by W. Dexter Bellamy
Waste recycling has been advanced as a method for preventing environmental decay and increasing food supplies. The potential benefits from a successful recycling of agricultural wastes are enormous. It may be the only method for largescale protein production that does not require a concomitant increase in energy consumption. In addition, it may be the most effective method for producing animal and human food from lignocellulose materials that are now of little nutritive value and are therefore used as fuel.
This article discusses the present status of microbial protein (singlecell protein) production from agricultural wastes and describes some of the technical and economic problems that must be overcome before large-scale application is possible.
The author is with the Environmental Unit, Physical Chemical Laboratory, General Electric Company, Corporate Research and Development, Schenectady, New York 12301, United States.
The volume of the various wastes available for production of singlecell protein (SCP) annually in the United States is presented in Table 1. While most other countries are not as profligate as the United States, the worldwide volume of lignocellulose wastes must exceed the volume of agricultural products used directly for human consumption. It is evident that the conversion of photosynthetically produced organic compounds into human and animal food is the limiting process in human food production. The worldwide annual production of organic material by photosynthesis has been estimated to be between 25 and 50 tons per caput. Any practical method capable of converting a small fraction of this yield into human food should find wide application and go a long way in reducing chronic food shortages.
The growth of microorganisms, more rapid than that of the higher plants, makes them very attractive as high-protein crops; while only one or two grain crops can be grown per year, a crop of yeasts or moulds may be harvested weekly and bacteria may be harvested daily. The use of microorganism as a source of protein for human and animal food is not a new development. Traditional foods and feeds such as cheese, sauerkraut, miso and silage have a high content of microorganisms to which their nutritional properties are due in part. The high-quality proteins synthesized during the growth of these microorganisms compare favourably with those derived from the better grains. There are many references to the amino acid composition and the protein quality of SCP (Shacklady, 1975). While there is little data on animal feeding trials using SCP produced from lignocellulose wastes, there is a large and growing body of information about SCP from petroleum. This information should be applicable to SCP from agricultural wastes with proper allowance for the undesirable contaminants in the sources. In petroleum there has been concern for accumulation of carcinogenic hydrocarbons. In agricultural wastes, there is concern for accumulation of pesticides and herbicides. The International Union of Pure and Applied Science has published a report (IUPAC, 1974) on guidelines for testing of SCP as a major supplement in animal diets and should be consulted for further details on feeding trials. Many more feeding trials will be needed before SCP from lignocellulose wastes is accepted for routine feeding.
Rapidly growing organisms such as bacteria and yeasts contain a higher uric acid content than slower growing plants and animals. While the uric acid limits the daily intake of SCP for humans and monogastric animals such as pigs and chickens, ruminants such as cattle, sheep and goats can tolerate higher levels. There are several methods available for removal of uric acids (Sinskey and Tannenbaum, 1975).
Some of the proposed methods for conversion of agricultural wastes into animal feed are presented in Table 2. These methods will be briefly evaluated in the following. First, a distinction should be made between the production of SCP from the lignocellulose parts of the plant and the production of SCP from the soluble carbohydrates of many agricultural wastes. Several processes for utilization of waste solubles have been proposed (Sloneker et al., 1973; Anthony, 1971), and some are now being tested. The Ceres Ecology Corporation of Chino, California in the United States will process waste from over 100 000 dairy cattle for feed recycling and for control of salt in ground water. Other plants in Toulouse, France, Zacantecos, Mexico, and Sterling, Colorado in the United States use processes that depend upon an anaerobic fermentation in a silo or covered ditch. The manure undergoes a lactic acid fermentation due to the action of anaerobic bacteria (chiefly streptococci and lactobacilli), and a typical silage odour results in place of the odour of manure. These short-time anaerobic fermentation processes do not utilize the fibre, and are therefore a partial solution to the waste problem. The fibrous residue may be used as a soil conditioner before or after composting. It appears that under special conditions, the fermented solubles can be substituted for 8 to 10 percent of the high-protein component of the feed with a saving of 10 to 20 percent in total feed costs. There appears to be no major technical obstacle to commercial development of this type of waste recycling, although the pesticide accumulation question has not been completely settled.
Table 1 Solid wastes in the United States
|Waste type||106 tons per year|
|Agricultural and food wastes||400|
|Logging and other wood wastes||60|
|Muncipal sewage solids||15|
|Miscellaneous organic wastes||70|
SOURCE: Humphrey, 1975.
The more important and difficult problem of waste fibre utilization requires microorganisms that can utilize lignocellulose. Unfortunately at present there are no known microorganisms that will utilize natural lignocellulose at rates of commercial interest. It is necessary, therefore, to pretreat the wastes. The purpose of the pretreatment is twofold: to expose the cellulose by removal or modification of the lignin, and to reduce the crystalline fraction of the cellulose. Several methods of pretreatment have been proposed, including wet and dry ball milling, wet and dry grinding, hot alkali and anhydrous ammonia. Both ball milling to micron-size particles and hot alkali (0.5 N H Cl 120°C for 15 minutes) are effective pretreatment methods for releasing most of the cellulose. Up to 90 percent of the cellulose is made available for microbial digestion. Alkali treatment based on recent modifications of the Beckman process has been proposed as a direct method for converting straw, maize cobs, etc., into a more digestible feed (Rexen, 1975). This method does not increase the protein content of the feed because there is no microbial growth. It does, however, increase the availability of cellulose to the rumen bacteria. It can be considered a pretreatment method for SCP production by the rumen microorganisms. The method consumes relatively large amounts of alkali and its economic value is still under investigation.
Table 2 Methods for conversion of cellulosic agricultural wastes into animal feed
|Treatment||Microorganism||Substrate||Protein produced||Fibre utilized||Reference|
|Ensiling||Mixed anaerobes||Wastelage||Slight||No||Anthony 1971|
|Dilute alkali||None||Straw||No||Yes||Rexen 1975|
|Aerobic mesophiles 25°C||Cellulomonas||Bagasse||Yes||Yes||Dunlap 1975|
|Mould growth 25°C||Trichoderma viride||Waste paper||Yes||Yes||Mandels 1974|
|Aerobic thermophiles 55°C||Thermoactinomyces||Fermented livestock wastes||Yes||Yes||Bellamy 1975|
Rates of soluble sugar utilization of 10 to 30 grams per litre per hour have been reported for SCP production by yeast. Rates of 5 to 15 grams per litre per hour have been claimed for utilization of selected hydrocarbons. In order for SCP to be an economic process under present market conditions, the rate of utilization of cellulose must be at least 1 to 5 grams per litre per hour. As no pilot plants have been operated, it is not possible to report commercial rates, but laboratory scale fermentors have been run at 1 gram per litre per hour on pretreated wastes. The studies reported by the Louisiana State University group (Dunlap, 1975) were on a gram negative aerobic bacteria, Cellulomonas sp., which grows rapidly on cellulose at 25 to 30°C, but cannot utilize lignin or lignocellulose. Therefore extensive pretreatment with hot alkali is required. The small (one micron) organism must be harvested by centrifugation. Amino acid analysis and small animal feeding trials have shown that a high-quality SCP is produced.
The group at Natick, Massachusetts (Mandels et al., 1974), has reported on the use of enzymes produced by the mould Trichoderma viride for production of soluble sugars from waste paper cellulose. These sugars can then be fermented by yeasts or bacteria for SCP production. As now envisioned, it is a fourstep process:
Pretreatment of waste by ball milling or hot alkali.
Enzyme production by growth of T. viride on pretreated cellulose.
Depolymerization of cellulose by T. viride enzymes.
SCP production by yeast or bacteria. Although mutated strains of T. viride with several-fold increase in enzyme production have been obtained, this process appears too complex to find wide application for agricultural use.
As indicated in Table 2, we have worked with thermophilic aerobic microorganisms (growth temperature 55°C). There are several advantages to the use of aerobic thermophiles:
The product is pasteurized after a growth period of 10 to 24 hours, i.e. after all bacteria, virus and parasites pathogenic to humans and animals have been killed.
The growth rates are faster.
Fermentation temperature using ground water is more easily controlled.
Aerobic microorganisms may utilize lignin and lignocellulose while anaerobes cannot.
Thermophilic microorganisms were collected from a wide variety of sources including local compost heaps and manure piles, lumber mill waste piles, salt water and fresh water mangrove swamps, the anomalous hot earth areas of Yellowstone National Park, and a tropical forest. After screening many organisms, the thermophilic actinomyces were selected as the most promising of the microorganisms growing on cellulose and lignocellulose. These organisms exhibit optimum growth at a temperature of 55°C and a pH of 7.5 to 7.8. They produce extracellular cellulases with a pH optimum of 6:0 and a temperature optimum of 65 to 70°C. A demonstration plant was built at Casa Grande, Arizona in the United States to produce enough SCP for animal feeding trials and to find answers to many questions that could not be answered by laboratory experiments. The engineering experiences of this plant were reported by Nolan and Shull (1973). Unfortunately, for a variety of technical reasons including culture instability, heavy metal contamination and product variability, SCP production was discontinued and the animal feeding trials were not completed. At that time, the U.S. Food and Drug Administration had not issued clear and complete guidelines for necessary feeding trials of recycled animal feed, and therefore the size, duration and cost of the trials could not be determined. The demonstration plant has not been reopened because of economic reasons.
In the long term, any process must generate enough profit above operating costs to attract investment capital. Production of SCP from lignocellulose wastes is no exception to this general rule. SCP must be produced at a price that is competitive with the major conventional protein sources of plant and animal origin. If there is a guaranteed payment for waste removal, the production costs will be reduced accordingly. But no credit can be taken for waste removal in the absence of a firm and long-term commitment. At mid-1975, SCP was unable to compete with soybean meal (US$112 to 140 per ton). The SCP from petroleum development is in a similar economic bind. Because of the increase in the cost of petroleum and the fluctuating prices of soybean meal and fish meal protein, the start-up of large-scale plants in Europe has been postponed for several years, despite heavy investment in large modern production facilities (Skinner, 1975).
Some factors that will contribute to reducing costs of SCP production from agricultural wastes are the following:
Payment for waste disposed (negative cost of raw material).
More effective and less expensive pretreatment.
Superior microorganisms with a more rapid growth rate.
Superior microorganisms that can rapidly utilize natural lignocellulose without pretreatment.
Methods of SCP production that do not require stirred tank fermentors.
Methods of SCP production from high solids-low water fermentations.
Methods that will decrease the capital investment.
It is obvious that the value placed on waste elimination is variable and will depend upon the laws and customs in each region. Local laws providing credit for waste disposal are an effective method for stimulating SCP production.
There is very little work in progress on a low-technology, labour-intensive, low-capital SCP system. It is an area with great potential for the developing countries, but of lesser interest to the industrialized countries or to the private sector of industrial countries because it does not appear to be a large potential market for sophisticated equipment and processes. It is in this area of controlled fermentation under non-sterile conditions with a minimum of equipment that methods may be developed for using agricultural wastes at the small farm level (Imrie, 1975). Instead of being grown under sterile conditions in expensive stirred fermentors, the moulds are grown in a plastic container under conditions where the desired organisms predominate.
An industry for the production of SCP from lignocellulose wastes based upon capital-intensive, high-technology, low-labour processes will not develop for several years because of technical problems and economic considerations. The technical problems can be overcome and will be solved when the economic climate improves.
The potential for a labour-intensive, low-technology, low-capital industry in the less developed countries will not be realized unless research in this area is greatly expanded.
Private sector capital from the developed countries cannot be expected to invest in the latter type of development because it will not provide a large market for sophisticated equipment and processes.
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Bellamy, W.D. 1974. Single-cell proteins from cellulosic wastes. Biotech. and Bioeng., 16: 869–880.
Dunlap, C.E. 1975. Production of singlecell protein from insoluble agricultural wastes by mesophiles in SCP II, p. 244– 268. Cambridge, Massachusetts, MIT Press.
Humphrey, A.E. 1975. Product outlook and technical feasibility of SCP, p. 1–3. Cambridge, Massachusetts, MIT Press.
Imrie, F. May 22, 1975. Single-cell protein from agricultural wastes. New Scientist, p. 458–460.
International Union of Pure and Applied Science. August 1974. Proposed guidelines for testing of SCP destined as a major protein source for animal feed, p. 1–25. Technical Report No. 12.
Mandels, M., Hantz, L. & Nystrom, J. 1974. Enzymic hydrolysis of waste cellulose. Biotech. and Bioeng., 16: 1471– 1484.
Nolan, E. & Shull, J. June 6, 1973. Engineering experiences during the demonstration phase of a nutrient reclamation plant. American Institute of Chemical Engineers. Symposium Series.
Rexen, F.P. January 5, 1975. The effect of a new alkali technique on the nutritive value of straw. Proc. Ninth Nutrition Conf. for Feed Manufacturers, p. 3–24. Nottingham, University of Nottingham.
Shacklady, C.A. 1975. Value of SCP for animals in SCP II, p. 489–504. Cambridge, Massachusetts, MIT Press.
Skinner, K.J. May 5, 1975. Single-cell protein moves toward market. Chem. and Eng. News, p. 24–26.
Sloneker, J.H., Jones, R.W., Griffin, Eskins K., Bucher, B.L. & Inglett, G.E. 1973. Processing animal waste for feed and industrial products. Symposium: Processing Agricultural and Municipal Wastes, p. 13–28. Westport, Connecticut, Avi Publishing Co.
Sinskey, A.J. & Tannenbaum, S.R. 1975. Removal of nucleic acids from SCP in SCP II, p. 158–178. Cambridge, Massachusetts, MIT Press.