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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderProduction of single-cell protein from cellulose
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Production of food, fuel, and chemicals is one of the primary requirements for the development and welfare of a society. During the past decade, the pattern of agricultural production has shifted in the world in such a manner that most of the countries around the world are not self-sufficient in their food production. The present shortage and impending depletion of fossil fuels place certain constraints on increasing agricultural productivity even in industrially developed countries. Further constraints are placed on the developing countries in meeting their food needs because of low productivity in agriculture and a steady increase in population. The race to catch up with the need for food by modernization of agriculture is steadily being lost to the burgeoning populations in several areas of the world. Thus, the necessity for exploring unconventional, non-agricultural means of food production, especially of proteins, cannot be over-emphasized. Production of microbial proteins or bioproteins by fermentation of agricultural waste products is one of the most promising approaches for increasing the availability of proteins in the world.

During the past several years, we have concentrated our efforts in developing fermentation of cellulose by micro-organisms. Cellulose is the major constituent of all agricultural wastes. Also, as a renewable resource, it is plentifully available for utilization as a substrate for fermentation of non-wood plant fibrous raw materials. The potential availability of such natural raw material is presented in Table 1 (1).

TABLE 1. Estimated Availability of Specific Non-Wood Plant Fibrous Raw Materials

Raw materials 1,000 metric tons
Sugar-cane bagasse 55,000
Different straws (wheat, rice, oat, etc.) 88,500
(Bast fibres kute, kenaf, etc) 6,099
Leaf fibres (sisal, abaca) 904
Reeds 30,000
Bamboo 30,000
Papyrus 5.000
Esparto grass 500
Sabai grass 200
Cotton fibre 13,500
Total 229,703

Source: Virkola (1).

Cellulose, however, does not occur in pure form. It is always associated with lignin and hemicelluloses. Thus, naturally occurring cellulose is not readily susceptible to microbial attack. Hence, it is essential that the cellulose be previously treated, by either physical or chemical methods, to facilitate the growth of micro-organisms (2).

Reduction of particle size of ligno-cellulosics leads to an increase in their susceptibility to microbial fermentation. This has been accomplished with both wet and dry raw materials. Either very fine grinding or ball milling has been the usual procedure. These methods require a high amount of energy, and hence add considerable cost to the overall production of biomass. Other physical methods for particle size reduction make use of sonic energy, cryogenic grinding, or extremely rapid depressurization with steam to cause multiple fractures within the cell walls of natural fibres, thereby increasing the total surface area. Some of the possible chemical methods of treatment include: (i) treatment with gaseous sulphur dioxide; (ii) alcoholysis of lignin with methanol or ethanol containing small amounts of hydrochloric or sulphuric acids; (iii) degradation of lignin under mild acid conditions; (iv) extraction of lignin by organic solvents such as acetone, dioxane, or others in the presence of mild acids, and (v) treatment with sodium hydroxide or ammonia to swell the cellulose fibres and solubilize lignin and hemicelluloses.

Alkali treatment has been the method of choice in all our investigations of cellulose fermentation. In this process, cellulose is steeped in 5 - 10 vol of 1 N sodium hydroxide and heated for 15 - 30 minutes at 100 - 120 C. In order to illustrate the efficacy of alkali treatment, some of the results obtained by Wilson and Pigden (3) on the in vitro digestion of wheat straw and poplar wood are presented in Figure 1.

Figure. 1. Effect of Sodium Hydroxide Pre-Treatment on the In Vitro Digestion of Wheat Straw and Poplar Wood

There are only a few organisms that can attack "native" cellulose. However, when cellulose is treated or modified before micro-organisms are introduced, the choice of available organisms increases greatly. Table 2 presents a few selected organisms that have been investigated in the biodegradation of ligno-cellulosics (4 8).

Peitersen (4).** Chahal and Wang (5).*** Eriksson and Larsson (6).+ Daugulis and Bone (7).++ Humphrey et al. (8).

This paper describes the "state of the art" in our laboratory investigations on the biodegradation of cellulose by microorganisms, leading to the production of single-cell protein.