Agricultural crop residues are abundant both in kind and quantity. Currently, most of them are wasted on the fields or ploughed back into the soil as conditioners and organic fertilizers. Because of their relatively good feed values, some are also used as animal feed. A much smaller quantity, through bioconversion, serves as raw material for some traditional fermented foods, as well as substrates for mushroom cultivation.
However, considerable amounts of copra and rubber-processing residues, e.g., coconut liquid and rubber serum, are still discharged into the environment, causing a serious pollution problem. Because of the lack of economic incentives, capital, managerial initiatives, and, in some cases, suitable technology, progress in wider use of residues is much slower than it would be if there were no such constraints.
A number of other reasons may make a residue undesirable. Logistic aspects and low dry matter content may be expensive to overcome. Seasonal variability often makes it difficult to manage the material by advanced technology. Chemical and microbial contamination and organoleptic or psychological unacceptability may preclude the use of some residues as food. The above characteristics all present problems that must be overcome if a residue is to be converted to food.
These residues form the major part of agricultural production and are susceptible to a common method of bioconversion. The bioconversion of residues is important for organic farming. Residues are not used as foods because they are inedible without some form of bioconversion.
Cellulose-Rich Substrates:
They form a total of more than 1,800 million tons annually of renewable resources. They are to a great extent found in Asia, and it is therefore not surprising that they consist primarily of rice straw. The present use is often none; in some areas it is used for fuel. Straw could form an extensive base for feeding ruminants.
There is no doubt that bioconversion would greatly improve the use of these materials, particularly in rural areas. To what extent in vitro SCP (single-cell protein) production can play a major role here depends greatly on local circumstances and on the results of research efforts in this field.
In practice, cellulosic residues are mainly lingo-cellulosic and sometimes include starchy and/or sugary components. The following are major waste materials under each of these categories with high potential for bioconversion. They are- rice straw, rice husks, corn stalks, corn cobs, bagasse, cane filter cake (mud), kenaf (Hibiscus cannabinus) stalks, animal manure, pineapple peels.
The cellulosic materials have high lignin components, digestion by animals is minimal. In such cases, a small amount of fermentable carbohydrates such as corn meal or molasses is added to produce silage, thus ensuring rapid fermentation. Silage making normally serves as one of the effective means of conserving high-moisture products for animal feed. Corn (maize) silage is most common in Thailand, and cassava is being used increasingly.
Starchy and Sugary Wastes:
The carbohydrates portion of these residues is more easily accessible. They require a somewhat less difficult form of SCP bioconversion. Starchy residues are obtained from cassava meal (residue), cassava effluent while sugary residues are obtained from molasses, distillery slops, pulp waste liquor, coconut water (juice). Cassava and sugar beets provide the greatest amount of residue. High productivity in relatively poor soil has made cassava a popular staple food, especially in countries most in need of food.
Non-cellulosic carbohydrate residues, such as starch in cassava and sugar-containing residues (molasses), are generally used as feeds for non-ruminants, i.e., pigs and poultry. Broken rice rejected in the milling process, barley, maize, and cassava meal are commonly used as feeds in Thailand. The use of these residues may be considered a form of bioconversion in common practice throughout the world.
Grass has a better composition, but if it were to be used for monogastric consumers like man, there would still be severe problems because of the relatively high crude fibre content. Poor digestibility is another reason for rejecting unprocessed residues. Many crop residues are deficient in protein and minerals. It is therefore not uncommon for farmers to add products such as urea and minerals at the time of ensiling.
Source of Organic Residues:
1. Rice:
Rice (Oryza saliva) is the staple food of Indonesia. It is widely cultivated in India wet as well as on dry lands. Its cultivation and processing are labour-intensive and require a large capital input. Because of the magnitude of the harvest and the fact that the method of harvesting and post-harvest processing is largely manual, paddy production generates a huge amount of residues. These consist of straw or stalks, husks or hulls, and bran.
The bulk of the straw is left rotting in the field, sometimes after burning, or is ploughed back into the soil as a conditioner and organic fertilizer. A large proportion of the straw is also used as cattle bedding and feed. As a feed it contains approximately 66 per cent dry matter, 3.4 per cent crude protein, 0.73 per cent digestible protein, and 38.1 per cent total digestible nutrients.
A significant quantity of the straw also serves as raw material for board and paper manufacturing. Straw is an excellent substrate for the cultivation of paddy straw mushrooms (Volvariella volvacea). Some of the rice husks are used for fuel. Some of the ash generated is used as washing powder, but the bulk is wasted.
Most of the husks around large rice mills, however, are also wasted. In some areas, significant quantities of husks serve as a component of cattle feed. The husk has a quality comparable to that of straw because it contains 91 per cent dry matter, with 3.8 per cent crude protein, 1.2 per cent digestible protein, and 19.55 per cent total digestible nutrients.
2. Maize:
Maize (Zea mays) is the second staple for many Indians. It is mainly cultivated on semi-arid lands in a multiple cropping system. As compared to rice, its cultivation and processing demand less labour and capital input. Most harvesting and post-harvest processing of maize are still manual operations that generate a large amount of residues, including stalks, husks, skins, cobs, pressed cake, and bran. It is estimated that stalk production per hectare is between two and four tons.
Most of the stalks, husks, and skins are left to rot or ploughed back into the soil as a conditioner and an organic fertilizer. A significant proportion of the stalks-particularly the fresh, young ones are used as green fodder. Stalks have relatively good feed nutritive values, containing about 21 per cent dry matter, 1.7 per cent crude protein, 1.1 per cent digestible protein, and 16.3 per cent total digestible nutrients. Dried stalks are often used as fuel.
Some of the husks and skin trimmings are also fed to cattle. In fact, they have better feed nutritive values than those of fresh stalks, because they contain higher digestible protein (18.1 vs.1.1 per cent) and higher total digestible nutrients (67.2 vs. 16.3 per cent). A by-product of the oil extraction process is press cake. With its content of 18.1 per cent digestible protein and 74.1 per cent total digestible nutrients, it has a feed nutritive value comparable to that of the whole grain.
3. Cassava:
Cassava (Manihot utilissima) is an important food crop, particularly in places where the soil is relatively poor or in times of food scarcity. It is also an important export commodity in the form of pellets and chips. The plant is widely cultivated in the South India as a monoculture or in a multiple-cropping system. It does not need special attention or large capital input.
Since only the roots are collected, harvesting cassava yields large quantities of residue in the form of woody stems, leaves, and soft plant parts. If a mature cassava plant consists of 50 per cent roots, 30 per cent woody stems, and 20 per cent soft plant parts and leaves, the total residues generated at harvest time. The roots are usually further processed into pellets, chips, or tapioca flour.
The processing generates residues as peels and tapioca flour wastes. Most of the woody stems are generally burned as fuel, while the soft plant parts and leaves, particularly those of non-toxic varieties are fed to cattle or left on the soil as a conditioner and fertilizer. A significant amount is also used as green fodder, which has good feed values, containing 26 per cent dry matter, 23.1 per cent crude protein, and 13.86 per cent digestible crude protein.
Significant proportions of the peels are fed to cattle, and contain about 5.3 per cent crude protein, 20.9 per cent crude fibre, 1.6 per cent ether extracts, 65.7 per cent nitrogen-free extract, and 6.1 per cent ash. Most of the peels, however, are not used. The bulk of the flour waste is used as a component of feedstuff. It contains about 90.3 per cent dry matter, 5.6 per cent crude protein, and 3.73 per cent digestible protein. A small amount is also used as a component for the preparation of traditional fermented foods such as noncom (fermented peanut press cake) and tempeh.
4. Soybeans:
Soybeans (Glycine max) are one of the important crops of India, especially Madhya Pradesh. It is usually planted in monoculture in rotation with rice. In places with low rainfall, multiple cropping of soybeans with other crops is commonly practiced. Compared to rice, soybean cultivation requires less attention and capital input. It serves as the raw material of many soybean-based traditional fermented and non- fermented foods, such as tempeh, kecap, taoco, and tahu.
Harvesting and separating the seeds from the pods generate residues in the form of straw and shells. Significant amounts of residues are produced as the result of processing soybeans into foods. In tempeh preparation, for example, about 28 per cent of the raw material is wasted. Approximately the same proportion (27.4 per cent) is wasted in tabu (soy curd) production.
No information is available regarding the proportion of residues generated in kecap preparation. The dry straw and shells are usually burned as fuel or left on the field and ploughed back into the soil as a conditioner and fertilizer. The young plant parts and leaves, however, are fed to cattle as green fodder. They have good nutritive value, containing 27.9 per cent dry matter, 24.7 per cent crude protein, and 12.53 per cent digestible crude protein.
In the case of tahu residues, the solid portion is usually fed to pigs as a feed component. It contains 22.84 per cent protein, 7.27 per cent fat, 65.42 per cent carbon, and 4.47 per cent ash. The liquid portion is usually discharged into the sewers, causing a considerable pollution problem.
5. Groundnuts:
Groundnut (Arachis hypogaea) is an important crop in India. It is widely used as an ingredient of many Indian foods and also as a source of vegetable oil. As in the case of soybeans, they are usually planted in monoculture in rotation with rice, while in places with low rainfall they are part of a multiple-crop system. Their cultivation requires about the same amount of labour and capital input needed for soybean production.
The residues from groundnut production are mainly generated at harvest and during separation of the seeds from the pods, i.e., stems and shells. Moreover, oil-extraction processing of the seeds also generates residues in the form of press cake. The stems and leaves are mostly fed to cattle as green fodder or as plant meal. In both forms they have relatively good nutritive values.
Green fodder contains 17.6 per cent dry matter, 19.9 per cent crude protein, and 13.08 per cent digestible crude protein, whereas plant meal contains 88 per cent dry matter and 16.6 per cent crude protein. Stems that are not suitable for feed are usually left on the field and ploughed into the soil as a conditioner and fertilizer. The dry shells are commonly burned as fuel.
The bulk of the press cake is utilized as an ingredient of feed. It contains 90.15 per cent dry matter, 44.9 per cent crude protein, and 41.32 per cent digestible protein. A significant amount of high-quality press cake constitutes the main ingredient in oncom (fermented groundnut press cake) preparation.
6. Coconuts:
Coconut (Cocos nucifera) is cultivated in India. Coconut is a general- purpose fruit, as practically all parts of it are usable. Husks, coir dust, and shells are the residues after the shelled nut has been obtained. In the preparation of copra from shelled nuts, additional residue is generated in the form of liquid.
As the result of oil extraction from copra, coconut press cake is obtained. The amounts of the respective residues in 1976 were approximately as follows: husks, 2.2 million tons, consisting of 1.5 million tons of coir dust and 0.7 million tons of fibre and outer rind; shells, 0.4 million tons; liquid, 0.7 million tons; press cake, 0.4 million tons.
Most of the husks are wasted, but a significant quantity are used for mats and matting, floor coverings, brushes, strong ropes, etc. The shells usually serve as fuel for drying copra. In recent years there has been some interest in using shells to produce activated charcoal. Practically all of the liquid is wasted, and in many cope-producing centres it causes a serious pollution problem.
In some places, the press cake is consumed as a side dish, but the majority serves as a feed ingredient. It has relatively good feed nutritive values, containing 91 per cent dry matter, 21.3 per cent crude protein, and 18.2 per cent digestible crude protein.
7. Oil Palm:
Oil palm (Elaeis guineensis) fruits and the oil-extracting process generate considerable amounts of residues as empty stalks, fibres, pulp (pericarps), shells, and cake. The residues generate empty stalks, fibre, pulp, shells, cake, etc. Most of the empty stalks, pulp, fibre, and shells are burned as fuel in the factory. The cake generally serves as an ingredient in cattle feed.
8. Sugar Cane:
Sugar cane (Saccharum officinarum) is an important crop in India. It is mostly planted on large plantations. Cane sugar production generates a large amount of residues, both at the time of harvesting and during the sugar-extracting process. At harvest the main residues are cane tops, while those associated with sugar extraction are molasses, bagasse, and pith. Fresh cane tops are commonly used as green fodder.
They contain 2.7 per cent digestible protein and 45.7 per cent total digestible nutrients. The dry tops and leaves, however, are left on the soil and (sometimes after burning) ploughed back into it as a conditioner and fertilizer. Large amounts of the bagasse and pith are used as fuels in the factory.
They are also important as raw materials for the production of board and paper, but the amount used is not large. Only a small proportion of it is fermented into alcohol, and a still smaller proportion serves as a component of cattle feed. Molasses has a relatively good feed value, containing 74.7 per cent dry matter, 6.5 per cent crude protein, and 3.7 per cent digestible protein.
9. Rubber:
Rubber plant (Hevea brasiliensis) is also cultivated in South India. Latex contains about 4 per cent non-rubber constituents, most of which are in the serum fraction. During latex processing, the serum fraction is washed away. These non-rubber components are protein (1 per cent w/w), lipids (1 per cent w/w), carbohydrates, mostly quebrachitol (1 per cent w/w), inorganic salts – largely phosphates, K, Mg, Na, Ca, Fe, and Cu (0.5 per cent w/w), among others.
Except for the lipids, most of the carbohydrates, inorganic salts, and 67 per cent of the latex proteins are in the serum. Thus, approximately 3.5 per cent of the latex is washed away during rubber processing. The non-rubber constituents wasted are proteins, carbohydrates, inorganic salts. They are usually discharged into open streams, thus causing a serious pollution problem.
10. Coffee:
Coffee (Coffea robusta) is one of the most important export commodities of India. Coffee processing generates residues in the form of pulp or husk, hull, and bean. These represent approximately 78, 6, and 16 per cent of the berry. The pulp has a relatively good feed value, containing 91.8 per cent dry matter, 9.2 per cent protein, 3.4 per cent digestible crude protein, and 42.2 per cent total digestible nutrients.
11. Forestry Products:
The forest products are important source of organic residue. Logging and wood processing generate a considerable number of residues in the form of logging wastes, defective logs, sawdust, edgings, slabs, and trimmings.
12. Livestock:
The important domesticated animals in India include cattle, buffaloes, horses, goats, sheep, pigs, and poultry (chickens and ducks). The manure produced by these animals is used as organic manure. Per year, they produce about (in kilograms per head)- cattle, 5,400; horses, 2,700; pigs, 1,500; sheep and goats, 500; and poultry, 13.
Except for human wastes, most manures are used as organic fertilizers. Only recently has there been an increased interest in using manure as a raw material for biogas production. This practice is still very limited, however.