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Processing cassava for animal feeds


Processing of cassava into chips and pellets
Artificial dryers
Processing of cassava leaves and stems
Production of single cell protein from cassava
References

G.B. Oguntimein

Animal feed has always been a major limiting factor in the growth of the livestock industry in developing countries. Most of the feed ingredients are imported and a large proportion of foreign exchange is spent for this purpose. This paper discusses the unit operations in the production process for cassava chips, pellets, and feed grade single cell protein from cassava roots and by-products, together with current research efforts to improve these processes.

Availability of animal feed is one of the greatest constraints to the expansion of the livestock industry in developing countries. Apart from the high and fluctuating costs, some of the ingredients used in mixed feeds, notably cereal grains, are in high demand for human consumption. In view of the dwindling supply of the conventional feed resources and the shortage of foreign exchange for importation, alternative sources produced locally within these countries are being investigated.

This paper discusses the processing of cassava into animal feed in the form of chips, pellets and feed grade single cell protein. The cassava plant, made up of the roots, leaves and stem, is a good source of carbohydrate and protein as shown in table 1. The different parts of the plant can be used as animal feed. The leaves can be used as silage, dried for feed supplementation and as leaf meal for feed concentrates. The stem can be mixed with leaves and used as ruminant feed, or dried for feed concentrates. The roots can be chipped or pelletized and used as feed, while the root peel, broken roots, fiber and baggase from starch extraction and gari processing can be dried and used directly as animal feed or as substrate for single cell protein production. The use of cassava root as animal feed is increasing in importance in the developing countries of Latin America and Asia where an export market for this commodity has developed. The European Economic Community imports about 6 million tonnes of cassava annually in the form of pellets or granules. Thailand and Indonesia are the world's largest exporters of dried cassava products, largely in the form of pellets. In Thailand, cassava is almost entirely utilized as cassava pellets and starch for export.

Table 1. Percentage composition of cassava plant


 

Root

Leaf

Stem

Fresh

Dry

Fresh

Dry

Dry

Moisture

66.7

12.6

nd

nd

nd

Crude protein

2.6

2.0

7.1

24.1

17.2

Crude fiber

4.9

4.0

1.4

26.0

23.5

Soluble carbohydrate

88.2

75.7

nd

nd

nd

Fat

1.0

0.7

nd

5.0

nd

Ash

3.3

5.0

nd

8.0

nd

Dry matter

nd

87.4

nd

16.1

nd

Nitrogen free extracts

nd

nd

nd

39.9

nd

Note: nd = not determined

Processing of cassava into chips and pellets

The flow chart for this process is shown in figure 1. The production of chips is an intermediate stage in the production of pellets. There is very little difference in the technologies used at different scales of chip and pellet production. The main difference is in sun-drying and mechanical drying. Chips can be produced by very simple techniques in the household or village as well as on a large mechanized scale.

About 2. 5-3.0 tonnes of fresh roots are required for 1 tonne of pellets giving a conversion rate of 33-40 %. The first step can be washing and peeling, depending on the quality of the harvested roots. The amount of soil that passes into the final product is largely determined by soil type and weather conditions during harvesting; wet clay soils tend to adhere to the roots. This leads to an increase in the conversion rate but dirt and peel reduce the quality of the final After washing, the roots are dipped in a 3% lime solution to neutralize the acid juice and prevent deterioration. The roots are usually cleaned manually in concrete tanks or mechanically in troughs with agitating paddles on a horizontal shaft. The rotating paddles push the roots from one end to the other, while they are washed. Chain conveyors are used to move the roots from the washer outlet to the chipper or to a holding tank where the washed roots drain. This unit is not advisable for a processing plant handling less than 25 tonnes per day because of the huge capital investment needed to establish it.

Figure 1: Flow chart for the production of cassava chips and pellets

The next unit operation is chipping. As is common in household processing this is done by hand or by a simple machine which consists of a driven disc with radial chipping slots fitted with cutting blades. There are two common types, the Malaysian and the Thailand models. The Malaysian type consists of a heavy rotating circular steel plate about 12 mm thick and 1m in diameter to which six blades are attached. The blade consists of a 1-1.5 mm steel plate that is corrugated at the cutting edge. The chipping wheels are usually mounted in wooden frames incorporating feed hoppers and driven by petrol, diesel, kerosine or electric motors. The Thailand model consists of a thin circular plate made from the ends of a 200-litre oil drum into which cutting edges are chiseled. These crude cutting plates are usually mounted on a fairly standard machine, frequently equipped with small wheels for mobility and a short elevator that deposits the chipped roots into hand carts (Booth and Wholey 1978). In Nigeria, manually operated chippers have been designed and fabricated by the Rural Agro-industrial Development Services (RAIDS) and IITA's Postharvest Unit. The length of the chips depends on the angle of contact of the roots with the blade. The size of the chips varies but generally they are 36 mm thick, 6-10 mm wide, 100-250 mm long. The chips produced by the Malaysian type chipper are more uniform with better geometry and they partially separate the thin brown root skin, which falls to the base of the machines from the chips (Booth and Wholey 1978). The cost of chipping increases as the size of chips gets smaller, since more energy is expended in breaking up the same amount of material (Manurung 1974). The choice of a chipper depends on the scale of operations in the processing plant.

The next unit operation is drying. Drying methods can be classified according to the technological level and cost. Natural drying, one of the methods, is done on cement floors which are sometimes painted black for better absorption of radiant energy or on trays for artificial drying. The factors that affect cassava drying time are the geometry (shape and size) of the cassava chips, the chip loading per unit drying area, air speed, temperature, humidity, radiation, as well as dry matter content of the fresh chips. In artificial heat dryers, all these parameters can be optimized to minimize the drying time and guarantee a high quality product. In natural drying methods, in which the heat source is solar radiation, air speed, temperature, and humidity depend on the environmental conditions, and very little control can be exerted over them. The optimum cassava chip size for natural drying on cement floor or trays is a rectangular shape with dimensions 8 x 8 x 50 mm according to Roa (1974). When three different geometrical shapes-rectangular bars 10 x 10 x 50 mm, slices 10 mm thick, and cubes 10 x 10 x 10 mm-were compared in drying trials using static bed dryers with 100 mm layers it was found that the cube-shaped cassava chips had the highest drying efficiency. The load of cassava chips per unit area measured in kilograms of fresh product per square meter is a function of the air flow through the chip layer. Chip load for natural drying on cement floor is restricted due to the reduced airflow at the soil level, and depending on the climatic condition, the optimum load is 510 kg/m². For horizontal trays it is 20-30 kg/m² and for vertically loaded trays it is 3040 kg/m² As a result of the higher loading on trays, the capital cost per unit throughout for tray drying is 30 percent less compared to concrete floor drying (Best 1978). In addition, chips dried on trays are better in appearance and more uniformly dried than those dried on concrete floors.

The dry matter content of fresh cassava is affected by several factors such as the variety, harvesting age, and the agronomic conditions, but in general, it ranges between 30-40 %. The selection of varieties with high dry matter content is important because drying time, and labor requirements per tonne of dry cassava are reduced. In case of artificial drying, fuel cost is reduced.

Sun-drying is a very labor intensive operation, requiring about 35-40 laborers per hectare of drying floor.

Artificial dryers

Three types of artificial dryers are commonly used for cassava drying:

1. Static bed dryer which is a batch system with low throughput and low heat efficiency. In addition, the product has a nonuniform moisture content.

2. Moving bed dryers which allow continuous feeding of wet material from one end and continuous withdrawal of dried product from the other end. They have a higher throughput with uniform moisture content because of better temperature control and higher heating efficiency. The only disadvantage is high fuel consumption.

3. Rotary dryers in which the wet material rotates within a cylindrical chamber through which hot air circulates while the product is continuously mixed. The interior surface of the chamber is provided with agitating blades that mix the product as the chamber rotates, forcing the product to fall through the hot air flow. Both concurrent and countercurrent flow configurations are possible. This system has a high drying rate as high air temperatures can be used, though this sometimes results in case hardening and scorching.

The selection of any of these methods of drying depends mostly on the amount of cassava to be dried, the availability of capital and labor cost, as well as the availability of relatively cheap energy.

After drying, the cassava chips are packed in either jute or polyethylene bags, or processed further into pellets. The commercial purpose of pelletizing cassava root products is to decrease the volume by 25-40 percent to produce a uniform product, to facilitate bulk handling during transportation, loading and reloading and to eliminate the dustiness of the product. Pelletizing contributes significantly to the density, durability, and quality of the product. If the chips are big, the cassava chips are first harmmer-milled and then preconditioned. During the preconditioning the moisture content is increased to between 16 and 18 %. This is usually achieved either by spraying water or by adding steam. The addition of moisture and heat increases the effectiveness of the pellet-making machine in terms of output, die life, savings, volume reduction, and nutritive value of the product. According to Fetuga and Tewe (1985), the heat generated by steam treatment and the high pressure during pelletization can release the cellulose from the lignin-cellulose bonds, thereby increasing the digestibility of starch and fiber. Pelletization of cassava diets also increases the nutrient density and in this form, about 50 percent root and 20 percent leaves can be used to replace almost all of the cereals and about a third of the soybean meal in a broiler diet.

Pelletization is done in continuous die presses with capacities from 2 to 8 tonnes per hour. The chips are forced through small holes in the die causing a rise in temperature through friction. This gives the pellets cohesion, but also causes considerable wear on the die and makes pelletizing energy intensive, about 60-140 kwh per tonne. The best results are obtained with rather small chips with 13-14 % moisture content which are heated to 65°C and moistened to 15-17% just before pressing. After pressing, the pellets are cooled, during which the moisture content drops to 14%, and packed in jute or polyethylene bags.

Factors affecting the quality of pellets are the composition of the material, protein, starch, fiber, and fat content. Protein-rich materials plasticize when heated and act as a binder to produce strong pellets. Starches gelatinize when heated in the presence of water and also act as binder to produce strong pellets. Fibers are difficult to compress but when they are present in sufficiently fine strands in the pellet, they give toughness to the product. Fats act as lubricants, resulting in easy pressing and therefore high capacity and lower power consumption.

Processing of cassava leaves and stems

Dried cassava leaves and stems have been fed to pigs, poultry, and dairy cattle. The meal produced from them has a nutritive value similar to that of alfalfa though deficient in methionine, isoleucine and threonine (Peyrot 1969, Rojanaridphiced 1977, Normanha 1962). Cassava leaves are a good source of about 20% protein. The amount of protein depends on the stage of growth. The processing of the aerial part of the cassava plant made up of both the leaves and the stem is shown in figure 2. For the extraction of cassava leaf protein, the leaves and the stem are interacted in a chopper or grinder and the juice pressed out. The extracted juice is then coagulated with injection of steam. The pressed cake is sent to the dehydrator. The coagulated juice is then sent to a separator where the soluble fraction is separated from the green curd and moved to the evaporator where it is concentrated to 50% by volume. The curd is sent to the drier to produce the cassava protein concentrate which is 50% protein (Müller 1977).

Figure 2: Flow chart for processing cassava leaves and stems

Pellets and cassava meal can be produced from either the pressed cake or whole leaves and stem by first passing them through a dehydrator to reduce the moisture content to about 15-20 %. The dried cake is then passed through a hammer mill to produce the cassava green meal which contains about 24% protein. The dried meal can be further processed into pellets by passing through a pellet mill to produce cassava green pellets. Antioxidant is sometimes added at the milling stage.

Production of single cell protein from cassava

The use of cassava as substrate for single cell protein has been investigated since the mid-1960s. Gray and Abou-El-Seoud (1966) grew some filamentous fungi on ground cassava roots, supplemented with ammonium chloride and corn steep liquor, to obtain biomass containing 13-24% crude protein.

Shrassen et al. ( 1970) described a process in which the yeast Candida utilis fermented enzymatically hydrolyzed cassava in a submerged culture to produce a product containing 35% crude protein on a dry weight basis. Gregory (1977) using Aspergillus fumigatus 1-21 A fermented whole cassava in a nonaseptic continuous fermentation system to produce single cell protein containing 37% crude and 27% true proteins. The fungi was a nonrevertible sporogonous mutant of A. fumigatus 1-21. This product was fed to rats and produced good growth responses. Rhodopseudomonas gelatinosa, a photosynthetic bacterium, was cultivated on cassava starch medium under aerobic dark and anaerobic light conditions. The optimum temperature for growth was found to be 40°C with maximum growth rate and growth yield of 0.23h-1 and 0.40g cell/g starch and 0.13h-1 and 0.83g cell/g starch, respectively for the aerobic dark and anaerobic light conditions (Norparatharaporn et al. 1983). Ghoul and Engasser ( 1983) developed a process for protein enrichment of cassava by enzymatic hydrolysis and Candida utilis fermentation. In this process, cassava starch is first liquefied by thermostable -amylase and then saccharified by glucoamylase before fed-batch fermentation of the hydrolyzed cassava by Candida utilis. A product with 70g/L yeast concentration was obtained after 20 hours of fermentation at a maximal 50% biomass conversion, with 40% crude protein.

Single cell protein can be produced by two types of fermentation processes, namely submerged fermentation and semisolid state fermentation (figure 3). In the submerged process, the substrate to be fermented is always in a liquid which contains the nutrient needed for growth. The substrate is held in the fermentor which is operated continuously while the product biomass is continuously harvested. The product is filtered or centrifuged and then dried. For semisolid fermentation, the preparation of the substrate is not as elaborate; it is also more conducive to a solid substrate such as cassava waste. Submerged culture fermentations are more capital intensive and have a higher operating cost when compared with semisolid fermentations which, however, have a lower protein yield. The major proportion of the production cost in most fermentation processes is the cost of the raw materials which can be up to 25-70 percent (Moo-Young et al. 1979). In effect, the cost of cassava will determine the economic feasibility of any single cell protein processed from cassava. In view of the present cost of fresh cassava roots in Nigeria, its use as a substrate is not economic. Thus, the potential of single cell protein from cassava can be realized when fresh cassava roots are cheap.

Figure 3: Flow chart for single-cell protein production

References

Best, R 1978. Cassava processing for animal feed. Pages 12-20 in Cassava harvesting and processing, edited by A. Ghoninard, J. H. Cook, and E. J. Weber. CIAT/IDRC: 114e.

Booth, R.H., and D.W. Wholey. 1978. Cassava processing in Southeast Asia. Pages 711 in Cassava harvesting and processing, edited by A. Ghoninard, J.H. Cook and E.J. Weber. CIAT/IDRC:-114e.

Fetuga, B.L., and O.O. Tewe. 1985. Potentials of agroindustrial by-products and crop residues as animal feeds. Nigerian Food Journal 3: 136- 142.

Ghoul, M., and J.M. Engasser. 1983. Nouveau procédé d'enrichissement protéique du manioc par hydrolyse ensymatique et culture de Candrola utilis. Microbiologie Aliments 1: 271283.

Gray, W.D., and M.O. Abou-El-Seoud. 1966. Fungal protein for food and feeds. 3. Manioc as a potential crude raw material for tropical areas. Economic Botany 20: 251.

Gregory, K.F. 1977. Cassava as a substrate for single cell protein production: microbiological aspects. Pages 72-78 in Cassava as animal feed. Proceedings, Cassava as Animal Feed Workshop, edited by B. Nestel and M. Graham, University of Guelph, 18-20 April 1977, Canada. IDRC-095e: Ottawa.

Manurung, F. 1974. Technology of cassava chips and pellets processing in Indonesia, Malaysia and Thailand. Pages 89-112 in Cassava processing and storage, edited by E.V. Argullo, B. Nestel, and M. Campbell. IDRC-03e.

Moo-Young, M.A. AS. Daugullis, D.S. Chalel, and D.G. MacDonald. 1979. The Waterloo process for SCP production from water biomass. Process Biochem. 14(10):38-40.

Müller, Z. 1977. Improving the quality of cassava root and leaf product technology. Pages 120-126 in Cassava as animal feed. Proceedings, Cassava as Animal Feed Workshop, edited by B. Nestel and M. Graham, University of Guelph, 18-20 April 1977, Canada. IDRC-095e: Ottawa.

Nommanha, E.S. 1962. Meal of cassava stalk. Chacaras e Quantais 105(3): 279-283.

Norparatharaporn, N., Y. Nishizawa, M. Hayshi, and S. Nagai. 1983. Single cell protein production from cassava starch by Rhodopseudomonas gelatinosa. Journal of Fermentation Technology 61(15): 515-519.

Peyrot, F. 1969. Nutritional role of some edible tropical leaves, cassava, yam, baobab, and kapok tree. Thesis, Faculté des Sciences, Paris Université, Paris, France. 61 pp.

Roa, G. 1974. Natural drying of cassava. PhD thesis. Department of Agricultural Engineering, Michigan State University, East Lansing, Michigan, USA.

Rojanaridphiched, C. 1977. Cassava leaf pellets as a protein source in Thailand. Cassava Newsletter 1:5.

Shrassen, J.J., A. Abbot, and RF. Battey. 1970. Process enriches cassava with protein. Food Engineering-May: 112-116.


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