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Solid fermentation of starchy substrates

by J. C. Senez

Tempeh, and many other food preparations obtained by solid fermentation of soybeans or other materials with filamentous fungi (Refs. 1 - 3), are traditionally used in various parts of Asia and Africa. Procedures for direct protein enrichment of cassava by liquid (Refs. 4, 5) or solid-state (Ref. 6) fermentation have been described. However, protein enrichment by these methods does not exceed 3- 4 per cent, and therefore the end product is insufficient for use as a complete feedstuff. The principle of a new procedure developed by Drs. M. Reimbault and F. Deschamps (of ORSTOM, Office de Recherche scientifique et technique d'Outre Mer [Office for Overseas Scientific and Technological Research], and IRCHA, Institute de recherche en Chimie Appliquée [ Institute for Applied Chemistry Research], both French government organizations) for protein enrichment of cassava and other starchy materials is summarized in Table 1.

All the operations are conducted in a commercial bread-making blender of 10-kg capacity, modified for the purpose. The coarsely ground raw material, with 30 - 35 per cent moisture, is gently steamed for 15 - 20 minutes to break the starch granules. After cooling to 40 °C, the preparation is mixed with water containing the inoculum (spores), the nitrogen sources (ammonium sulfate and urea), and mineral salts, to a 60 per cent final moisture content. By mechanical stirring, the inoculated substrate spontaneously takes the form of well-separated and uniform granules of about one millimeter in diameter.

Aeration is performed by passing humidified air through the perforated bottom of the tank. Conventional probes are used to monitor, by mixing and water-spraying, the temperature, pH, and moisture content. Up to now, all experiments have been performed with a selected strain of Aspergillus niger having high amylolytic activity and suitable amino-acid composition. However, it should be pointed out that other filamentous fungi could be utilized as well.

This method of protein enrichment has already been used successfully with a variety of starchy materials, namely, cassava, whole potatoes, potato wastes from industrial facula works, and banana refuse. The results are summarized in Table 2, showing that after 30 hours of incubation, one obtains a product containing an average of 20 per cent true proteins, measured by the Lowry method, and 25 per cent residual reducing sugars. The rate of conversion of carbohydrates to protein is 20 to 25 per cent. The process can be seen in a series of scanning micro-photographs showing the initial shape of starch granules in crude potato flour, the striking modification of the substrate structure after steaming, the spores of the organism, the early stage of spore germination, the growing mycelia, and, after 24 hours of incubation, the final appearance of the preparation completely converted into a dense mycelial network. The organism currently utilized is rather thermotolerant, with an optimal temperature at 40 °C, but growth still takes place at 30°C and 45 °C without extensive change of the final protein concentration.

Because of the aerobic and highly selective conditions, no aseptic precautions have to be taken and the sporulation of the mould is totally inhibited. Preliminary results of nutritional and toxicological tests on rats and chickens are quite satisfactory, showing a nutritional value similar to that of soybean meal. It is intended that the experimentation will be extended to the setting up of experimental production units in tropical Asia and Africa in order to adapt the procedure to local climatic and agro-economic conditions.

TABLE 1 Protein enrichment of cassava by solid-state fermentation

¹ carbohydrates: 90%; protein: 1%; water: 30 - 35%
² determined by the Lowry method
³ determined by enzymatic hydrolysis (amyloglucosidase) and Somogyi-Nelson titration.

TABLE 2 Protein enrichment of various raw materials

TABLE 3 Agro-economic prospects

A. Productivity of raw material and of protein:

B. Conversion into animal product (pork)⁴:

C. Overall agro-economic prospect:

¹ 34% protein
² data from USDA
³ for 20% protein enrichment
⁴ from C.A. Shacklady, in Proteins from Hydrocarbons, Academic Press, New York, 1972, pp. 1 15 - 28
⁵ birth to weaning: 70 days; +25 kg; diet with 15% protein
⁶ weaning to slaughter: 130 days; +85 kg; diet with 10% protein
⁷ total: 200 days; 110 kg

Agro-Economic Perspectives

The two main sources of starch potentially available for protein enrichment are cassava in tropical regions and potatoes in temperate climates. Protein enrichment of cassava is of special interest in those semi-arid regions of Latin America and Africa where climatic conditions are not suitable for the cultivation of soybeans or other protein-rich feed grains.

The productivity of cassava per hectare varies widely from one region to another depending on climatic and agro-technological conditions. On the basis of a productivity of 40 tons per hectare and 20 per cent protein enrichment via solid-state fermentation, cassava or potatoes may provide 2.4 tons of protein per hectare: i.e., the supply required for the feeding of 65 pigs (Table 3). This is about four times the quantity of protein per hectare provided by soybean cultivation in the United States The crop yield and protein productivity per hectare of other protein sources conventionally utilized for animal feeding are reported in Table 4.

TABLE 4 Optimal productivity of protein-rich feeds

¹ 40 tons per hectare of cassava with 70% moisture content

The figures cited in Table 5 strikingly demonstrate the economic value in protein enrichment by solid-state fermentation. Actually, in the case of cassava, the value of the residual sugars (25 per cent dry weight) should increase the gross product figure by about 10 per cent. On the other hand, for a rural community combining the production of raw material with protein enrichment and direct utilization for animal feeding, the actual gross product should be estimated, not from the commercial value of protein, but from the value of the feedstock produced. Moreover, one of the major agro-economic interests in protein-enriched cassava is to provide the possibility of feedstock production in regions where no other suitable source of conventional feed protein is available.

TABLE 5 Compared productivity and gross product per hectare

¹ on September 29, 1978
² cassava: 40 tons per hectare, with 70% moisture, dry product containing 20% protein
³ estimated from current price (US$213.6, Rotterdam, c.i.f.) of soybean meal with 44% protein.

References

1. Raimbault, M., and Germon, J.C., Procédés d'enrichissement en protéines de produits comestibles solides, Patent B.F. n° 76.06.677, March 9, 1976.

2. Raimbault, M, Deschamps, F., Meyer, F., and Senez, J.C., "Direct protein enrichment of starchy products by fungal solid fermentation," 5th International Conference on Global Impacts of Applied Microbiology, Bangkok, November 1977.

3. Hesseltine, C.W., "A millenium of fungi, food and fermentation," Mycologia, 67, 149 - 97, 1965.

4. Martinelli, A., and Hesseltine, C.W., "Tempeh fermentation," Food Technology, 18, 167 - 71, 1964.

5. Gray, W.D., "The use of fungi in food and in food processing " Chemical Rubber Co., critical review in Food Technology, 1, 225-329, 1970

6. Reade, A.E., and Gregory, K.F., "High temperature production of protein enriched feed from cassava by fungi," Applied Microbiology, 30, 897 - 904, 1975.

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