PART II (Contd.)

CHAPTER 13
UTILIZATION OF AGRO-INDUSTRIAL BY-PRODUCTS IN INTEGRATED SYSTEMS OF PLANT AND ANIMAL PRODUCTION

by

T. R. Preston

Centro Dominicano de Investigación Pecuaria con Cana de Azúcar
CEAGANA, Apartado 1258, Santo Domingo, R D
Centro de Investigación y Experimentación Ganadera
Calle Alvaro Obregón 227, Chetumal, QR
México

Resumen

El empleo eficiente y económico de los nuevos recursos en materia de piensos exige la aplicación de sistemas de alimentación y ordenación del ganado distintos de los que se emplean actualmente en países de zonas templadas. Entre los objetivos de tales sistemas son de señalar la producción de carne y leche, el ahorro o la adquisición de divisas, la creación de puestos de trabajo, el control de la contaminación, etc.

La composición de la mayoría de los subproductos agroindustriales impone restricciones a su utilización. En su mayor parte se caracterizan por un bajo contenido de nitrógeno, en general, y de nitrógeno libre en particular y por la presencia de carbohidratos, que son azúcares solubles o carbohidratos muy insolubles, que pueden fermentar solamente en el rumen. Por ello, los piensos son convenientes para los rumiantes, pero no para los cerdos y las aves, en cuya alimentación hay que utilizar casi siempre, como piensos básicos, recursos que se destinan también a la población humana, especialmente granos cereales.

Considerando que es durante la producción de leche y en las primeras fases de crecimiento del ciclo de producción cuando mayor es la necesidad de aminoácidos y glucosa y, mayor también por tanto, la necesidad de productos que son también de consumo humano, el objeto principal de la reestructuración de los sistemas de producción de ganado vacuno para la utilización de subproductos agroindustriales y piensos tropicales, debería ser la evitación de dichas fases. Por consiguìente, en este contexto, la producción especializada de leche o de carne es antieconómica y debe sustiuirse por una producción que tenga la doble finalidad de satisfacer la demanda nacional, tanto de leche como de carne.

También es posible una producción multifuncional en que se recojan y elaboren los excrementos para producir fertilizantes de N, P y K, así como biogás. Para demostrar las ventajas económicas de estos sistemas, se han calculado relaciones insumos/producción para una pequeña unidad de producción intensiva planeada de forma que pueda ser explotada por una sola familia. Los datos sobre producción y costos se basan en los resultados obtenidos en la República Dominicana. A pesar de los índices moderados de producción, la unidad puede ser muy rentable en general, a condición de que se aprovechen completamente los subproductos del sistema de producción de ganado vacuno.

Introduction

This book is concerned with the utilization of new feed resources in the sense of materials not used normally in livestock feeding programmes in most of the agriculturally advanced countries. In this category fall the agroindustrial by-products and new feed crops, mainly of tropical origin.

The efficient and economical use of these new feed resources requires an approach in the organization of feeding and management systems different from that in traditional feeding programmes in current use in temperate-zone countries. On the one hand, account must be taken of the general constraints that apply to the conditions of developing countries; more specifically, there are other limitations related to the nature of the feed resources to be used.

Among the general objectives to be sought in setting up animal production systems in developing countries are - a) production of meat and milk in order to cover the minimum protein requirement of the population as a whole, and to satisfy the demand for quality foods by that sector of the population with adequate purchasing power (including tourists), and eventually by the export market; b) savings or earnings on foreign exchange; c) creation of jobs, particularly in rural areas; d) improving the quality of life by reducing pollution and providing a balanced strategy of development that takes ecological factors into account; e) contributing to regional development; and f) developing systems that are biologically, economically and ecologically appropriate for the country concerned.

More specifically, consideration must be given to the fact that the composition of most agroindustrial by-products and the new tropical feeds imposes constraints as to the most appropriate species of livestock to utilize such feeds and to the productivity that can be obtained within a species.

Accordingly, the first step in any programme designed to maximize use of new feed resources is to determine the most appropriate animal species and the most appropriate management systems.

Of particular relevance in selecting the species is the fact that in most developing countries there may be quantitative, and almost certainly are qualitative, shortages in the human diet. Competition between animals and humans for the same basic nutrients then poses a problem in resource utilization and must be taken into account.

For planning purposes, domesticated animals can be divided into the two broad categories of ruminants (cattle, sheep and goats) and the non-ruminants (pigs and poultry). The latter have digestive systems (and therefore nutrient requirements) similar to those of humans. The former possess an additional compartment in the stomach - the rumen - through which all solid foods pass, and where a pre-digestion takes place by anaerobic fermentation. This feature gives the ruminant the capacity both to degrade (digest) and to use (for purposes of synthesis) substances which are not usable by humans, so that they need not compete with humans for available feed supplies. If they do compete, it is probably a result of bad planning and/or the application of inappropriate technologies.

In nutritional terms, the special virtue of the synthesizing capacity of rumen microorganisms is their ability to transform inorganic ammonia nitrogen into microbial protein of excellent biological value, protein which subsequently becomes available to the host animal for formation of milk, meat and wool. The degrading properties of rumen microorganisms are used to advantage on feeds containing structural carbohydrates, chiefly cellulose and related compounds, for which the nonruminant digestive system possesses no enzymes.

These two characteristics enable ruminants and humans to occupy different ecological niches, so that instead of competition there can be true symbiosis between the two species. In other words, by transforming inorganic nitrogen and carbohydrate, made available to them by man, into animal protein, ruminants enable humans to live adequately in conditions where, otherwise, they would suffer protein deficiency.

In marked contrast with ruminants, pigs and poultry must nearly always compete with the human population for basic feed resources, especially cereal grains.

Unfortunately, the very processes that give ruminants a competitive advantage over non-ruminants also exert certain constraints on their potential productivity.

Nutritional constraints imposed by the rumen function

Recent research has shown that, even when the digestible energy supply is not limiting, productivity in ruminants is governed by the availability of essential amino acids and glucose precursors at the site of metabolism. The importance of these rate-limiting nutrients is illustrated in figures 13.1 and 13.2, which show the overall requirements of ruminants for amino acids and for glucose at the various stages of their production cycle.

It appears that the pattern of these two requirements closely parallel each other, and that both are directly related to the productive rate, The significance of this relationship lies in the fact that the rumen function processes are apparently incapable of supplying the amounts of amino acids and glucose needed to support high levels of production, specifically the points in the production cycle reflecting rapid growth in young animals and high levels of milk production.

Two sets of factors are responsible for this situation. If amino acid supply is to depend only on rumen synthesis, then it will be governed by the intake of fermentable energy and the efficiency of microbial synthesis. Present evidence indicates that even when these two factors are operating at maximum attainable levels, the amount of microbial protein synthesized (indicated by the dotted line in Figure 13.1) approaches adequacy only for late growth, early and mid-pregnancy and mid- and late lactation. Both fast growth in young animals and early lactation represent critical points when considerable supplementation with preformed protein is required.

A similar situation applies to glucose. The principal precursors of glucose are propionic acid produced by fermentation in the rumen, starch arriving at the small intestine where it can be hydrolyzed directly to glucose, and amino acids surplus to requirements and deaminated. This latter source is the least efficient of the three, since only certain fragments of amino acids can serve as glucose precursors.

If the animal has to depend on the rumen function, the only available source of glucose will be propionic acid, since there will be no surplus amino acids, and in fact, there is more likely to be a deficiency. The picture with respect to propionate as the only supply of glucose is not conclusive, but recent evidence appears to indicate that even with molar proportions of propionic acid as high as 35%, the rapidly growing ruminant will still respond to glucose precursors in the form of dietary starch arriving at the duodenum (Silvestre et al., 1976) or to glucose that by-passes the rumen via the oesophageal groove reflex (Fernandez et al., 1976).

It appears, therefore, that high rates of production in ruminants can only be ensured by dietary supplies of preformed protein and glucose precursors in such a form that they will arrive, in part, at the sites of metabolism as amino acids and glucose, respectively.

It is now easy to see why many of the new feed resources present important constraints limiting rate of ruminant productivity. Most of these feeds are characterized, in terms of their composition, by low content of nitrogen in general and of true protein in particular, and by carbohydrate component which can only be fermented in the rumen, since most agro-industrial by-products are composed of either soluble sugars or highly insoluble structural carbohydrate, or combinations of the two.

In contrast, the traditional feeds given to livestock in agriculturally advanced (mostly temperate) countries will support high levels of animal production since they are rarely, or only marginally, deficient in total nitrogen, most of which usually is true protein; moreover, a large part of the carbohydrate fraction is present as starch, which can act as a glucose precursor via both rumen fermentation and intestinal hydrolysis.

FIGURE 13.1 Pattern of ruminants' amino acid requirements in relation to the production cycle (N retained per unit of digestible organic matter consumed) (according to Ørskov, 1970). Source: Leng and Preston, 1976.

FIGURE 13.2 Pattern of sheeps' glucose requirements in relation to the production cycle (rate of glucose synthesis per unit of metabolic body weight) (according to Leng, 1975). Source: Leng and Preston, 1976.

The need for new cattle production systems

Two possibilities exist for developing animal production systems based on new feed resources. The first is to maintain existing management procedures and provide considerable supplementation with sources of preformed protein and glucose precursors for the peak points in the production cycle, but in developing countries this abandons the policy of avoiding competition with human food supplies, since both these sets of nutrients can be used in human nutrition.

A more reasonable approach is therefore to be sought in the second possibility: to develop new management systems that avoid the peaks in the production cycle and thus enable the feeding programmes to be based on available feed supplies with supplementation only by inorganic or other substances inedible by humans which can enter the nutritional cycle via fermentation processes.

The principal aim in redesigning cattle management systems for use with agro-industrial by-products and tropical feeds must be to avoid those phases in the production cycle where requirements for glucose and amino acids are highest; these critical stages are high levels of milk production and early growth in the calf. The problem is particularly serious with milk production, in which both amino acid and glucose requirements are considerably higher than in growth and fattening.

In many respects, these problems have resulted from increasing specialization in cattle production systems and relate specifically to the conditions of high levels of milk secretion, as typified by modern high-producing Holstein cows, and to the change from natural suckling to artificial rearing of calves.

It is commonly believed and widely argued that modern cattle management necessarily requires speciallzation of milk production on the one hand and beef production on the other. This point of view has a sound biological and economic justification in terms of milk production in temperate countries, for there can be no doubt as to the efficiency of the modern dairy cow in terms of converting feed energy into milk. However, there is no comparable argument to support the specialization of beef production, since the basic factor determining efficiency in this programme is the reproductive rate of the female, and there are no indications that this can be changed within the forseeable future. Thus beef production in the reproductive phase remains biologically and economically inefficient, irrespective of the nature of the available feed supplies. Even for the fattening process, specialization is not supported by the evidence, since the most efficient live-weight gains and feed conversions are associated not with fattening of conventional beef cattle (conversion rates of 7 to 10) but with the surplus male calves of the dairy industry (feed conversions of 5 to 6) (Preston and Willis, 1974).

The most logical cattle management system for utilizing agro-industrial by-products and tropical feeds efficiently will be an integrated multi-purpose programme that not only combines milk and beef production, but also maximizes utilization of wastes and effluents. The justification for this approach does not depend only on the nutritional arguments advanced above. Such a strategy also provides a basis for balanced development, in terms of national consumer demand for the principal products, beef and milk, as well as of the need to conserve energy and to protect the environment.

Demand for beef and milk

The obvious starting point is the national requirement for beef and milk. Here there must be a differentiation between what might be consumed if purchasing power was not a limitation, and what is actually consumed by the population as a whole. Fortunately, for the purpose of this calculation, absolute figures are not needed, but rather the ratio between the two products. Also, as a general reference point, these estimates can be based on data from developed countries, on the assumption that this is what most developing countries aspire to when purchasing power is adequate. These theoretical demand figures are 42 kg carcass beef per year and 0.55 kg fresh milk equivalent daily (USDA, 1976).

The annual milk/beef demand ratio represented by these consumption rates is 4.78:1. As an example it is possible to take the use of specialist dairy cows

(Holstein) giving 4500 kg per lactation and producing a calf which eventually ends up as beef, either as a culled female or a fattened bull/ steer. Assuming that the weight of carcass produced in either case is 250 kg, then the milk/beef production ratio is 18:1. Since the demand ratio is only 4.8:1, this means that if milk production is based on a specialist system, then either beef must be imported to make up the deficit, or there should be a parallel specialist beef production industry.

General policy in the developed countries has been to opt for the latter alternative. In countries like the United States, Brazil, Argentina and Australia, such a policy may be acceptable, since there are large areas of pasture land on which inexpensive ranching is feasible without recourse to supplementary feeding. In almost all other countries, single-purpose beef production is a luxury operation which cannot be afforded.

The reasoning behind this argument is this; If the milk/beef demand ratio is approximately 5:1, and the specialized dairy cow gives a production ratio of 18:1, then three additional specialized beef cows are needed for every dairy cow. But specialized beef cows, because of their low reproduction rate, are inefficient both biologically and economically. This is particularly so when there are no extensive grazing areas. Therefore, in an industry based on specialist milk and specialist beef production, the greater the degree of self-sufficiency in both products, the less efficient becomes the overall industry (since there are three inefficient beef cows for every efficient dairy cow) and the greater, in the end, is the burden on the taxpayer. As witness to this, the present government subsidy to the beef industry in Britain is almost US$400 million. To achieve the desired milk/beef ratio without having to support an inefficient beef industry, it is necessary either to import beef from countries that produce it more cheaply or to restructure the cattle industry. For developing countries, particularly those situated in the tropics, with rich agricultural potential, the latter course is the more attractive.

Dual-purpose cattle

Such a restructuring involves the substitution of both specialist beef and dairy cattle by dual-purpose animals, which produce both milk and beef, in accordance with consumer demand. For a milk:beef ratio of 5:1, the specifications become a net lactation yield of 1250 kg, plus additional milk to suckle the calf to a weaning weight of 160 to 200 kg, at which point it can enter the fattening programme at a stage when supplement needs are minimum.

Such a yield level, in terms of milk alone, may be viewed with derision by the majority of cattle breeders as representing an unacceptable reversal of the technological clock. Nevertheless, there are a number of very sound reasons why such a policy is particularly appropriate for developing countries in general and tropical regions in particular:

1. A net lactation yield of 1250 kg is equivalent to an average of 5 kg/day during a 250-day lactation. Adding the milk consumed by the calf (2.5 kg/day) by restricted suckling gives a total lactation yield of approximately 1900 kg, or an average of 7.5 kg/day. Such a yield level is compatible with the nutritional potential of the agroindustrial by-products and tropical feeds.

2. The milk obtained by the calf by suckling constitutes an excellent source of both amino acids and glucose precursors. High growth rates can therefore be achieved on the same basal ration (carbohydrates and inorganic nitrogen) fed to the cows.

3. Combined milking and restricted suckling have been shown to reduce mastitis in the cows and diarrhoea and mortality in the calves (Preston and Ugarte, 1973).

4. Mean yields of 5 kg/day can be achieved by once-daily milking; milking could even be avoided on Sundays and holidays, leaving the calves to consume all the milk on these occasions.

5. A dual-purpose animal of intermediate milk-producing potential is easily produced by crossbreeding almost any type of native (therefore adapted) cow with recognized dairy or dual-purpose bulls. Such a crossbreeding programme - through the manifestation of heterosis - leads to better adaptability, improved fertility, reduced mortality and more efficient growth and feed conversion.

6. In dual-purpose cattle there is no need to pursue a genetic selection programme for milk and therefore to use expensive proven sires or semen; improved beef traits can be incorporated easily by selecting bulls produced in the herd, on the basis of performance to weaning and slaughter.

Multi-purpose production

In addition to producing milk and beef, cattle can also help to alleviate the effects of the oil crisis. When a tropical feed such as sugarcane is consumed by cattle, some 40% in terms of dry weight is excreted as feces and urine. This effluent contains undigested carbohydrate and also N,P and K, the three mineral elements which make up standard fertilizers, plus valuable trace elements. If all of this effluent is collected and passed through a simple anaerobic fermenter, it is possible to produce biogas rich in methane which can be used as a source of fuel, light and power. This process utilizes only part of the carbon, hydrogen and oxygen present in the effluent and leaves as residues the mineral elements, which can be recovered after the fermentation process and used for fertilizer.

To demonstrate the economic advantages of a multi-purpose approach to cattle management systems, input/ output relationships have been calculated for a small intensive-production unit, designed to be managed by one family. Sugarcane and cassava forage are used as energy and protein sources respectively, with urea and minerals as the only importations. Production data and costs are based on results obtained in the Dominican Republic. With the yields assumed in Table 13.1 (higher rates of production have in fact been obtained in practice), the land requirement for an integrated unit of 16 cows and followers is approximately 3 ha, two of which for sugarcane and the third for cassava forage.

An intensive building is projected, with partial slatted floors and with facilities for the complete collection of effluent and its fermentation in an anaerobic digestor using a 20-day detention cycle. The partially digested effluent (containing all the fertilizer elements) is applied to the sugarcane/cassava, obvlating completely the purchase of artificial fertilizer.

The cattle are crossbred Holstein or Brown Swiss or Zebu. At any one time, 11 of these cows are in production while 5 are dry. It is expected that 15 calves will be produced annually.

The production coefficients are an average saleable yield of 5 kg milk per day for 250 days, the calves being reared by restricted suckling to a weaning weight of 160 kg at 250 days. It is expected that the males will reach a slaughter weight of 380 kg at 20 months of age.

Production of milk, beef, biogas and fertilizer is set out in Table 13.2, together with estimated costs of operation and cash flow. No value is credited to the gas or the fertilizer. The former is intended to be used to drive the forage chopper and the pump for distributing the digestor effluent, as well as to provide fuel and light for the family. Similarly, all the organic fertilizer is returned to the crops. It is probable that production of both biogas and organic fertilizer will exceed the requirements of the unit.

The results of this analysis show that despite the moderate rates of production, in terms of milk yield and live weight gain per animal per day, the unit can be highly profitable overall, providing complete advantage is taken of the by-products from the cattle production system to provide inputs to help grow the basic crops and to operate the enterprise. The contribution of the biogas to help reduce living costs is an additional bonus.

¹ Mean value from birth to weaning (160 kg)
² Mean value from weaning to 380 kg
³ Refers to a unit defined as 1 cow plus followers
⁴ Requires 2 ha of sugarcane and 1 ha of cassava for forage at yields of 120 and 60 ton/ha respectively

¹ Based on production of 114 litres biogas/kg effluent dry matter (Fernandez et al, 1976)
² Labour provided by the family
³ See Table 13.³

References

Fernández Angela, MacLeod, N.A. and Preston, T.R. 1976. Voluntary intake and rumen fermentation in calves fed sugarcane, urea and protein and supplemented with a solution of glucose given by suckling from a teat. Trop. Anim. Prod.

Hernandez, C., MacLeod, N.A. and Preston, T.R. 1976. Production of biogas from different substrates: cattle effluent fresh and stored, bagasse and sugarcane. Second Annual Meeting, CDIPCA.

Leng, R.A., 1975. Factors influencing net protein production by the rumen microbiota. Rev. Rural Science II from plant to animal protein. (Ed. T.M. Sutherland and R.A. Leng) University of New England, Australia.

Leng, R.A. and Preston, T.R. 1976. Sugarcane for cattle production; present constraints perspectives and research priorities, Trop. Anim. Prod. 1:1.

Ørskov, E.R. 1970. Proc. 4th Nutrition Conf. Feed Manufacturers, University Nottingham (Ed. Swan, H. and Lewis, D.) Churchill, London.

Preston, T.R. and Willis, M.B. 1974. Intensive Beef Production, 2nd Edition Pergamon, Oxford.

Preston, T.R. and Ugarte, J. 1972. Rearing calves by restricted suckling. Wld. Anim. Rev. No. 3:28,

Silvestre, R., MacLeod, N.A. and Preston, T.R. 1976. Supplementation of sugarcane/urea diets for growing cattle: different levels of maize grain and a protein concentrate. Trop. Anim. Prod. 1:206-214.

USDA 1976. US Department Agriculture National Food Situation Report. Feedstuffs. Oct. 4, 1976, p. 19.

CHAPTER 14
ECONOMIC BENEFITS OF BY-PRODUCT UTILIZATION IN ANIMAL FEEDING SYSTEMS IN DEVELOPING COUNTRIES: THE PRODUCTION OF SINGLE CELL PROTEIN FROM AGRICULTURAL WASTES

by

A.J. Forage

Tate & Lyle Ltd.
Reading, Berks, United Kingdom

Resumen

En este trabajo se examinan los aspectos económicos de la producción de proteínas unicelulares (PUC) a partir de residuos agrícolas, se describen las tecnologías necesarias y se resumen los beneficios que pueden obtenerse.

El renovado interés que en los últimos años ha suscitado la producción de PUC y que ha dado lugar a la creación de una serie de fábricas de levaduras para alimentos o piensos, obtenidas a partir de aguas residuales sulfíticas, aguas amiláceas, suero, melazas, etc., en parte se debe a las previsiones relativas a escaseces de alimentos y proteínas en los próximos decenios, y en parte responde a la preocupación general por la conservación del medio, ya que estos residuos son muy contaminantes y su eliminación resulta cada vez más costosa.

Dadas las necesidades de capital y mano de obra que entraña la compleja tecnología de la producción de PUC, ésta es practicable, en la situación actual, solamente a gran escala y en países desarrollados, donde a causa del costo total del proyecto no siempre las proteínas unicelulares son competitivas con otras proteínas. Sin embargo, si se considera como un medio de reducir los costos de la eliminación de residuos, el proceso puede resultar ventajoso.

En los países en desarrollo donde se dispone de grandes cantidades de carbohidratos semielaborados (por ejemplo, los procedentes de industrias de elaboración de frutas y hortalizas), las operaciones pueden ser también rentables, si se desarrollan en escalas adaptadas a las condiciones locales.

Los organismos utilizados para la simple recuperación de biomasas de residuos, en teoría, deberían poder desarrollarse a temperaturas superiores a 35°C y valores extremos de pH; deben tener elevados índices de crecimiento en una amplia gama de fuentes de carbono; han de tener una elevada eficiencia de conversión del sustrato en biomasa y un alto contenido de proteínas; y no deben ser tóxicos.

Se describe una fábrica establecida recientemente en Belize para producir PUC a partir de residuos de una fábrica de elaboración de cítricos. Como la fábrica ha empezado a producir recientemente, no se dispone todavía de ningún informe acerca de sus resultados técnicos y económicos.

El tratamiento de aguas residuales difiere en varios aspectos del que se aplica a los residuos sólidos. Normalmente la reducción de la demanda de oxígeno biológico es más importante que la producción de PUC. Se están desarrollando sistemas para tratar los residuos azucarados por fermentación continua o por reciclaje de la biomasa; la primera de las dos técnicas está más adelantada y los ensayos preliminares utilizando un efluente simulado han dado resultados alentadores.

La producción de PUC puede completar la disponibilidad de piensos en países en desarrollo, posiblemente con costos inferiores a los de las proteínas importadas, así como estimular la producción local y acrecentar así la ingestión de proteínas. Puede servir de trampolín para llegar a una tecnología más compleja de elaboración por fermentación. Sus necesidades de terrenos son pocas. Es importante para reducir la contaminación y contribuye, en general, a la conservación de las materias primas.

Introduction

When assessing the economic benefits that accrue from making a product, several factors must be taken into account. It is proposed here to take the production of microbial protein for animal feed as a specific example, to discuss the economic considerations, to describe the technologies involved, and then to summarize the economic benefits that can be gained.

Economics

During the past 10 to 15 years there has been a revival of interest in the production of single cell protein (SCP), with the result that several plants are producing food or feed yeasts from sulphite waste liquor, ⁽¹⁾ starch waters, ⁽²⁾ whey ⁽³⁾ and molasses ⁽⁴⁾. The present interest in SCP has come about for two reasons. The first is a result of the alarming forecasts of an increase in world population without a concomitant increase in food supply, expected to culminate in a severe food shortage during the latter part of the century. Although the total world food production is increasing steadily, per caput production is stationary, and the increase in production occurring mainly in the developed countries, the standard of nutrition in the less developed regions is not improving.

What is true for the production of food in general can be seen to be true also for protein. A serious problem in the developing nations is the lack of protein, which characterizes the diet of much of their population. This protein deficiency ultimately threatens the health and productivity of the future generations more directly than would a general food shortage, since a deficiency of proteins mainly affects children and expectant mothers. Numerous schemes have been put forward for increasing the more conventional forms of food production such as farming, and SCP production is seen as sone of the less conventional ways of increasing food reserves: indeed, the production of microbial biomass has often been referred to as “microbial farming”.

The second reason for the renewed interest in SCP has come from a general concern for the environment. It was realized, not only by environmentalists but also by governments and populations as a whole, that wastes disposed on land and into watercourses were polluting the environment. The biological oxygen demand (BOD) of many of these wastes are due to carbohydrates - potential substrates for microbial growth. It would be reasonable therefore to use the carbohydrate to produce biomass which could be recovered, thereby reducing the BOD of the waste.

During the last decade or so it has become clear that a number of companies throughout the world are now in a position to operate full-scale SCP production units. However, there now seems to be a certain hesitancy and a slight slowing down of development in the SCP area, probably due to two main causes, both economic: (i) the cost of raw materials and energy, and (ii) the cost of sophisticated fermentation equipment.

The costs of raw materials, that is, the carbon feedstocks for the production of biomass, have risen disproportionately to the value of the protein materials into which they are to be converted. These price rises include both carbohydrate and hydrocarbon feedstocks. It is therefore reasonable to consider the use of wastes which have little or negative value.

Similarly, costs for energy have markedly increased in the last few years, not surprisingly in view of the fact that many generating stations are run on oil.

Modern fermentations, unlike the age-old process of brewing, have become very sophisticated, and with sophistication come the inevitable cost rises. In addition, the fermenters and their ancillary items of equipment generally have to be fabricated out of expensive materials such as high-grade stainless steel, all adding up to a high capital cost per plant. Technologically sophisticated plant requires skilled manpower to operate it. These two factors, capital and labour, make high-technology SCP production feasible only on a large scale, and in developed countries.

The demand and sale price of microbial protein is governed by the cost of its competitors. Yeast and fungal SCP must compete with soybean meal, and bacteria possibly with fishmeal. Taking soybean meal as an example, three years ago its cost in the UK was $ 350–400/ tonne, now it is only $ 200–250/ tonne. Since production costs incurred in making SCP have risen proportionately to the cost of the raw materials and energy necessary for the fermentation process, perhaps two-or three-fold, it is now questionable whether production could be economical. Whether it would be depends very much on the local situation.

In the present economic climate, however, there are still two situations in which there is a future for SCP production:

(1) In areas of the Third World where large volumes of low-and negative-cost carbohydrate feedstocks are available, and in particular where there are fruit and vegetable industries whose culls, peelings and washings are thrown away. The scale of operation would be appropriate to the local conditions.

(2) In those parts of the world, usually the industrialized developed countries, where food-processing industries are finding waste disposal a particularly expensive matter. In these cases, the production of a saleable material from wastes may offset the costs of effluent treatment.

Techniques

The organisms used for simple recovery of biomass from wastes ideally must have a number of special properities. They should be able to grow at high temperatures (about 35°C) and at extremes of pH, conditions which provide the fungus with an environment hostile to contaminant yeasts and bacteria. They must have high growth rates to minimize the size of the fermentation system, and must be capable of growth on a wide range of carbon sources, preferably simultaneously. Many microorganisms are capable of utilizing a wide variety of substrates, from simple sugars such as sucrose and glucose to the more complex polysaccharides like cellulose and starch. The omnivorous nature of some microbes raises the possibility of general-purpose waste fermentation plants, capable of coping with seasonal changes and variations in carbohydrate feedstocks. Organisms should have a high efficiency of conversion of the substrate to biomass; this restricts the choice to aerobic microbes, since in anaerobic growth several times more carbohydrate is consumed to obtain the necessary energy. A further requirement is a high protein content: most microorganisms are capable of producing large amounts of protein, most of which is available to animals. Protein contents of up to 50% of cell weight can be obtained with yeast and fungi, and still higher yields can be obtained with certain bacteria. It goes without saying that the microorganisms must be non-toxic.

One particular advantage of SCP production is its low space requirement: fermentation plant requires little area compared to conventional farming or conventional effluent treatment plants such as aeration lagoons and trickle biofiltres, which take up valuable farmland required for livestock and cereal production. Fast growth rates and short doubling times, of the order of 2 to 4 hours for yeast and fungi, mean large production capabilities. For example, one 100-m³ vessel could produce up to three tons dried feed every day.

Tate and Lyle have been developing two types of waste treatment processes,⁽⁵⁾ a low-level technology designed for operation in Third World countries and a continuous process to treat high-volume effluents in developed countries, but which of course would be applicable to other areas of the world in the future.

Treatment of solid wastes

A pilot plant has recently been set up in Belize, in Central America, using waste from a citrus-processing factory.

Belize is in the situation of being a developing country in which a low-grade carbohydrate waste is available in quantity, in particular the peel from oranges and grapefruit. Also, during the closed season for citrus, there are sufficient other carbohydrate sources (cane molasses, citrus molasses and cull fruit) with which operations can be carried forward.

The plant itself is essentially a simple one.

The equipment is set up outdoors, on a concrete platform approximately 14m×7m, covered by a corrugated metal roof that protects it from the worst of the weather. It consists of:

(a) a medium preparation tank, manufactured out of stainless steel and with a capacity of 2.5m³. The tank could also be fabricated out of high-density reinforced polypropylene. A mixer is suspended in the tank in order that nutrient salts and peel suspension can be mixed together;

(b) a two-way pump, used for transferring sterile liquor from the make-up tank to the fermenter and also for extracting the final culture suspension from the fermenter and passing it to the filtre;

(c) a fermenter tank, approximately 4.3m high and 2m in diameter, with a working capacity of 10m³. It is fabricated out of high-density polypropylene reinforced with fibreglass to give rigidity. Situated on top of the tank is a motor and gearing system to power the impellers. Filtred air enters the tank from the air compressor via a stainless steel sparge ring;

(d) a rotary vacuum filtre to separate the mycellal product of the fermentation from the exhausted media. It is usually found that a double cheesecloth covering the drum of this filtre is sufficient to remove the fungus;

(e) a granulater to extrude the wet cake (at approximately 30% solids) into thin strands after filtration. This has the effect of increasing the surface area of the product for ease of drying;

(f) a simple oven dryer for final drying of the product down to less than 8% moisture. The wet extruded cake is spread out on aluminium trays, stacked inside the oven on shelves.

After drying to between 6 and 8% moisture, the feed granules can be either bagged directly or powdered in a pin mill.

The fermentation itself is basically a semi-continuous operation. The process is run on a 24-hour basis. Approximately 90% of the fermented broth is discharged from the fermenter and filtred. Fresh medium is then pumped into the fermentation tank, the 10% fermented broth left behind after the previous run acting as the inoculum for the next.

The fungus used is Aspergillus niger; a range of feeding trials on rats, chickens and pigs have shown this material to be non-toxic and in most cases equivalent to soymeal in its feed value.

The plant has just been commissioned and it is hoped that the technical and economic success of the process can be assessed over the next year.

Treatment of watery effluents

The treatment of watery effluents from agricultural processing plants differs in several respects from that of solid wastes. Effluents are often produced 24 hours per day and for much of the year. The concentration of substrates for fermentation is low, normally less than 5g/l in wastes from the canning industry and from starch processing, although in certain industries (e.g. palm oil processing) it can be higher. In most cases the reduction of the BOD is more important than the production of SCP.

Effluents containing in excess of 10g/l BOD could be treated by straightforward continuous fermentation. Effluents with lower BODs could probably be treated by a process including recycling of the biomass from the fermenter back to the fermenter. Both systems are being developed for treating sugary wastes by using the yeast Candida utilis. The conventional continuous process is the more advanced in development.

Using a simulated effluent, consisting of 10g/l glucose supplemented with nutrients for yeast growth, virtually complete removal of the glucose was achieved at operating dilution rates 0.3-0.4 h^-1. If there was any sugar left it was less than 0.01g/l. The yield of yeast was 0.45– 0.48 and the crude protein content of the dried product 48%. A small-scale pilot plant study using a trade effluent is now in progress.

Conclusions

The profitability of producing SCP in plants such as those at Belize may be questionable and is dependent on the local situation. There may be parts of the world which import protein at premium prices or consider self-sufficiency to be politically desirable. But even in these situations it is important that the plant operate throughout the year, to keep production costs of the protein as low as possible.

Treatment of effluents in developed countries presents a different set of problems. Throwing wastes down the drains can lead to heavy penalties. If the industry treats its wastes, then conventional treatment plants occuy large areas, and in a city or town this could be difficult. Treatment of the wastes to produce SCP may not be profitable in most cases but could offset much of the costs without using large amounts of land area.

What are the economic benefits of these processes?

1) In the Third World, a successful plant will provide farmers in the vicinity with a feed supplement, possibly cheaper than imported protein. Tables 14.1 and 14.2 show the operating costs of SCP production from solid wastes and process effluents, respectively.

In both cases labour costs are a major part of the operating costs. The cost/ton of the product must be low enough so that it can compete satisfactorily with alternative protein sources.

2) The availability of an additional feed source is likely to stimulate local poultry and meat production, and this could lead to increased protein intake of the local population. Also, the introduction of this technology could be a stepping stone to the subsequent fermentation production of more valuable products, such as amino acids and ethanol.

3) The land area required for the processes is slight as compared to conventional methods. In terms of the area occupied it can be shown that yeast produces protein over 100 000 times as efficiently as a growing bullock.

4) Both processes help to reduce pollution, leading to cleaner watercourses, higher levels of general hygiene, better fish farming, etc. Where industries have to pay to get rid of their wastes, the costs of treatment could be lower, leaving more money available for reinvestment.

5) Finally, recycling brings general conservation of raw materials. In essence, therefore, there is a more complete utilization of the raw materials or capital input of the in-house process.

References

1. Oy Tampella Ab - Report “Pekilo”, a new protein fermentation process. Protein from spent sulfite liquor.

2. Jarl, K. 1969. Production of microbial food from low-cost starch materials and purification of industry's waste starch effluent through the Symba yeast process. Food Technology 23 26.

3. Humphrey, A.E. 1974. Current developments in Fermentation. Chem. Engineering, Dec. 98.

4. Peppler, H.J. 1967. ed. Microbial Technology, P. 74 New York: Reinhold Publishing.

5. Righelato, R.C., Imrie, F.K.E., Vlitos, A.J., 1976. Production of single cell protein from agricultural and food processing waste. Resource Recovery and Conservation 1 257.