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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderSolid state fermentation of starchy substrates
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View the documentIntroduction
View the documentMaterials and methods
View the documentAgro-economic perspectives
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Materials and methods

Tempeh and many other food preparations obtained by solid state fermentation of soybeans or other materials with filamentous fungi (3 - 5) are traditionally used in various parts of Asia and Africa. On the other hand, procedures for direct protein enrichment of cassava by liquid (6, 7) or solid state fermentation have been described. However, protein enrichment by these methods does not exceed 3 - 4 per cent, and therefore is insufficient for use as a complete feedstuff. The principle of the new procedure devised by ORSTOM and IRCHA for protein enrichment of cassava and other starchy materials is summarized in Table 1.

TABLE 1. Protein Enrichment of Cassava by Solid State Fermentation

Initial substrate (g)  
Cassava flour* 100
SO4 (NH4)2 9
Urea 2.7
PO4 KH2 5
water 100 - 120
Optimal growth conditions  
T: 35 - 40°C; initial pH: 3.5  
Inoculum: 2 x 107 spores/g flour  
Incubation time: 30 hours  
Composition of the product  
Protein** 18 - 20% in dry matter
Residual sugar*** 25 - 30% in dry matter
Water 63%

* Carbohydrates: so per cent; protein: 1 per cent; water: 30 - 35 per cent.
** Determined by the Lowry method.
*** Determined by enzymatic hydrolysis (amyloglucosidase) and Somogyi-Nelson titration

All of the operations are conducted in a commercial dough mixer of ten kg capacity, modified for that purpose. The coarsely ground raw material, with 30 - 35 per cent moisture content, 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 sulphate and urea), and mineral salts, to 55 per cent final moisture content. After mechanical stirring, the inoculated substrate spontaneously takes the form of well separated and uniform granules of about 1 mm diameter.

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

With the organism currently utilized, the optimal temperature is +40 C, but growth still takes place at temperatures from +30 to +45 C without significant changes in the final protein yield. The initial moisture content is critical, with an optimum of 55 per cent. In the course of fermentation, the water content is progressively increased to a final value of 70 75 per cent.

This method of protein enrichment has already been worked out with a variety of starchy materials, namely cassava, whole potatoes, potato waste from industrial starch works, and banana refuse. The results are summarized in Table 2, showing that, after 30 hours of incubation, a product is obtained that contains, on average, 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, corresponding to 40 to 50 per cent conversion into dry weight biomass.

TABLE 2. Protein Enrichment of Various Raw Materials

  Initial composition Final product








Cassava 2.5 90 18 30
Banana 6.4 50 20 25
Banana waste 6.5 72 17 33
Potato 5.0 90 20 35
Potato waste 5.0 65 18 28

The kinetics of a fermentation on potato waste are shown in Figure 1, illustrating the production of protein, reducing sugars, water content, and the pH of the preparation. The curve, marked by crosses, is of special interest. It shows that, during a total incubation time of 30 hours, the monitoring devices for mechanical stirring and water spraying had to operate for only five hours, thus demonstrating the excellent efficiency of the cooling device. Additionally, it requires a remarkably low expenditure of power, a fact of obvious importance in regard to the production cost of solid state fermentation and to its economic feasibility at the farm level in tropical regions.

Figure.1. Solid State Fermentation of Potato Waste

It has been pointed out that, owing to perfectly aerobic and highly selective conditions, no aseptic precautions have to be taken, and sporulation of the mould is totally inhibited. Nutritional and toxicological tests on rats and chickens are in progress, and the preliminary results are quite satisfactory, showing a nutritional value similar to that of soybean meal.

Currently, the studies on solid state fermentation are being actively developed in France by ORSTOM and IRCHA in close collaboration with the Applied Scientific Research Corporation of Thailand. The scaling-up of the process to a fermenter unit of 1 m³ has been undertaken and is in progress. This equipment, which is expected to be operative in the coming months, will be utilized for large-scale nutritional and toxicological testing on target animals (pigs and poultry), for further optimization of substrate preparation and growth conditions, and for determination of the actual investment and operation costs. 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.