatures.
However, the taxonomic similarity to the resting yeasts suggests that there is a class of biodegradative phenomena involving lignin degradation and utilization that might be explored further for economically useful species of microorganisms. One that comes to mind is the resting of jute, Corchorus indicus, though other tropical fibre plants such as Hibiscus spp. might also provide environments for the requisite yeast flora.
The author is indebted to Baker (P. Baker, Novo. Enzym, Kuala Lumpur, personal communication) for suggesting that, under relatively anaerobic conditions (resting is relatively anaerobic), pseudomonads might be commensal with, or replace, the yeasts. However, further research into this multistage, biodegradative SCP-building process appears to be appropriate for encouragement on a collaborative basis, in which the Microbiological Resource Centres (MlRCENS) would be involved by assisting in isolating ring-compound-using yeasts and making isolates available to microbial technologists engaged in formulating feeds based on ligno-cellulosic materials. The use of a second organism might also accelerate the cellulose-cleaning operation, performed by cellulase-deficient white-rot organisms such as those produced by Ericksson and co-workers at the Swedish Forestry Institute in Stockholm.
Further references to this subject are given in Stanton,2 and the author is indebted to Zadrazil (H. Zadrazil, Universität Braunschweig, Federal Republic of Germany personal communication, 1977) for pointing out that references to "palo podrido" are to be found in nineteenth-century literature.
References
1, H. Zenk. Kuhlwein, Bakteriol. Parasitenk. Abt
11: 116, 294, (1963).
2. W.R. Stanton, "The Microbes' Potential in Resource
Economy," in Microbial Energy Conversion, H.G.
Schlegel and J. Barnea (eds.) (Erich Gottze KG, Gottingen, West
Germany, 1976), pp. pp. 95-215