Cassava wastes as feedstuff 3: Cassava leaf processing, and nutrient composition

Published Date:  22nd June 2020

Introduction

Cassava has come to be a favored crop for both, small and large-scale farmers as it requires low input of time, labor, and cost. It is mostly produced for its starchy roots, however, the stem, leaves, and petioles are also edible and are widely used as food in Africa where it forms an important year-round vegetable. Therefore, cassava leaves merit consideration as a source of protein and nutrients for human and animal nutrition. In Nigeria, substantial amounts of cassava leaves are produced annually and freely available as a by-product at the time of harvesting the roots.

The potential yield of cassava leaves varies considerably, depending on cultivars, the age of the plant, plant density, soil fertility, harvesting frequency, and climate. In early studies conducted in Asia, scientists harvested 6.75-ton dry matter/ha of cassava leaves by defoliating once during a 7- month production season and obtained about 86% of the normal yields of roots. Approximately 10 tons of dry cassava foliage per hectare have been reported in more recent times in Asia. Researchers, however, recommend a harvesting frequency of 2 - 3 months, starting from 4 months, for the best all-around yield of roots and leaves in 12-month cultivars. The leaf dry matter yields will usually be lower if cassava leaves are to be obtained as a by-product at root harvest. When the cultivation of cassava is exclusively aimed towards leaf production, the planting density could be increased and the harvesting frequency could be shortened. Under such a scenario, foliage can be harvested from 4 months of age in a cycle of 60 - 75 days. However, whether the purpose of cassava cultivation under a given circumstance should be roots, leaves, or production of both would depend upon the comparative prices of cassava roots, cassava leaf meal, and traditional feedstuffs at the farming location.

Toxicity of cassava leaves

As in other cyanogenic plants, the cyanide poison concentration in cassava leaves decreases with age. The nutritional status of the plant; for example, increased nitrogen and potassium content of the soil may influence the production of cassava poison in the leaves. Again, leaves produced during prolonged drought are reported to contain high amounts of cyanide, while shading of young plants also causes an increase in the leaf cyanide levels. Sheep and goats may browse cassava leaves at certain times of the year without any signs of toxicity, however ingestion during certain other times lead to poisoning and death. Acute cyanide toxicity in animals usually results in sudden death, but in less severe cases it may lead to gastrointestinal disorders and growth depression. Tannin contents in cassava leaves are reported to increase with maturity and vary between cultivars. The presence of condensed tannins in cassava leaves may also present grounds for some health concerns of feeding unprocessed cassava leaves to animals.

Processing of cassava leaves for livestock feeding

Several attempts have been made to expand the livestock industries in the developing countries of the tropics. The ever-rising cost and acute shortages of traditional feed resources, however, remains a major hindrance to these efforts. All the same, in these countries, there exists a wide spectrum of agricultural by-products, such as cassava leaves which are available in large quantities and have considerable nutritional potentials and applications in reducing the cost of finished feeds. The high content of hydrogen cyanide in cassava leaves, however, acts as a poisonous agent, preventing excess consumption of cassava leaves by grazing animals. Substantial work has been done on the nutrient composition and feeding value of cassava leaves in livestock, especially monogastric and ruminant animals, in which the leaves were differently processed into dehydrated leaf meals.

Sun-drying: Simple sun-drying is capable of eliminating almost 90% of the initial cyanide content in the cassava leaves. When this is combined with chopping, cyanide in the dried meal was reduced to levels that are safer for monogastric animals. This reduction is due to the action of endogenous enzymes in the leaves following the loss of cell integrity or tissue damage. Free tannin contents of cassava leaves are also considerably lowered during sun-drying. Similar reductions during sun-drying have been reported for other leaf meals, indicating that this simple approach could be applied to several tropical leaves. It appears that sun-drying also irreversibly fixes tannins to other cell chemicals, thus reducing the total analyzable tannin content of the leaf. From a practical and economic point of view therefore, sun drying should be the method of choice for improving the nutritive value of cassava leaves in the developing countries of the tropics.

In general, cassava leaves dry easily, and drying is completed to about 10 – 12 percent moisture content in 2 days during dry, sunny weather. It is important, however, to note that oven or other artificial drying methods are less effective in eliminating the cyanide than sun-drying, due possibly to the shorter duration of drying. Processing has a limited impact on the crude protein content of the cassava leaf meal, while chopping, however, has been reported to cause a slight reduction in the protein content.

Asian studies have shown that at room temperature, cassava leaf meal exhibits excellent storage stability, and even after eight months of storage, mold or insect infestation were not detected. Cyanide content was also reduced during storage, although there was a gradual decline in the crude protein content. Moreover, the storage of cassava leaves reduced the vitamin C content, since, at ambient tropical temperatures, vitamin C is usually lost rapidly. A reduction in polyphenol content to about 48 percent in oven-dried cassava leaves has been reported, while it was retained at 62 percent in sun-dried, shredded and sun-dried, or steamed and sun-dried cassava leaves.

Ensiling and fermentation: Cassava leaves can be processed into 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 production. In a study at the Hue University of Agriculture and Forestry, Vietnam, researchers compared different ensiling techniques of cassava leaf to determine the nutrient compositions of the end products. Fresh cassava leaves collected after the roots have been harvested were mixed with 0.5 percent salt and 0, 5 or 10 percent of molasses, rice bran or cassava root meal and sealed in plastic bags as shown in table 1 below. Samples of silage were taken after 0, 7, 14, 21, 28, and 56 days and assessed for their nutrient compositions. Good quality silage was obtained by ensiling the leaves with as little as 5 percent of cassava root meal, rice bran or molasses, and hydrogen cyanide concentrations were reduced by about 60 percent. The concentration of hydrogen cyanide decreased rapidly for all treatments from initial values of about 300 mg/kg fresh silage and stabilized after 56 days at around 150 mg/kg for the control treatment (5 percent salt) and at a mean of about 100 mg/kg silage for the other treatments. Crude protein contents, however, decreased in all treatments by between 2 and 5 percent, probably due to bacterial degradation.

In another study by scientists at the Federal University of Agriculture Abeokuta, Nigeria, fresh cassava leaves were chopped into 4 -5 cm length and ensiled either alone, or with 5 percent (w/w) molasses or caged layer waste as additives. The set-ups were manually compacted in polyethylene bags, placed in covered plastic containers, and were allowed to ferment for 30 days. The addition of molasses and caged layer waste caused minimal reductions in the cyanide and crude protein contents of the cassava leaf meal from 95.8 mg/kg in non-additive silage to 89.3 - 84.7 mg/kg, and from 20.76 to 19.85 percent respectively in the additive treated ones.

Production of leaf protein concentrate

Although the potential for the use of cassava leaves in the feeding of monogastric animals is high, factors such as high fiber and cyanide contents limit its use as a major source of protein in monogastric animal feeding. These limitations could be largely overcome if the protein is separated from the fiber and a protein concentrate is prepared by juice extraction and steam coagulation. Nigerian scientists were able to extract leaf protein from 15 cultivars of cassava and attributed the inconsistency in the extractability of cassava leaf protein to differences in tannin contents between cultivars and the extraction techniques employed. More than 75 percent of the crude protein was true protein, and the digestibility ranged from 52.8 to 60.9 percent. The amino acid content of the leaf protein concentrate was superior to that of oil seed meals, and comparable to that of animal protein supplements, except sulfur-containing amino acids. Other studies have also confirmed that cassava leaf protein concentrate is a more effective protein source than fishmeal in the diet of chicks. The fibrous residue that remains after leaf protein extraction can be used as a feed for ruminants.

Researchers at the Federal University of Technology Akure, Nigeria, have described a simple low-cost village-level fractionation scheme for processing fresh cassava leaves into cassava leaf protein concentrate. The freshly harvested cassava leave was pulped in a pulping machine, followed by pressing with a screw press, to press-out the leaf juice. The leaf juice was then heated at about 80 – 90OC for 10 minutes to coagulate and pasteurize the leaf protein. The coagulated protein was then separated from the fraction by filtering through appropriate cloth material and followed again by pressing with a screw press. The protein concentrate produced was washed with water, pressed again, pulverized, and sun-dried. The flow chart for the leaf protein production process is shown in figure 1.

The protein content of the produce leaf protein concentrate was high at 47.0 percent, while crude fiber content was 2 percent. The crude fat was however high at 21.6 percent, while nitrogen-free extract was low at 15.9 percent. The gross energy value was also encouraging at 52.4 MJkg-1. The leaf protein concentrates on analysis, recorded favorable levels of both essential and non-essential amino acids, especially lysine, leucine, valine, and tryptophan, while the limiting amino acid appeared to be methionine. The nutritive potential, low-cost, and the simplicity of this technology make cassava leaf protein concentrate attractive as a source of protein in local food production systems as a practicable and ameliorative intervention strategy for the endemic protein under-nutrition in most developing regions. Pellets and cassava meal can also be produced from either the pressed cake or whole leaves and stem by first passing them through a dehydrator (hydraulic press) to reduce the moisture content to about 15 - 20 percent. The dried cake is then passed through a hammer mill to produce cassava green leaf meal, which contains about 24 percent protein. The dried meal can be further processed into pellets by passing through a pellet mill to produce cassava green pellets. An antioxidant may be added at the milling stage to enhance the stability of the product.

Scientists at the Universidade Estadual de Maringá, Paraná, Brazil, also evaluated four methods of protein extraction to obtain protein concentrate from cassava leaf. These included coagulation of proteins by lowering the temperature, extraction by isoelectric precipitation, solubilization of proteins, and fermentation of filter leaf juice. They reported that the solubilization of proteins method recorded higher extraction yield, although with lower concentrate quality. The juice fermentation method produced concentrates of higher quality and lower costs, while the isoelectric precipitation method required a shorter time to produce the concentrate.

Nutrient composition of cassava leaves

Although cassava leaves are rich in protein, factors such as high fiber content may limit their nutritive value for monogastric animals. The protein content of cassava leaves is essentially high for a non-leguminous plant. The cassava leaves contain an average of 21 percent crude protein, but values could vary from 14.7 to 40.0 percent, primarily due to differences in stage of maturity, differences in cultivars, sampling procedure, soil fertility, and climate. For example, Asian researchers found that the crude protein content decreased from 38.1 percent in very young leaves to 19.7 percent in mature leaves. In common with all plant materials, the stage of maturity is the major factor influencing the fiber content of cassava leaves. Very young leaves may contain about 8.3 percent crude fiber, which increases to more than 26 percent in mature leaves. Sampling procedures, such as the inclusion of petioles, may also influence the fiber levels, with fiber content in mature leaves being as low as 17 percent when the petioles are removed during preparation. Cassava leaves are also high in mineral content, as well as being a valuable source of vitamin B1, B2, C and carotenes, indicating that dry cassava leaves can be fed to poultry as a source of protein and carotene. The metabolizable energy is however moderate at a range of 1,590 to 1,800 kcal/kg.

Studies on the protein quality of cassava leaves show that it is deficient in methionine and marginal in tryptophan, but rich in lysine. The variation in the amino acid content has also been reported and attributed to the stage of leaf maturity, processing procedures, and the analytical methodology employed. As the leaves mature, the general trend is for the amino acid concentrations to decline, with lysine content declining as much 75 g kg-1 protein in very young leaves to 38 g kg-1 protein in mature leaves. The digestibility of cassava leaf protein has been reported to be low and ranging from 0.55 for valine and isoleucine to 0.84 for serine in monogastric animals. Again, only about 59 percent of the methionine is biologically available, indicating a biological value, which could be improved with methionine supplementation. The low protein digestibility of cassava leaves may be partly due to their high fiber and tannin contents. The availability of methionine, in particular, is known to be negatively influenced by tannins and promotes the worsening of the inherent sulfur-containing amino acid deficiencies in cassava leaves.

Scientists at the Institute for Oceanography and Marine Research, Lagos, Nigeria studied the nutritional and anti-nutritional compositions of cassava leaf protein concentrate from six cassava varieties for use in aquafeed. They reported variations in chemical composition and anti-nutritional factors across cassava varieties, especially crude protein and β-carotene levels, while ash, moisture and carbohydrate levels for all six varieties were relatively the same, indicating the need to use such analysis in arriving at the choice of cassava variety best suited for aquafeed production.

Conclusion

Cassava leaves can be processed into nutritious feedstuffs for feeding different classes of livestock. The currently available processing technologies can reduce the endogenous anti-nutritional substances in cassava leaves to safe limits. The significant amounts of cassava foliage produced annually in the tropics as waste products at the time of harvesting cassava roots should be gathered and processed into high-quality feedstuffs. Information on feeding trials with processed cassava leaf meal in poultry and other livestock will be discussed in the next article.

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