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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderMicro-organisms as tools for rural processing of organic residues
View the document(introduction...)
View the documentIntroduction
View the documentMicrobial utilization of mono- and di-saccharide residues
View the documentMicrobial conversion of starchy residues
View the documentMicrobial conversion of complex mixtures of compounds (Polysaccharides, Proteins, Lipids, etc.)
View the documentMicrobial utilization of cellulose and ligno-cellulose residues
View the documentAlgal culture as a source of biomass
View the documentMicrobial utilization of silviculture biomass
View the documentMicro-organisms and marine and freshwater biomass
View the documentInternational studies on processing organic residues
View the documentReferences

Microbial conversion of starchy residues

There is extensive literature on the utilization of starch-containing materials by microorganisms. Although not al) microbes are capable of producing enzymes (amylases) that attack starch, amylases have been found in many species of bacteria, streptomyces, yeasts, and moulds. The following species appear to be the most active (9). Bacteria:

a-amylase: Bacillus subtilis, B. macerans, B. amyloliquefaciens, B. stearothermophilus, Clostridium acetobutylicum

b-amylase: Bacillus cereus, B. megaterium, B. polymyxa

Moulds: Aspergillus oryzee, A. niger, A. fumigatus

Two examples may be mentioned briefly where substances rich in starch are converted by the organisms mentioned above to useful products.

Aspergillus fumigatus, a thermophilic mould, has been used to make single-cell protein from cassava (10). Because this process is not complex and produces good yields of protein, it could be adapted to rural areas. Cellulolytic fungi (Trichoderma viride, basidiomycetes) have been employed with commercial amylases to enhance the saccharification of cassava starch; the hydrolysate served as a better substrate for the alcoholic fermentation by yeast (1 1).

The second example is currently a successful commercial process, but because of its nature it could possibly be adapted to the production of sugar "sweetener" in rural regions. In the United States, in 1977, over 2 million tons of fructose-sweetener corn syrup were manufactured from corn starch, using over 1,000 tons of microbial amylases and glucose isomerases.

The manufacture of high fructose corn syrup is now a continuous process, employing immobilized enzymes. The saccharification of the starch is accomplished by the combined action of acid and microbial amylases from bacilli, and the resulting maltose-glucose solution is then subjected to isomerization to yield fructose (42 per cent), glucose (50 per cent), and some higher saccharides (Figure 2). Several commercial processes employ preparations of isomerase from Streptomyces sp. (S. albus, S. olivaceus, S. wedmorensis, and mutants of several kinds), but several bacterial species (Bacillus coagulans, Pseudomonas hydrophilia, Escherichia freundii, Nocardia asteroides, etc.) and aquatic actinomycetes (Actinoplanes missouriensis) yield considerable amounts of glucose isomerase (12, 13).

Figure. 2. Flow Chart for the Production of High-Fructose Corn Syrup from Cornstarch (From Mermelstein [12])

Many other agricultural residues and agro-industrial wastes belong to this group of substances, which are rich in starch, pectin, sugars, organic acids, and even some nitrogenous compounds. They include cull and wash materials from fruit, vegetables, meat, and other foods being processed. In the United States, Pimentel and associates (3) estimate these materials to be several million tons annually (Table 3). For example, Aspergillus niger will convert 97 per cent of the sugars from brewery-spent grain liquor to fungal mass suitable for feeding purposes (14). Similarly large quantities of wastes occur from washing and pulp waters from coffee processing in Central and South American, Asian, and African nations (1,15-17) These substances offer considerable challenge and promise for future developments, and micro-organisms play a part in their utilization. According to a review by Han and Smith, they can best be utilized for what they call one of the five F's: fuel, fibre, fertilizer, feed, and food (18).