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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

Characteristics of residues

Residues are not used as foods because they are inedible without some form of bioconversion. Table 3 shows that the chemical composition of most residues is not well balanced. Straw contains 48 per cent crude fibre and 3 per cent crude protein.

TABLE 3. Chemical Composition (% dry matter) of Various Residues

  Grain*

straw

Leaf*

(grass)

Citrus**

pulp

Manure***

(poultry)

Organic matter 95 91 93 77
Ash 5 9 7 23
Crude protein 3 17 7 32
Crude fibre 48 27 14 -
Nitrogen-free-extract 43 44 69 27

* O. Kellner and M. Becker, Grund├╝ge der Futterungslehre (1959).

** CVB Cattle Feed Table.

*** F. de Boer and A. Steg (Hoorn), report, "Megista mestdag," part II.

It is hardly a good product for human consumption. Grass has a better composition, but if it were to be used for monogastric consumers like man, there would still be severe problems because of the relatively high crude fibre content. Poor digestibility is another reason for rejecting unprocessed residues.

Table 4 gives the digestibility coefficients of some residues for ruminants. The organic matter of straw is only 38 per cent digestible even for ruminants. For monogastric organisms like man, poultry, and pigs, the coefficients are even lower. Grass is more digestible but less suitable for monogastrics. Digestibility of citrus and animal wastes is reasonable, but not particularly good.

TABLE 4. Digestibility Coefficients in Ruminants

  Grain*

straw

Leaf*

(grass)

Citrus**

pulp

Manure***

(poultry)

Organic matter 38 72 - 72
Crude protein 12 75 42 78
Crude fibre 40 65 80  
Nitrogen-free 38 77 95 69

* O. Kellner and M. Becker, Grundz├╝ge der Futterungslehre (1959).

** CVB Cattle Feed Table.

*** F. de Boer and A. Steg (Hoorn), report, "Megista mestdag," part II.

A number of other reasons may make a residue undesirable. Logistic aspects and low drymatter content may be expensive to overcome. Seasonal variability often makes it difficult to manage the material by advanced technology. Chemical and microbial contamination and organoleptic or psychological unacceptability may preclude the use of some residues as food. The above characteristics all present problems that must be overcome if a residue is to be converted to food.