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close this book Bioconversion of Organic Residues for Rural Communities (1979)
close this folder Solid state fermentation of starchy substrates
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Agro-economic perspectives

Agro-economic perspectives

As already pointed out, 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 feeds.

The productivity of cassava per hectare varies widely from one region to another, depending on climatic and agro-technological conditions. From about 16 tons (harvested weight) per hectare in northeastern Brazil, the yield can be easily increased by the use of fertilizers and by improved cultural practice to 40 and even 60 tons per hectare. Other advantages of cassava are low production costs, easy storage in the soil for several months, and the fact that cassava is also an excellent source of calories.

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 3. Agro-Economic Prospects of Cassava Enrichment

A. Productivity of raw material and of protein Cassava Soybeans*
Raw material (tons/ha) 40 1.8**
Moisture content (%) 70 -
Protein (tons/ha) 2.4*** 0.6  

B. Conversion into animal product (pork)****

Alimentary conversion rate (units of feed per unit liveweight gain) 3:1
Protein consumption:  
birth to weaning† 11.3 kg
- weaning to slaughtered†† 25.5 kg
- total††† 36.8 kg

C. Overall agro-economic prospect

(1) Protein productivity per hectare: protein-enriched cassava versus soybeans:

c.a. 4:1

(2) One hectare of cassava can produce, via solid state fermentation, enough protein for feeding:

c.a. 65 pigs

* 34 per cent protein.
** data from U.S. Department of Agriculture.
*** for 20 per cent protein enrichment.
**** From: C.A. Shacklady, in G. de Pontanel (ed.), Proteins from Hydrocarbons, pp. 115 - 128, Academic Press, New York, 1972.

† Birth to weaning: 70 days; + 25 kg; diet with 15 per cent protein.

†† Weaning to slaughter: 130 days; + 85 kg; diet with 10 per cent protein.

††† Total: 200 days; 110 kg.

TABLE 4. Optimal Productivity of Protein-Rich Feeds

  Total yield





Soybeans 1.8 34 0.6
Rapeseed 3.0 23.3 0.7
Sunflower 2.5 22 0.6
Horse bean 3.2 28 0.9
Peas 3.0 25 0.75
Protein-enriched cassava 12.0* 20 2.4

* 40 tons per hectare of cassava with 70 per cent moisture content.

Based on prices in October 1978 and on the average yields of agricultural products, a comparison can be made of the gross product per hectare of corn, wheat, soybeans, and protein-enriched cassava. The figures in Table 5 strikingly demonstrate the economic advantage of protein enrichment by solid state fermentation. 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, as already pointed out, one of the major agro-economic advantages of proteinenriched cassava is the possibility of feedstock production in regions where no other suitable source of conventional feed protein is available.

It is obvious that the economic competitiveness of protein enrichment by solid state fermentation depends ultimately on the investment and production costs of the process. It would be premature to propose a really accurate estimate in this regard, and information must be obtained from pilot operations at the farm level. However, based on its present state of technological development, and on the data reported in Table 5, the conclusion is that the process will prove valuable.

TABLE 5. Comparison of Productivity and Gross Product per Hectare












Corn 6 82.9 497.4 114
Wheat 5 127.7 638.5 147
Soybeans 1.8 241.8 435.2 100
cassava 12** 485.4*** 1165.0 268

* On 29 September 1978.
** Cassava: 40 tons per hectare, with 70 per cent moisture, dry product containing 20 per cent protein.
*** Estimated from current price (US$213.6, Rotterdam, cif) of soybean meal with 44 per cent protein.