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CLOSE THIS BOOKBioconversion of Organic Residues for Rural Communities (UNU, 1979, 178 p.)
Micro-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

Bioconversion of Organic Residues for Rural Communities (UNU, 1979, 178 p.)

Micro-organisms as tools for rural processing of organic residues

Introduction
Microbial utilization of mono- and di-saccharide residues
Microbial conversion of starchy residues
Microbial conversion of complex mixtures of compounds (Polysaccharides, Proteins, Lipids, etc.)
Microbial utilization of cellulose and ligno-cellulose residues
Algal culture as a source of biomass
Microbial utilization of silviculture biomass
Micro-organisms and marine and freshwater biomass
International studies on processing organic residues
References

J.R. Porter

Department of Microbiology University of Iowa, Iowa City, Iowa, USA

Introduction

Micro-organisms have been closely associated with transforming or cycling organic matter in nature for as long as such material has existed. But it has only been within the past 100 years that certain of these associations have become known, or that advantage has been taken of helpful microbes in rural agricultural practices. Predictions are, however, that greater use will have to be made of beneficial micro-organisms.

I need not discuss the important part micro-organisms play in the production of humus, nor how they help cycle all elements or substances in the soil and thereby provide the nutrients necessary for healthy crops. These topics are beyond the scope of this paper. Neither will I discuss in depth how microbes fix an estimated 150 to 175 million tons of atmospheric nitrogen per year, which is several times more than the total commercial production of nitrogen fertilizer in 1977; nor how micro-organisms may be degrading over 1,500 million tons of pesticides and large quantities of other complex synthetic substances that find their way into the environment each year.

The concept of utilizing excess biomass or waste from agricultural and agro-industrial residues to produce energy, feeds or foods, and other useful products is not necessarily new. For centuries agricultural residues and wood have been used as sources of fuel, food, construction materials, and paper-making, as well as for other purposes. Recently, fermentation of biomass has gained considerable attention because of the forthcoming scarcity of fossil fuels, and because it is necessary to increase the world food and feed supplies - especially those high in protein.

Most attention today is being given to the possible use of micro-organisms to convert relatively high-quality biomass (corn and grains, sugar-cane juice, etc.) to fuel. Although this topic will be discussed later, certain technical and economic restrictions exist that must be removed if significant fuel production is to result from fermentation of such high-quality biomass, because these substrates have other important possible uses. This does not mean, however, that residues from farm crops, livestock feedlots, agro-industries, forest operations, and other similar practices should be excluded. This is especially so in circumstances where their removal does not eventually reduce the quality of the land, permit soil erosion, or produce other harmful effects on crops.

A logical classification of agricultural and agro-industrial materials has recently been published by Rolz (1) His data (Table 1) illustrate the variation in the structure of the substances, and the nature of the by-products that may be available for utilization by micro-organisms

TABLE 1. Classification of Agricultural and Agro-Industrial By-products

Group Predominant compound Activity By-product
I High proportion of di- and mono-saccharides Sugar-cane growing and processing Molasses
    Pulp elaboration Sulphite liquors
    Cheese-making Whey
II Di- and mono-saccharides with some structural polysaccharides Fresh fruit collection centres Rejected or damaged fruit
    Rum and liquor making Wash waters
III Mixture of soluble organic compounds, including starch,

sugars, proteins, pectin,

acids, etc.

Fruit and vegetable processing Wastes from washing, peeling, and blanching
    Tuber and grain processing Wastes from sorting and washing
    Coffee processing Washing and pulping waters
    Meat processing (beef, pork, poultry) Washing and scalding waters
IV Complex mixtures of structural

polysaccharides and other

compounds such as proteins,

lipids, etc.

Fruit and vegetable processing Peels, insoluble solids from pulp and seeds
    Animal and poultry production Manure
    Animal slaughtering and meat processing Suspended solids
    Coffee processing Pulp
    Sugar-cane and oil palm processing Residual solids
    Alcohol and alcoholic beverage processing Residual solids
V Structural cellulose and lignin in high proportion Cereal, sugar-cane, and rice growing and processing Straw, husks, bagasse
    Corn growing and processing Stocks and cobs
    Citronella and lemon grass processing Bagasse
    Coffee and cacao processing Husks
    Cotton seed processing Hulls, linters
    Forest processing Bark, sawdust, wastes

Source: Rolz (1).

Several calculations have been made of the quantities of biomass produced annually in the world by photosynthesis, and the resulting agro-industrial wastes One estimate is that 1.7 x 10(11) tons of biomass are produced annually, and that 98 per cent of this amount is not used in an economically sound manner DaSilva, Olembo, and Burgers (2) present data on some agricultural residues in six European countries (Table 2); these constitute about 98 million tons each year. For other countries, the three authors estimate the following: Malaysian oil palm and rice mill wastes in 1974 were 3 million and 250,000 tons, respectively. In Egypt, 600,000 tons of maize cobs, 1.5 million tons of dry rice straw, and 40,000 tons of sugar pith residues accumulate annually. About 100,000 tons of sugar-cane bagasse are burned in Bangladesh each year. In Western Australia, 10 million tons of wheat and barley straw and chaff are produced annually. Recent similar estimates for the United States by Pimentel and associates (3) are presented in Table 3.

TABLE 2. Agricultural Wastes in Certain European Countries

  Amounts in tons    
Country Cereal straw* Corn stover Beet pulp by-product
Belgium 1,760,000 - 120,000
Federal Republic      
of Germany 8,600,000 - 700,000
France 26,000,000 9,000,000** 1,100,000
Italy 8,850,000 4,000,000  
Netherlands 1,350,000 - 110,000
United Kingdom 26,000,000 - 375,000

Data from Battelle Document 75712, Courtesy DaSilva, Olembo, Burgers (2).

* Amount varied in different countries from 1.5 to 5.0 tons/ hectare.

** Yield 1.8 tons/hectare.

TABLE 3. Sources of Biomass Available Annually in the United States

Source Available

Hypothetical

Mt, dry

Ready

Mt, dry

Energy

conversion

kcal x 1012

Livestock manure 255 128 27
Crop remains 430 0 0
Food-processing wastes(20 - 70% moisture) 4 4 18
Food-processing wastes(70 - 90% moisture) 14 14 10
Forestry remains 340 44 50
Forestry-processing wastes 81 20 85
Fuel-wood production 10 10 40
Fuel-wood plantations 60 60 52
Hydrocarbon plantations 16 16 13
Aquatic plants 17 3 0.6
Urban refuse 123 66 41
Industrial wastes 40 5 4
Municipal sewage 13 2 1 3
TOTAL 1,403 372 341,9

Source: Pimentel et al. (3).

Before any organic residue or high-quality biomass material is considered for microbial conversion to other substances, a number of factors must be taken into consideration. For instance: (i) Is there a ready and continuous supply of the raw product to be converted? (ii) If the material is removed from cropland or forests, will this contribute to soil erosion and depletion of plant nutrients? (iii) Are expensive equipment and large amounts of capital necessary for the processing? and (iv) Are such things as an external energy supply and large amounts of water necessary?

After considering the above factors, the following question may be asked: Which microorganism or microorganisms possess potentials for the bioconversion of the organic material under consideration? First, we must keep in mind that in natural conditions the indigenous microbial flora is only one component of a complex, dynamic biomass undergoing interaction in the transformation of organic matter. Only in a few cases can any one species or genus be given sole credit for natural bioconversions. For example, in the transformation of green fodder or forage crops into silage, the complex fermentation process involves plant enzymes as well as several groups of microorganisms present in the fodder and in the environment. Likewise, in the production of biogas from organic wastes, methane bacteria may be responsible for the gases produced, but this is not the only biological process taking place in the digester.

Even though mixed cultures of micro-organisms are usually involved in the transformation of organic residues, there are cases where pure cultures of bacteria, yeasts, moulds, or enzyme preparations can be used for processing such materials; these will be discussed later

Because other papers in these proceedings are also devoted to topics that can be included under the broad title of this paper, the following discussion will be restricted to a few possible microbial processes involved in the transformation of by-products or residues listed in Tables 1-3. Some of these processes have been, or can be, adapted on a small scale to rural regions, but others currently require fairly sophisticated knowledge or equipment for operation.

One of the major problems facing us today is how to adapt technical skills to various regions where people differ in their cultural or social customs, where natural resources vary, where the economy is dissimilar, or where environmental conditions may limit certain processes. More careful thought must be given in future developments as to whether the so-called "high technology" will be the best choice for people in every nation, or whether more attention needs to be given to what Norman refers to as "soft technologies" (4); Hedn speaks of as "self-reliance in an equilibrium society" (5); or what DaSilva, Olembo, and Burgers consider "low capital" vs. "high capital" technologies (21. The results presented in this Symposium can help point the way for leaders in various countries to make certain important decisions for future development.

An extensive discussion of even the major organic residues that can be utilized by microorganisms in a rural environment cannot be covered in one article. So I have selected only a few substances from the groups used for classification in Table 1.

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.

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).

Microbial conversion of complex mixtures of compounds (Polysaccharides, Proteins, Lipids, etc.)

The residues in this group (Table 1) consist of mixtures of various complex compounds resulting from several agro-industrial activities. Some of the compounds are soluble, others are colloidal or solid (1). In certain cases the residues may be fairly uniform in character (peels from potatoes or apples), whereas in other instances the material may be of varied composition (manure).

Under natural conditions, rarely are substances in this group transformed by a single microbial species. Rather, a mixed flora is usually responsible for the conversions that occur. Thus, it is generally impossible to single out separate species as being responsible for any transformations that take place. Some of the residues are such, however, that they could be utilized for alcohol fermentation or SCP production by yeast, or they could serve as a substrate in rural areas for biogas production, and for algal culture. Because these processes are currently receiving considerable attention, and it is not easy to single out other unique processes where pure cultures can be employed, the reader is referred to reviews by Rolz (1) and DaSilva et a). (2), and the treatise on methane generation from human, animal, and agricultural wastes (19).

Microbial utilization of cellulose and ligno-cellulose residues

The most abundant renewable biomass on earth consists of cellulose, with between 5 and 15 tons per person being synthesized annually by photosynthesis. Much of the cellulose in nature is bound physico-chemically with lignin.

Because lignin is highly resistant, it protects cellulose against attack by most microbes, and it must be degraded by chemical or biological means before the cellulose can be utilized. Some higher fungi such as the basidiomycetes (Planerocheate chrysosporium) can degrade lignin, and mush rooms (Lentinus, Volveriella, and Pleurotus species) convert ligno-cellulose directly into fungal protein suitable for human consumption.

Table 4 lists some cellulose-utilizing organisms together with the general products they form, and their current status of development as useful agents. Brief mention may be made of each of these groups.

TABLE 4. Products of Some Cellulose-Utilizing Organisms

Group Product formed Current status
Volvariella sp. Human food (mushrooms), animal feed Produced commercially
Lentinus edodes Human food (mushrooms), animal feed Produced commercially
Pleurotus sp. Human food (mushrooms). animal feed Produced commercially
Bacidiomycetes:    
Phanerochaete chrysosporium Delignified cellulose for use as feed, fibre, or further conversions Under research
Thermoactinomyces sp. and other thermophilic actinomyces Human food (SCP), animal feed Under research
Trichodenma viride Cellulases for converting cellulose to sugars, animal feed (SCP) Under development
Clostridium thermocellum Cellulases for converting cellulose to sugar, ethanol, acetate, lactate, and H2; animal feed (SCP) Under research
Pseudomonae fluorescent var.

cellulosae and similar bacteria

Animal feed, cellulases for converting cellulose to sugars Under research
Cellulomonas sp. plus Alcaligenes faecalis Animal feed Under research
Candida utilis Animal feed Under research
Thermophilic sporocytophaga Animal feed, ethanol, acetic acid Under research

Volvariella Species

Mushrooms of the genus Volvariella (V. volvacea, V. esculenta, and V. diplasia) are cultivated mainly on rice straw and similar cellulosic materials by individual families in Asia and Africa Commercially, mushrooms in this genus account for about 4 per cent of the world production of some 916,000 tons. They have promise of expanded use in regions of the tropics where the grain is grown Production usually involves simply inoculating pre-soaked straw in flat stacks with spores (spawn), maintaining optimal moistures, and harvesting several crops of mushrooms. The spent straw is used to inoculate new straw stacks, and is a rich animal feed (20).

Lentinus edodes

The mushroom Lentinus edodes has been cultivated for centuries in China and Japan, where it is commercially produced in a multi-million-dollar industry; it accounts for about 15 per cent of world production. (Both in Asia and more especially in western countries, Agaricus bispora is the most important mushroom species and accounts for about 75 per cent of world production [20].)

L. edodes has potential for bioconversion of lignified residues and low-quality wood into fungal protein. Such protein is easily digested by ruminants, but its use as a feed supplement has received little attention.

Pleurotus Species

Mushrooms of the genus Pleurotus (P. ostreatus, P. sajorcaju, P. florida, P. cornucopiae, etc.) are called "White-rot" fungi, and they decompose lignin and polysaccharides in wood. They have potential in the conversion of waste and low-grade wood into protein-rich food for human consumption. P. cornucopiae is grown commercially in Japan, but none of the species is grown in western countries. P. ostreatus and P. florida have temperature optima near 30 C, making them promising for processing organic residues in the tropics. All can be cultivated on mixtures of sawdust and grain, manure, and food processing wastes (20 - 22).

Basidiomycetes

Wood-decaying fungi, such as Phanerochaeta chrysosporium, are widely distributed in northern countries where they are commonly called "white-rot" fungi. P. chrysosporium decomposes both the lignin and cellulose in wood; it is unique in that (i) it produces copious quantities of spores, making it easy to transfer; (ii) it is thermotolerant, growing rapidly at 35 to 40 C, but also well at 25 C; and (iii) it has simple nutritional requirements. This fungus has been fed to fish and rats as a source of protein, but it has not been studied extensively as a nutrient for other animals. It should be considered as a means of converting wood processing residues and other lignified wastes into partially de-lignified products for feed or fibre use, or for further conversions (23).

Thermoactinomycos Species

The thermophilic, cellulolytic, and starch-utilizing actinomyces, such as Thermoactinomyces sp., may provide an opportunity to produce single-cell protein for feed supplements in tropical climates. The thermoactinomyces do not utilize ligno-cellulose directly, so treatment of such complex materials would be necessary. However, they grow rapidly at 55 to 65C under aerobic conditions on a variety of cellulosic and starchy materials plus other simple nutrients. According to preliminary results of Humphrey and associates (24), cell yields of 0.45 9 cell/g cellulose utilized can be obtained in 20 to 24 hours; apparently four cell-bound enzymes are involved in the degradation process.

Trichodorma viride

Although a number of fungi are cellulolytic, only a few produce cell-free cellulose in sufficient quantity to be useful for large-scale development. Trichoderma viride and a number of its mutants do produce a stable cellulose that is capable of degrading cellulose (Figure 3). The fungus grows rapidly on simple media in the pH range of 5.0 to 2.5, thus reducing to a minimum contamination from other microbes. The broth containing the enzyme is then mixed with pure cellulose, or with treated lignocellulose to remove the lignin, and a glucose syrup results.


Figure. 3. Enzymatic Conversion of Waste Cellulose to Glucose Sugar

The Japanese are producing cellulose commercially from J. viride on a limited scale using the Koji process, and considerable research is being done in several laboratories to obtain mutants that yield more enzyme. The use of cellulose from T. viride holds great promise as a tool for processing cellulose residues (25 - 27).

Clostridium thermocellum

The only known thermophilic, anaerobic bacterium that degrades cellulose is Clostridium thermocellum. The organism has simple nutritional requirements and grows at higher temperatures (50 C) than do most bacteria, which has the advantage in a fermentation process of being less prone to contamination by other organisms. In pure culture, the chief products from cellulose (or treated ligno-cellulose) are cell mass (protein), acetate, ethanol, lactate, H2, and CO2. In a mixed culture, C. thermocellum and Methanobacterium thermosutotrophicum, yield from cellulose are cell mass, methane, and acetate (27, 28). This is not a process that could be easily adapted to rural areas unless proper equipment were available but it could be used to produce either ethanol or biogas (methane) from cellulosic wastes.

Pseudomonas flvorescens var. cellulosee, Cellumonas Species, Cellvibrio Species, and Other Cellulose-Degrading Organisms

Species in the genera Pseudomonas, Cellulomonas, and Celivibrio utilize cellulose, but they apparently are unable to degrade ligno-cellulose. Co-fermentative studies on cellulose have been conducted using P. fluorescens and Candida utilis, Cellulomonas species, and Alcaligenes faecalis, and with Cellulomonas flavigena and Xanthomonas campestris Supposedly, cofermentation of cellulose with a non-cellulolytic organism increases the rate of utilization of soluble sugars produced in the process and thereby hastens the reaction (29). In fact, Casas-Campillo and colleagues (personal communication, 1978) have found that C. flavigena and X. campestris together are much more active against cellulose than are T. viride or combinations of other organisms.

Thermophilic Sporocytophaga

The Sporocytophaga (Sporocytophaga myxococcoides, etc.) digest cellulose and other components of cell walls, but not ligno-cellulose. A thermophilic strain that grows at 55 to 65 C has been found. This organism might be useful for the production of cell mass, ethanol, acetate, and lactate from cellulose.

Algal culture as a source of biomass

For many years algae have been used by the shoreside populations of Lake Chad and Lake Texcoco (Mexico) as a source of food, and one algal species (Spirulina sp.) is now produced commercially in Mexico at the rate of several thousand tons per year.

Certain big-engineers and microbiologists believe that carefully selected genetic strains of algae (Scenedesmus acutus, Spirulina maxima, Cosmarium turpinii), or photosynthetic bacteria (Rhodospirillum sp. or Rhodopseudomonas sp.) are the organisms of choice for the production of single-cell protein (30, 31). Such microbes contain about 65 per cent crude protein of moderately high biological value. The protein appears to be well utilized by animals. In addition, most species are as good a source of the B vitamins as yeast, and they contain ascorbic acid.

In addition to algal biomass having many uses, the process can be used to help purify sewage, livestock manure, and other agricultural wastes. A unique feature proposed by Colombo (31) is the cultivation of the algae in plastic tubes that can be extended for considerable distances in arid regions over land that is not useful for cultivating crops. Such a system takes advantage of solar energy, saves water, requires little capital and labour for development, and can be used either in the rural areas of less developed countries or on a larger industrial scale.

Microbial utilization of silviculture biomass

Several excellent reviews have been published on biomass production from forest ecosystems (32, 33), and data (Table 3) indicate that forestry remains and processing wastes are not fully utilized today. Chemical products that can be obtained from forestry biomass are ammonia, methanol, ethanol, fuel gas and oil, and charcoal. Ethanol fermentation by yeast from wood hydrolysis could become competitive within 10 to 15 years (32). But, as with methanol production from wood, the demand for ethanol will be satisfied for the near future by existing synthetic sources, unless it becomes necessary to use it in gasoline blends.

Micro-organisms and marine and freshwater biomass

Other sources of biomass deserving mention are marine plants, such as giant seaweeds (kelp), from the waters of the tropical and temperate oceans, and the so-called weeds, such as the water hyacinth.

Although the technology has not been proven, ocean-based kelp farming has some attraction for two reasons: (i) kelp is fairly efficient in converting sunlight into stored energy (2 per cent), and (ii) land and terrestrial waters are not constraints. Experimental data on marine plants have been collected by Wilcox of the US Navy's Ocean Food and Energy Farm Project (34). All aspects of this programme are interesting, but the only data involving microbiology are those concerned with the production of methane as a source of energy from kelp by anaerobic microbial digestion. In addition to methane, certain by-products remain in the sludge and liquid of the digester, which can be used for nitrogen fertilizer or animal feed supplements

There are no major nutritional deficiencies in kelp for mesophilic, anaerobic, methaneproducing microorganisms, so they grow readily on a slurry of the material. One precaution is that the salt concentration of the raw slurry must be reduced. An interesting research project would be to develop strains of methane-producing micro-organisms that are more salt-tolerant.

Economic studies indicate that the cost of methane production from kelp fermentation may range from US$2 to US$7 per GJ (per million BTU), depending on credit values received from feeds and other by-products. Thus, entry to the fuels market for kelp-derived methane will require research to provide a cheaper product, and capitalization. If preparatory methods for handling the kelp, and the fermentation, could be carried out in the open ocean where wind, wave, and solar energy could be used, the cost of the methane could be reduced below current land based fermentation processes.

One of the most prolific plant colonizers of rivers and lakes is the water hyacinth, which has spread in recent years from its natural habitat in South America to at least 50 tropical and sub-tropical countries around the globe. A few plants can multiply and spread over an area of 120 yd(2) of water in several months, depending on the nutrients in, and temperature of, the water, and the plant mass may represent many hundred tons of hyacinth.

Such water weeds are an environmental disaster in some countries because they interfere with water transportation and fishing, and they can be a health hazard as well by providing a suitable breeding site for the malarial mosquito. Ironically, the water hyacinth may be a promising candidate for solving needs of animal feed, energy, and control of water pollution, and in this regard, micro-organisms can play an important part.

Water hyacinths contain most of the essential nutrients for animal growth, but making a palatable feed from them is not easy because of the high moisture content of the plants. Research indicates, however, that the plants can be converted into silage by placing chopped plants in a closed container and allowing them to undergo microbial fermentation for about a month. Such silage has been shown to be highly palatable to sheep and other animals.

Potentially, the water hyacinth may be used as a source of energy, and for the purification of sewage. Considerable research has been done on these subjects, especially by Wolverton and McDonald at the NASA Space Technology Laboratories in Bay St. Louis, Mississippi (35). Biogas or methane production from the microbial anaerobic decomposition of water hyacinths has been investigated only on a laboratory scale. Many factors, such as carbon to nitrogen (C/N) ratios and temperature, affect the amount of gas and residue produced from the microbial digestion of the plant material. Based on research, it has been calculated that one hectare of water hyacinths can produce enough biomass each day to generate between 90 and 180 m of methane gas, and at the same time 0.5 ton of residue useful as a fertilizer. Further research is needed on the use of water weeds as a substrate for microorganisms.

International studies on processing organic residues

Considerable work is being done in several countries on the microbial production of food, energy, enzymes, and other useful substances from natural and agro-industrial wastes.

Some of these processes are, or could be, adapted to rural areas where the residues originate. A few examples are listed in Table 5, from the paper by Olembo (36) in the monograph on the Global Impacts of Applied Microbiology and Its Relevance to Developing Countries (37).

TABLE 5. Products Obtained in Various Countries from Residues Using Micro-organisms

Country Product Residue material Organism
Egypt Microbial protein Bagasse. rice hulls, distillery slops Candida utilis

C tropicalis

Chile Microbial protein Fruit peels, papaya wastes Yeast
Guatemala Animal feeds, alcohol,

enzymes, etc.

Bagasse, fruit wastes, coffee-bean by-

products, cotton cake, etc.

Bacteria, yeast,

fungi, algae

Indonesia Ontjom, tempe mate, kedele Soybean, peanut presscake Neurospora sp Rhizopus sp.
Israel Fodder yeast Citrus peels, cannery wastes C. tropicalis
Malaysia Fish sauce, poultry feed,

glutamate, vitamins

Fish wastes, tapioca rejects, rubber and

palm oil effluents

Bacteria,

Chlorella sp.

Philippines Vinegar, nata di, coco Copra extraction waters Torula sp. Leuconostoc sp.
Sri Lanka Vinegar, acidulants Molasses, copra waters Torula sp.
Thailand Microbial protein, fish sauce, etc. Fish rejects, tapioca, coconut, vegetable wastes, etc. Chlorella sp.

Torula sp.

Source: Olembo (36); data from UNEP/U/ICRO Training Courses.

The main objectives of the studies reported range from the need for an increase in protein food production to pollution abatement, and from industrial expansion to innovative research on the use of beneficial micro-organisms to improve the environment and welfare of human beings throughout the world.

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John Roger Porter, a much appreciated participant at the Guatemala conference and contributor to these Proceedings, died suddenly in May 1979. A highly productive and renowned microbiologist and former Head of the Department of Microbiology at the University of Iowa, Dr. Porter was a member of the Science Information Council of the National Science Foundation and a member of the National Board of Medical Examiners in Microbiology. He served as Chairman of the Advisory Committee on Scientific Publications of the National Institutes of Health, and was the recipient of the Pasteur Award in 1961. As Editor-in-Chief of the Journal of Bacteriology for ten years, he compiled and edited the 50-volume index for the Journal. His text Bacterial Chemistry and Physiology was widely used. A man of great warmth and personal charm, Dr. Porter will be greatly missed by his friends and colleagues.

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