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Microbial conversion

Alkali-treated straw can be given to small fermentation "plants" located in the rumens of cows, buffaloes, or goats The micro-organisms in these rumens are able to convert the treated residue into protein. The process has been fairly stable through the ages.

Microbial conversion can also be carried out outside the animal through fermentation processes By applying appropriate technology, we should then be able to produce protein products that could be converted into food by monogastric animals like poultry and pigs

If the micro-organisms used remain combined with the remnants of the organic residue that was used as a substrate, we call the product microbial biomass product (MBP). If the micro organisms are harvested and separated from the substrate, we refer to the product as single cell protein (SCP) The composition of SCPs compares favourably with the substrates on which they are grown, as shown in Table 6.

TABLE 6. Comparison of Chemical Composition (%) of SCP with Soybean Oilmeal

  Yeast Bacteria Fungi Algae Soybean oilmean
Dry matter 96 90 86 94 88
Ash 6 8 2 7 6
Organic matter 90 81 84 87 82
Crude protein (N x 6.25) 60 74 32 52 45
True protein (amino acid- N x 6.25) 47 55 22 46 38
Crude fat 9 8 5 15 1
Crude fibre - - 28 11 6
Nitrogen-free extract 20 - 20 12 30

Crude protein content and amino acid composition (Tables 6 and 7) put bacteria, yeasts, fungi, and algae into the category of high quality protein sources such as soybean oilmeal.

TABLE 7. Amino Acid Composition (9/16 9 N) of SCPs and Soybean Oil meal

  Yeast Bacteria Fungi Algae Soybean oilmeal
Lysine 7.0 5.5 4.8 4.6 6.2
Methionine + cystine 2.9 3.1 2.5 3.2 2.9
Arginine 4.8 4.7 5.2 - 7.2
Histidine 2.0 1.9 2.0 - 2.5
Isoleucine 4.5 3.9 4.1 3.1 4.9
Leucine 7.0 6.3 6.4 7.0 7.6
Phenylalanine + tyrosine 7.9 6.2 8.1 6.0 8.4
Threonine 4.9 4.2 4.4 4.9 4.2
Tryptophan 1.4 0.8 1.4 1.7 1.3
Valine 5.4 4.8 5.6 4.7 5.0

The digestibility of SCP (Table 8) again compares well with conventional high-quality protein sources like soya. Digestibility is lower for algae, and the data are inconclusive. Further evaluation is required.

TABLE 8. Digestibility Coefficients in Pigs

  Yeast Bacteria Fungi Algae Soybean oilmeal
Organic matter 92 90 79 - 83
Crude protein 90 93 71 54 91
Crude fat 95 87 34 - 34
Crude fibre - - 99    
Nitrogen-free extract 94 - - - 94
Metabolizable energy (kcal/kg) 3,860 3,720 2,940 - 3,190

It is often assumed that small-scale SCP production can be made operational relatively easily. This is a serious under-estimation of the problems involved. Development of low-key technology that can operate on the scale of a farm co-operative or a village, and that is nevertheless effective and stable, requires elaborate research efforts. Positive results are more likely to be achieved if experienced industrial fermentation research groups participate.

The animal conversion phase

In order to use SCP products successfully, a thorough nutritional and toxicological evaluation is necessary. Nutrient requirements and digestion in animals are species-specific and so is absorption of nutrients after digestion. The metabolism of nutrients and potential toxic substances is also species-specific, as is susceptibility to toxic substances.

The consequence of this specificity is that experimental data obtained in animal testing cannot be extrapolated with certainty to other animal species. The nutritional and toxicological evaluation must, therefore, be done in all animal species for which the product is destined, i.e., the target species. In guidelines for testing the nutritional and safety aspects of novel sources of protein, as formulated by the Protein Calorie Advisory Group of the United Nations System (PAG) (4, 5), this is taken into account.

Table 9 shows digestibility coefficients of a fungal product in two different monogastric species - poultry and pigs. The digestibility of the organic matter, which is close to 80 for pigs, is a mere 24 for chickens. Protein digestibility differs somewhat less dramatically. The table suggests that the difference is probably caused by the difference in digestibility of crude fibre. In the final analysis, the metabolizable energy available for chickens is only one-third of that for pigs. The difference between species is usually less marked, but the figures illustrate that specific reactions of animals must be taken into account in the evaluation of SCP.

TABLE 9. Digestibility Coefficients of a Fungal Product

  Pigs Chickens
Organic matter 79 24
Crude protein 71 59
Crude fat 34 18
Crude fibre 99 6
Metabolizable energy (kcal/kg) 2,940 1,000

When the basic nutritional and toxicological evaluations have been completed with satisfactory results, the product can be submitted for approval by government authorities.

When producing SCP products commercially, biological testing must ensure that they comply with the specifications of the product for which approval was originally obtained. If the product is modified after testing, the initial experimental data may no longer be applicable.

The final stage of testing should include optimum application of the product in the rations for the animal species that will consume these rations in the countries where the product will be applied. Here, too, specific environmental and social factors may play a major role. Table 10 presents the kinds and numbers of animals available for conversion of residues into protein, but the acceptability of their products differs greatly among regions and cultures.

TABLE 10. Number of Domestic Animals in World /in millions)

  World Africa Latin America South Asia S.E. Asia Near East
Cattle 1,214 160 266 199 23 46
Buffaloes 132 2 - 73 14 4
Sheep 1,038 159 120 77 3 137
Goats 413 127 41 87 9 62
Pigs 645 8 72 7 25 -
Chickens 6,116 488 721 192 307 236


For proper application of bioconversion systems, a detailed study of the optimum use that can be made of the biomass produced is necessary.

Only 5 per cent of the annual production of nutrients on land is used directly as food by man. Bioconversion systems making use of micro-organisms for SCP production can help the remaining 95 per cent to be utilized.

In most cases, food-producing farms are essential for making acceptable food for man from SCP. It is advisable to concentrate efforts, to a large extent, on the rural areas while making use of low-key technology that does not require large investments.

For the application of bioconversion, the development of stable village-scale fermentation technology and adequate evaluation of nutritional and toxicological aspects of the biomass produced form major obstacles.

A concentration on resources that are, in terms of quantity and allocation, particularly suitable for bioconversion seems advisable in order to avoid a dilution of efforts.

Of the cellulose-rich materials, straw is the most important. Among the starchy materials, the upgrading of cassava and its by-products to a protein-rich material shows good potential. Farm animal manure is considered a third major raw material that deserves high priority for upgrading.


1. E. Saouma, statement at the opening session of the Fifth International Congress of Food Science and Technology. Kyoto, Japan, 17 - 22 September 1978. (Proceedings in press.)

2. F. Aylward, "Food Safety: Novel Foods," paper presented at the Fifth International Congress of Food Science and Technology, Kyoto, Japan 17 - 22 September 1978. (Proceedings in press.)

3. E.M. Mrak, "The World Food Problem and Meeting the Challenge," paper presented at the Fifth International Congress of Food Science and Technology, Kyoto, Japan, 17 - 22 September 1978. (Proceedings in press.)

4. Protein-Calorie Advisory Group of the United Nations System (PAG) Guideline No. 6, "Preclinical Testing of Novel Sources of Protein," United Nations, New York, 1970.

5. PAG Guideline No. 15, "Nutritional and Safety Aspects of Novel Protein Sources for Animal Feeding," United Nations, New York, 1974.

6. P. van der Wal, "The Future of Single-Cell Proteins: The Nutritional and Safety Aspects," paper presented at the Fifth International Conference on Global Impacts of Applied Microbiology, Bangkok, 21 - 26 November 1977.

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