| Bioconversion of Organic Residues for Rural Communities (1979) |
|Solid state fermentation of starchy substrates|
In spite of a combination of currently unattractive economics and political opposition in some quarters, large-scale production of single-cell protein (SCP) will undoubtedly develop soon in industrialized countries in Western Europe and in Japan and the USSR, where the development of new protein sources is becoming an absolute, urgent necessity. A priori, it could also be expected that the SCP industry would provide a contribution to the problem of hunger in the Third World. In this regard, however, there are several major obstacles.
To be economically viable, an SCP production unit should have a minimal capacity of at least 100,000 tons per year, corresponding to a capital cost of US$50 - 70 million. On the other hand, a plant producing 100,000 tons of SCP from paraffins would require an equal supply of substrate and should thus be associated with an oil refinery having a minimal capacity of about 3 to 5 million tons of crude oil per year. Similar considerations apply to the production of SCP from natural gas or from methanol. Such facilities are obviously absent in most non-oil producing countries of Asia, Africa, and Latin America. Moreover, these countries may have neither a potential market nor a transportation and distribution network for the commercialization of 100,000 tons of SCP per year.
Clearly, those countries that cannot currently import food or feeds because of currency shortages will also be unable to import industrial SCPs. Consequently, it is of utmost importance for them to develop their own protein resources. In addition to hydrocarbons and methanol, a wide variety of raw materials potentially utilizable for SCP production might be considered. However, most of them are available at too high a cost to be economically competitive, or exist in quantities too low for protein production on a significant scale. Among the substrates suitable in cost and supply, special emphasis is usually put on cellulosic materials, but, at the moment, the many attempts made in this direction have had little success, the main difficulty being the lack of cellulolytic organisms with an adequate growth rate.
In contrast, starchy materials, more specifically cassava in the tropical regions or potatoes in temperate climates, are of obvious interest because of both high productivity per hectare and excellent rate of conversion to biomass by a great number of fast-growing micro-organisms.
In order to be economically competitive, the production of protein from starch should not be undertaken by classical fermentation in liquid medium, under aseptic conditions, followed by biomass separation and drying. As in the case of SCP production from paraffins or methanol, the optimization of such sophisticated technology would require a minimal production of well over the potential market of most developing countries, and would result in high investment and operation costs. Moreover, in the developing countries, the collection, transportation, and storage of large quantities of raw materials would result in major difficulties. Considering these factors, a more practical approach would be to enrich starchy materials with protein by means of a simplified technology that can be applied at the farm or village level, and that would allow the combination of cultivation of raw material, its conversion into protein, and its direct utilization for animal feeds. Economically, a great and decisive advantage of such an integrated procedure is to prevent intermediary profits and speculation that would inevitably take place if either the raw material or the final product were commercialized.
To be workable at the farm level, a protein enrichment process should not require aseptic conditions, and should be performed in a single operation. Additionally, the product must be sufficiently rich in protein to be utilizable as such, without a secondary fortification step. This last requisite creates a biotechnological difficulty that has been responsible for the failure of many previous attempts to achieve direct protein enrichment of starchy materials. In a mash of raw material dense enough to be directly utilized for animal feeding, the major problem is to maintain aerobic conditions and oxygen transfer efficiency so as to prevent anaerobic contamination of the culture.
A new procedure of solid state fermentation (1) fulfilling the above specifications was developed in collaboration with Drs. M. Raimbault and F. Deschamps at the French Office de la Recherche Scientifique et Technique d'Outre Mer (ORSTOM), and the Institut de Recherche en Chimie Appliquée (IRCHA), respectively. A preliminary report on this procedure has already been presented (2) at the 5th International Conference on the Global Impacts of Applied Microbiology, held in Bangkok in November 1977.