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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
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close this folderPerspectives on bioconversion of organic residues for rural communities
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View the documentSources of available nutrients
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close this folderAvailability of organic residues as a rural resource
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View the documentDiscussion summary: Papers by van der Wal and Barreveld
close this folderMicro-organisms as tools for rural processing of organic residues
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View the documentMicrobial utilization of mono- and di-saccharide residues
View the documentMicrobial conversion of starchy residues
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View the documentMicrobial utilization of cellulose and ligno-cellulose residues
View the documentAlgal culture as a source of biomass
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close this folderProduction of feed as an objective for bioconversion systems
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close this folderEnvironmental goals for microbial bioconversion in rural communities
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View the documentDiscussion summary: Papers by Porter, Berk and La Rivière
close this folderStrategies for developing small-scale fermentation processes in developing countries
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close this folderProduction of microbial protein foods on edible substrates, food by-products, and ligno-cellulosic wastes
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View the documentContributions to the solution of nutritional problems
View the documentDevelopment of protein-rich vegetarian meat substitutes in the western world
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close this folderThe role of ruminants in the bioconversion of tropical byproducts and wastes into food and fuel
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View the documentNutritional limitations in the use of tropical by-products and waste
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View the documentAn integrated system for converting tropical feeds and byproducts into milk, beef, and fuel
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close this folderPossible applications of enzyme technology in rural areas
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close this folderIndian experience with treated straw as feed
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View the documentAnnex 1. The energy efficiency of the two-stage, feed-fuel processing of straw in indian villages
View the documentAnnex 2. Method of calculating the value presented in table 2 for the efficiency of naoh energy usage
View the documentAnnex 3. Recommendations to farmers on the treatment of straw
View the documentAnnex 4. Calculated efficiency of milk production by straw-fed village buffaloes
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close this folderIndian experience with algal ponds
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close this folderOrganic residues in aquaculture
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close this folderBiogas generation: developments. Problems, and tasks - an overview
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View the documentWhat is biogas?
View the documentMicrobiology of CH4, or bio-methanogenesis
View the documentThe biogas plant-some technical considerations
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close this folderMushroom production technology for rural development
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close this folderThe combination of algal and anaerobic waste treatment in a bioregenerative farm system
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close this folderA continuous composting system for disposal and utilization of animal wastes at the village level
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View the documentA continuous composting system for land utilization of animal wastes at the village level
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close this folderBioconversion of fruit and vegetable wastes
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close this folderIntegrated research on agricultural waste reclamation
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View the documentProduction of yeast from soybean cooking waste at miso factories
View the documentApplication of soy waste as koji substrate for rice miso manufacturing (5, 6)
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close this folderSolid state fermentation of starchy substrates
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close this folderProduction of single-cell protein from cellulose
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close this folderAnalysis of energy cost of integrated systems
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close this folderAnalysis of bioconversion systems at the village level
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close this folderNutritional evaluation of bioconversion products for farm animals
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close this folderBioconversion products: toxicology - problems and potential
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close this folderNutritional evaluation in humans
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View the documentEvaluation of products of bioconversion for human consumption
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View the documentDiscussion summary: Papers by van Weerden, Shacklady, and Bressani
close this folderBiomass from organic residues for animal and human feeding
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close this folderAppropriate biotechnology - summary remarks
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close this folderOther UNU Publications
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Microbial utilization of mono- and di-saccharide residues

The by-products (molasses, sulphite liquor, whey) listed in group 1, Table 1, are rich in fermentable sugars, and they serve as a major source of carbon for a great variety of micro-organisms. At least 5,000 microbial metabolic products have been isolated from solutions in which the simpler sugars have served as the main source of carbon for metabolism by micro-organisms. These metabolites include not only simple alcohols, organic acids, gases, antibiotics, vitamins, enzymes, toxins, etc. but also some unique compounds whose use or function remains unknown. Great opportunities exist for finding uses for some of these substances, or for developing technologies that may be applicable to rural processing of such materials.

Molasses

Large quantities of molasses are produced in countries where sugar-cane is grown and processed. Rolz, for example, estimates that over 6.3 million tons are available annually in the major sugar-cane-growing countries of Latin America (1).

The sugar in molasses can be metabolized by many micro-organisms and by several known pathways. The particular pathway followed, and the end-products produced, depend not only on the particular microbe, but also on a variety of environmental factors.

Special strains of Saccharomyces cerevisiae, S. fragilis, and Candida utils are used in the baking industry, as feed and food supplements, and for other purposes. World production of such yeast is over 300,000 tons per year. The raw materials for cultivation of such yeasts are generally a mixture of molasses, ammonium salts, and other essential inorganic salts.

In recent years the production of filamentous fungi as a source of protein has been emphasized. Espinosa et al., for example, have shown that the growth of Verticillium sp. on cane blackstrap molasses and coffee-waste water is technically feasible (6).

Mushroom mycelium has also been grown in molasses, as well as in vinasse, a waste product from the distillation of fermented sugar-cane juice.

Perhaps the greatest potential use of molasses, other than as a sweetener in foods for human consumption, and as a livestock feed supplement, is for the production of ethanol by fermentation, or as a feedstock for the manufacture of other useful products. The fermentation of molasses to ethanol by yeast is not an especially complex process, and it can be easily adapted on a small scale to rural areas. In Brazil, however, the production of ethyl alcohol from sugar-cane, manioc, and other tropical plants has become a major project of the government to reduce petroleum imports (Figure 1). Approval was given by Brazil's National Alcohol Commission for government financing in the amount of US$800 million in 1977 for over 30 of the 170 proposed distilleries. The plan calls for increasing alcohol production to over 3,800 million I by 1982. As fossil fuels become scarcer, many nations may need to turn to the ethanol fermentation of waste saccharide materials as a source of energy (7).



Figure. 1. Fermentation of Biomass to Ethanol or Other Organic Chemicals, and Other Organic Chemicals (From Altepohl [7])

Sulphite Waste Liquor

Several million tons of sugar occur in the sulphite liquor that results from the production of paper products; most is discarded in the United States (Table 3), and similar amounts are probably considered waste in other countries. Apart from the fact that sulphite liquor from the paper mills causes a disposal problem, it is also an economic loss because it can be converted into single-cell protein (SCP), ethanol, or D-lactic acid.

Candida utilis has been used for alcohol and feed yeast production from paper mill waste because it has a high tolerance for sulphite and can convert both hexoses and pentoses into yeast protein. A commercial operation called the Pekilo Process has been developed in Finland for the production of single-cell animal feed. Spent liquor from sulphite pulp mills is used as the substrate, and the fungus Paecilomyces variotil, which consists of 55 to 60 per cent protein, is used in the fermentation process. The first Pekilo plant built produces about 10,000 tons of single-cell protein annually.

Lactobacillus pentosus seems superior to other bacteria for producing D-lactic acid from sulphite waste liquors. Estimates for a mill producing 100 tons of pulp daily are that over 3 million kg of lactic acid can be manufactured annually.

Mushroom mycelium has been grown in sulphite waste liquor, and the process has been granted a patent.

Whey

In countries where cheese-making is important, large volumes of whey accumulate and must be disposed of as a waste, as profitable uses have not been found for the material. Development of new uses for whey would do much to reduce the waste and avoid the loss of milk nutrients. The possibilities for such developments offer some of the most interesting challenges in applied science.

Whey has some limitations as a substrate for attack by micro-organisms because fewer microbes utilize lactose than other sugars such as glucose. The best suited organisms for fermentation of whey are lactobacilli and certain yeasts.

Lactobacillus bulgaricus is capable of converting over 90 per cent of the lactose in whey to DL-lactic acid, and the organism is now used commercially for this purpose. Various lactosefermenting yeasts (Saccharomyces fragilis, Candida pseudo-tropicalis, or Torula cremoris) can convert the sugar to various products without altering the other nutrients in whey; this has become a commercial process for producing lactose-free whey and ethanol (80 to 90 per cent conversion of the lactose).

Several hundred-thousand tons of yeast for baking, feed, and food supplements have been manufactured for many years, utilizing low-grade sugars as a substrate; the demand for such protein is increasing. Recently a new, continuous-flow, closed-system plant has been put into operation to produce the lactose-fermenting yeast Candida utilis from whey. The plant is capable of manufacturing 7,500 tons of yeast annually.

Other Saccharides

Juices from various fruits, leaves, and stalks of plants contain sugars that can be grouped in categories I and II (Table 1). Many of the materials are abundant and cheap, and could be readily converted by microbial processes to useful substances. One example may be mentioned.

Agave juice from plants growing on arid lands has been used experimentally as a substrate for SCP production (8). Both pure cultures of yeast (Saccharomyces carbajali, Candida utilis, etc.), and mixed cultures of yeast, fungi (Ustilago maydis), and bacteria (Corynebacterium glutamicum, Brevibacterium flavum) were used to produce the SCP biomass. The yields of high-quality microbial protein obtained were good (20 g/l) from a 24hour semi-continuous operation. Indications are that a plant would have considerable socioeconomic impact on production in Mexico, where protein feed and food are badly needed.