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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderPerspectives on bioconversion of organic residues for rural communities
View the document(introduction...)
View the documentIntroduction
View the documentSources of available nutrients
View the documentThe most suitable materials for bioconversion
View the documentCharacteristics of residues
View the documentBioconversion systems
View the documentPhysical and chemical treatments
View the documentMicrobial conversion
View the documentThe animal conversion phase
View the documentSummary
View the documentReferences

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.