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Mary Arlene P. Saquido, Verma A. Cayabyab, and
Flordeliz R. Uyenco
Natural Science Research Center, University of the Philippines, Quezon City, Philippines
One of our major scientific and technological advances has been in the area of harnessing the activities of micro-organisms. For years it has been known that numerous kinds of yeasts, fungi, and bacteria have a direct relation, either favourable or unfavourable, to operations such as brewing, wine-making, and cheese-making. These have emerged from small-scale or family arts to the present industrial scale. Only in the past few decades, however, have the advantages of exploiting microbial activity been fully appreciated, owing to advances in biochemistry,
One of the most recent applications of micro-organisms is in the search for additional sources of protein. The increasing world population results in a rising demand for protein for both human and animal consumption. The demand for protein is certain to become serious with overexploitation of the sea and the use of most of the available arable land as the rapid growth in population continues. We are therefore faced with the problem of finding new sources of protein that will not require agricultural land or costly and tedious means of production. The escalating prices of traditional protein ingredients for animal feeds - animal and plant proteins such as fish, meat, and soybean meals - have intensified the problem.
Micro-organisms as a source of protein are one solution to this problem. The single cell of a microorganism is a perfect protein factory. Under controlled conditions in a fermentor, the culture of single cells can effect a highly efficient transformation of simple substances into protein. Land use is negligible and the gain in time is great, because of the fast rate of reproduction by micro-organisms, Emil Mrak of Davis, California, has pointed out that a 1,000 lb steer can yield 1 lb of new protein per day, and 1,000 lb of soybeans can yield 80 lb of new protein per day, but the corresponding figure for 1,000 lb of yeast is 50 tons per day (1).
TABLE 1. Macromolecular Composition and General Properties of Micro-organisms
|Doubling time (hours) Crude protein||1-3||2-6||5-12||6-24|
|(% dry cell weight)||40-80||40-60||30-45||40-50|
|Nucleic acids (%)||8-20||5-15||6-13||45-51|
|Carbohydrates and fats (%)||10-30||10-40||10-45||34.6-45|
|Ash content (%)||4-10||4-10||4-10||5-8|
|Temperature range (°C)||22-55||25-40||25-50||25-32|
a. Percentage G.
Interest in microbial protein production is increased because micro-organisms can utilize waste materials that cause pollution problems and are sanitary hazards. Agricultural waste is a renewable resource of great variety and potential. In recent years the use of wastes like bagasse, rice straw, rice hulls, manure, and starchy residues as substrates for growing microbes has been studied. If the use of these materials is industrially developed, a vast bulk of them could be rendered economically useful, and this would help control pollution and eliminate some waste-disposal problems as well.
Protein of microbial origin, called single-cell protein (SCP), or microbial protein, can be derived from a variety of micro-organisms, both unicellular and multicellular - namely, bacteria, yeasts, fungi, or microscopic algae. The macromolecular composition and general properties of these organisms are shown in table 1. These potentially important food substances are not pure proteins but are, rather, dehydrated cells consisting of mixtures of proteins, lipids, carbohydrates, nucleic acids and a variety of other non-protein nitrogenous compounds, vitamins, and inorganic compounds. Microbial protein is a nontraditional protein, it is not a palatable, desirable food and must be incorporated directly or indirectly into other foods.
The use of waste products, specifically agricultural wastes such as rice straw, manure, and bagasse, has been the object of current studies on microbial protein production. Dunlap (2) stated that an agricultural waste, to be a useful substrate for production of microbial protein, must meet the following criteria: it should be non-toxic, abundant, totally regenerable, non-exotic, and cheap, and able to support rapid growth and multiplication of the organisms resulting in a biomass of high quality.
Rejects of Cavendish bananas (Muse cavendishii Lamb.), also locally known as tumok in the Philippines, constitute one of the major agricultural wastes in areas where this variety of banana is farmed for export. The Philippines is one of the top ten banana producing countries in the world (3). In 1975 alone, a total of 406,927 kg of the Cavendish variety, valued at US$35.25 million, were exported (4). This variety is grown and intensively cultivated in southern Mindanao. It has been reported that the yearly banana export constitutes only about 80 to 90 per cent of the total produce from about 22,000 hectares of land in Davao planted with this variety intended for export (5). The remaining 10 to 20 per cent is rejected because the size does not meet export standards. Aside from their wide use as dessert fruit, the rejected bananas may be used in cakes, muffins, soup, fried chips, or flower bud vinegar and in many other ways. The rejects are also used as animal feed (3), but most of them are simply thrown away, posing a sanitary hazard. Therefore, studies on the use of banana rejects for production of microbial proteins for feeds were begun.
Different methods of converting starch wastes into useful products are shown in figure 1. There are two major processes: chemical conversion of the substrate by treatment with dilute acid in the presence of heat (A), and enzymatic hydrolysis, either through the direct action of selected groups of fungi and yeasts or through the use of highly active commercial enzyme preparations (B). In either case, the starch is hydrolysed into lower saccharides, predominantly glucose, which in turn are used either as raw material for chemical industries or as substrates for micro-organisms in the production of microbial proteins for human or animal consumption. Both of these processes are two-step reactions; i.e., the hydrolysis of the substrate is an entirely different step from the propagation of microbial cells or the chemical process. The present study is directed towards the improvement of the fermentation process involving the overall transformation of the starch to biomass through a one-step reaction (C) instead of the traditional two-step reaction.
FIG. 1. Utilization of Starch Wastes
Yeast is currently the most commonly used organism in the production of biomass, probably because it is already accepted both in the human food and animal feed industries (6). Yeast-based processes are the farthest advanced towards commercial production, followed by bacterial processes (7).
Yeasts have many convenient characteristics, such as the ability to use a wide variety of substrates such as hexoses, pentoses, and hydrocarbons (8; 9); susceptibility to induced and genetic variation (10), ability to flocculate (11); and high nutritional value (12; 13). However, attention has often been drawn to the fact that yeasts appear to be deficient in essential sulphur amino acids (14; 15). Nevertheless, this deficiency can be corrected by the addition of synthetic methionine, as shown in several studies done by Yañez et al. (12) and Harris et al. (16).
Until recently, the yeast most commonly employed has been Candida utilis grown on molasses (17) and sulphite liquor (18), but other species have been used successfully with various substrates such as methanol (19), whey (20), and hydrocarbons (21; 22). A study of Candida utilis grown on rye grass straw hydrolysate by continuous fermentation was reported by Han and Anderson in 1974 (23). In that study, the workers obtained cell densities of about 4 g/litre of medium. In North America some 25 per cent of sulphite liquor solids are made into yeast, representing a production of about 50,000 tons per year. The present use is mainly for animal feed supplements, the normal recommendation for mixed poultry feed being about 50 lb dried yeast per ton of feed. A factory in Taiwan produces food yeast from cane sugar molasses with a daily capacity of 40 tons of dried yeast (24).
A limited number of bacterial species have been grown specifically for food purposes, Recently, these organisms have been used extensively for SCP production on hydrocarbons such as petroleum, gas oils, and alkanes. Cellulomonas and Alcaligenes faecalis have been used in symbiotic fermentation studies using cellulose substrates (25-27). Although bacteria have a slight advantage over the other microorganisms as a food source because of their higher growth rates and relatively higher protein content and sulphur-containing amino acids (28), they have been objected to because of their size, which makes harvesting difficult without the use of flocculants or thickeners (29).
Algae processes are still short of full-scale development because they are limited by the requirements for light over a major portion of the year and for a continuous supply of CO2 or other carbon source 17). In Taiwan, however, plants for the production of Chlorella feeds using methane generated from manure are now in operation.
The production of fungal protein is far behind; it has not been scaled up to commercial level yet (6). Fungal proteins have not been considered for industrial SCP processes until recently, when studies on their potential as SCP proved that they exhibit growth rates comparable to those of yeasts and that crude protein contents in excess of 50 per cent can be achieved. Fungi have the ability to provide form and texture (30), and hence can be harvested with ease; also, the cost of production may be reduced. Like algae, fungi generally have low nucleic acid content, and accordingly, the dangers of kidney stones and gout are not great even without processing the biomass to lower nucleic acid content (7). Another advantage is that microfungi can prosper on a variety of carbohydrates, although growth rates vary considerably with different substrates (31).
Most species of fungi produce a range of carbohydrate-hydrolysing enzymes (31), but the amounts of the enzymes vary enormously between different organisms and more particularly between strains. Aspergillus niger reportedly produces large amounts of alpha-amylase and alpha-(1,6)-glucosidase, but less cellulase. Only a few fungal species exhibit problems of sucrose assimilation (or inversion}, and growth rates on these carbohydrates are usually similar to rates on glucose. Since carbohydrates are the main carbon sources of organisms, it would be reasonable to predict that fungal growth and yeast growth on bananas, which are largely composed of carbohydrates, would be substantial.
A comparison of the protein composition of micro-organisms and traditional sources of protein shows that microbial proteins are comparable to animal and plant proteins. Although it has been customary to regard animal proteins as nutritionally superior to vegetable proteins (including microbial protein), investigations have shown that many plant proteins in appropriate mixtures with one another, or with small quantities of animal protein, have a high biological value.
Many feeding trials have been carried out in animals with yeast and other microorganisms to assess their value as a source of protein and vitamins. The majority of animal feeds consist essentially of three components - in descending order of magnitude, cereals, high-protein ingredients, and mineral/vitamin/drug supplements (32). SCP is evaluated in terms of its ability to replace one or more of the high-protein sources in existing feeds (soybean, fish, meat, and blood meals and poultry offals) wholly or in part. Shacklady (32) reports that the normal protein requirement of laying and breeding hens is met when yeast at a dietary level of 12 to 14 per cent forms the sole high-protein source. Yeast also performed well in rations for turkeys at levels of up to 10 per cent of the total diet. Pigs were fed for five successive generations with yeast SCP replacing 10 per cent of the protein requirement with good results, and yeast SCP has been used successfully as the sole high-protein source in the rations of young beef animals.
Roberts (33) showed that Escherichia coli grown in aerated culture on a simple medium and heat dried was a very good protein supplement for rats and chicks. Micro" fungal proteins, when used at levels of 10.5, 21.0, and 42.0 per cent for feeding rats and chicks, showed results comparable to those obtained in animals fed casein (34).
The economic feasibility of any SCP process depends on its being able to produce a protein feed supplement of comparable quality and at a competitive price with alternative protein feed supplements such as soybean meal or fish meal. In the economic analysis of small-scale SCP processes from wastes, several factors have to be considered. Substrate costs can probably be minimal and in some cases may be considered negative, such as definite costs involved in the use of acid hydrolysis in the conversion of cellulose to glucose. Capital costs of the process can be reduced if low-technology procedures are used. In the Philippines, where extensive supplies of cellulosic and carbohydrate wastes are available, operating costs can also be reduced because of lower labour costs,
On the other hand, considerable costs may be involved in the collection of waste materials from food factories or agricultural feedlots located far from the SCP plant. Low-technology fermentation probably will produce SCP that is of a more variable composition than that produced by high-technology (controlled) fermentations using well-defined substrates, as practiced in developed countries. Extensive toxicological and acceptability tests will have to be performed before the product is approved for sale, and it is likely to command a lower price in the market.
A number of processing industries in South-East Asia release effluents that are rich in fermentable substrates, and this has raised interest in SCP production. Microbial upgrading of solid wastes is becoming increasingly attractive in view of stricter environmental regulations and the unacceptability of alternative treatment methods. Although it is still too early to come up with any detailed feasibility studies, it is evident that local markets for protein feed supplements do exist to replace the currently imported soybean and fish meals. The exploitation of our agricultural wastes for microbial protein production will greatly minimize, if not eliminate, the immense cost of waste pollution control.
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