Various products, co-products and by-products that can be obtained in soybean solvent extraction processing (Cherry, 2004) are shown in Fig. 16.2. The main commercial products of soybean are oil and meal (protein). Soy meal contains 45-50% good-quality protein and can be processed into a variety of high-protein co-products such as livestock feeds, de-fatted soy flours, soy protein concentrate, soy protein isolates and TSP (Table 16.8).
Much soybean oil is used for edible applications such as for cooking, salad oil, dressing, shortening, margarine, mayonnaise and confectionery coating. Soybean oil is also being used for biodiesel, lubricants and cleaning
Fig. 16.2. Flow diagram for soybean solvent extraction processing and resultant products, co-products and by-products (modified from Cherry, 2004).
RBD, refined, bleached and deodorized.
Fig. 16.2. Flow diagram for soybean solvent extraction processing and resultant products, co-products and by-products (modified from Cherry, 2004).
RBD, refined, bleached and deodorized.
Table 16.8. Products, co-products and by-products of the soybean solvent processing industry (adapted from Cherry, 2004).
Oil Refined, bleached and deodorized oil: cooking oil, salad oil, shortening, margarine, mayonnaise, biodiesel. Lecithin, soy oil derivatives Protein Soy meal: livestock feed, de-fatted soy flour, soy protein concentrate, soy protein isolates, texturized soy protein, soy protein derivatives
Hull, germ, gum, feed fat, soapstock, spent catalysts, distillates/sludge, spent bleaching earth agents. Lecithin, once treated as a by-product of crude oil refining, has grown in use and value to become a co-product of soybean processing. It is a valuable ingredient, primarily a mixture of phospholipids including phos-phatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphati-dylinositol and phosphatidic acid. These components have outstanding functional properties when applied to food, feed and other products. The pharmaceutical and cosmetic industries have benefitted greatly from this ingredient. The unique lipophilic and hydrophilic properties of lecithin components make them very useful.
It is estimated that there are currently 400-500 non-food soy and soybean-containing products being manufactured and marketed by 200-300 companies all over the world. These products can be placed into three categories: consumer products, ingredient intermediates and institutional or industrial products. Examples of these products are listed in Table 16.9.
During soybean processing into various products and co-products, by-products such as hull, molasses, soap stock, deodorizing distillates, spent bleaching earth and spent catalyst are obtained. These by-products are natural compounds and their use or disposal has been a challenge to the soybean-processing industry. A number of these by-products are returned to the land as fertilizer or soil modifiers, fed to livestock as feed, burnt for energy or applied to value-added conversion. The by-products are rich in carbohydrates, protein, lipids, lignins and phytochemicals. More focused research is needed to utilize these by-products as value-added co-products, similar to as has been successfully done with the once by-product, now a co-product, lecithin. Some soybean processing by-products are briefly described in Table 16.10.
Soybean continues to earn its worldwide importance as an accepted food and feed resource. The ability to use processed soy products and ingredients in many applications as biodegradable substitutes for petroleum-based products is a rapidly growing industry. Moreover, some selected soybean byproduct-based ingredients are gaining recognition as major nutritional sources of nutraceuticals and functional foods.
Soybean oil is extracted using solvent and expeller technologies. Hexane is the most common solvent used in the soybean oil industry. Expander
Table 16.9. Examples of soybean-based non-food products and ingredients (adapted from Cherry, 2004).
These products are sold by companies in packaged forms of different sizes through stores and used every day. Examples: Lubricant and greases Biodiesel Auto polish Building material composites Candles Carpets Coatings Hair care Motor oil Adhesives Engine oil Waxes
These are not finished products, but chemicals and ingredients used by manufacturers in making finished products and sold by companies in bulk. Examples: Industrial oil for paint or varnishes Industrial protein for cosmetics Industrial solvent Lubricant base stocks Surfactants/emulsifiers Waxes for specialty applications
These are used by companies in construction and by institutions such as schools, hospitals and farms. They are packaged in large quantities for multipurpose application, depending upon the needs of the users. Examples: Agricultural adjuvant Dielectric fluid Dust suppressants Fuel oil emulsifier Lubricant Industrial cleaner Metal-working fluid Odour reduction Printing ink Saw guide oil
Table 16.10. Soybean processing by-products and their potential uses (adapted from Cherry, 2004).
Hulls A by-product of the seed-crushing process. Hulls are made up of cellulose, hemicellulose, lignin, protein and minerals, and can be converted to valuable co-products such as feed that can be used as a replacement, up to 10%, for forage fibre in dairy cow rations to increase milk yield. Hulls can also be processed and used as dietary fibre. They lower blood serum cholesterol. Ground soy hull fine particulate matter treated with citric acid works as an excellent metal absorbent, especially of copper ions, for cleaning waste streams. Another component of soy hulls is the enzyme peroxidase, which has industrial applications.
Soy molasses Obtained while preparing soy protein concentrate from soy meal.
An ethanol extract of soy molasses called phytochemical concentrate (PCC) has been shown to repress genomic DNA elastogenic damage and point mutations in mammalian cells. Isoflavones, genistein, genisten, daidzein and daidzin have been identified in PCC. These compounds express a wide range of growth suppression of selected human colon cancer cells.
Table 16.10. continued
Description and potential uses
Deodorizer distillates or sludge
Spent bleaching earth/clay
Feed fat and soapstock
Obtained during soybean oil refining, and also known as scum oil. Sludge is composed of a complex mixture of aggregated compounds, tocopherol (vitamin E), sterol esters (campestene, sitosterol), squalene and free fatty acids (FFA). Tocopherol, the basis of the demand to process this by-product, can be used by the vitamin market and as a nutraceutical, sterol or feed stock for hormone production; squalene, a high carbon source for cosmetic and cholesterol biosynthesis; and fatty acids for biodiesel. A supercritical CO2 extraction method may be economically feasible for preparation of tocopherol from deodorized distillate/sludge. Contains a substantial amount of absorbed oil. The by-product is prone to spontaneous combustion and is a source of recoverable oil using steam, aqueous, solvent or pressure extraction treatments. It can also be used as a feed supplement, pellet binder and metabolizable energy source for poultry diets. Feed fat is obtained while de-gumming soybean crude oil for refining and it is used for animal feed. Further refining produces soapstock. This is a mixture of FFA, phosphatides and unsaponifiable materials. It is also used in animal feed. Results from the use of nickel and clay support in the hydrogenation of fats and oils. The recovered material contains nickel, clay, residual oil and filtration media used to aid in separation of the physically fine spent catalyst. Incineration is being used as one way to recycle nickel into the stainless steel industry, while the released energy is used for generating electricity.
technology has improved plant capacity and reduced solvent and energy losses. The technology of supercritical (SC) fluid extraction (SCFE) is emerging as a safe and viable extraction system. Extrusion-aided mechanical extraction of soybean oil is an environment-friendly technology for the production of soybean oil and edible-grade soy cake. Membrane technology may be used for separation of oil from miscella. Physical refining offers an increased oil yield and reduced energy consumption. Refined soybean oil is used in various types of cooking and may also be converted into margarines, shortenings and other products. Soy lecithin has many usages. Soy by-products such as soy meal, with 50% protein, offer a wide range of products for food and livestock feed. Soy hull is a good source for the production of single-cell protein for animal feed.
The purpose of oil milling is to separate oil from the protein-rich residual mass/cake. Oil and cake are then used for food, feed or fertilizer. Whether the oil is to be separated by pressing, solvent or a combination of the two, the seed is usually first cooked and flaked to rupture the cell walls, reduce oil viscosity and increase the rate of diffusion.
Oil extraction technology has not changed much and both expeller and solvent extraction technologies are standard options for the oil industry throughout the world. Solvent extraction technology is normally preferred for low-oil seeds such as soybean. Better mechanical and thermal designs leading to improved processes and equipment for desolventizing and miscella distillation are offering substantial energy savings. A flash-desolventizing technology is available to produce edible-grade, high-protein meal. For mis-cella distillation, waste heat utilization and three-stage evaporation with a spherical liquid film system is now the state of the art technology to reduce energy consumption as well as solvent loss.
One of the new technological developments is in seed preparation using an extruder/expander. This offers better energy and solvent efficiency and an increased plant capacity. Expander technology for direct extraction plants, processing low-oil-bearing seeds such as soybean offers steam saving up to 60 kg t-1 of seed, lower solvent loss and about 0.3% more oil recovery. This technology may also be adapted for high-oil seeds. Membrane separation of oil from solvent offers scope for energy saving. SCFE is emerging as a safe and viable extraction system. Extrusion-aided mechanical extraction of soybean oil has emerged as an environment-friendly and decentralized small-scale technology that is suitable for low soybean-production catchment areas. It results in chemical-free, high-quality edible soybean oil and cake.
The function of the preparation process of soybean is to prepare the seed for expelling and/or extraction of the oil by mechanical methods, solvent or a combination of both. If possible, the hulls and other materials should be removed from the seed kernel or meat. The unit operations involved in the preparation of seed and their purposes are given in Table 16.11.
An innovative and new technology in pre-extraction is the addition of an expander, also known as an extruder or enhancer press, to the conventional preparation after flaking (Singh and Ali, 1992; Ali, 2004). The flakes are extruded to form pellets, which enhance the extraction and drainage properties of the flakes. The extruder consists of a worm screw in a barrel. Flakes with about 18% moisture are fed in and high temperature and pressure are generated as the flakes pass through the barrel. When the material leaves the barrel, the sudden drop in pressure causes expansion of steam, which puffs the final product to produce favourable drainage and extraction properties. Excess moisture is removed and the material is cooled to 60°C before extraction. Success has also been found by extruding soybean at 10-14% moisture (Rittener, 1984). The subsequent increase in extraction efficiency and oil quality justifies the additional cost of an extruder or expander.
Oil is removed or extracted from the flakes or pellets by an organic solvent (n-hexane) to form an oil/solvent mixture called miscella. The oil is recovered from the miscella by removing the solvent. Since hexane is explosive, flammable and expensive, efforts have been made to investigate alternate solvents such as alcohol, isopropanol and CO2. Pressurized CO2 (named as an SC fluid) (Freidrich and Pryde, 1984)-extracted oil contains less phospholipids than hexane-extracted oil, but they are otherwise similar. It is expensive because of equipment cost and the need to generate high pressure. The de-fatted flakes from the extractor contain about 30% hexane by weight, which is removed through a desolventizer-toaster. The process also
Table 16.11. Major unit operations in seed preparation before extraction (adapted from Ali, 2004).
Cleaning Removes foreign materials from soybean. Includes the separation of plant tissues, pebbles, dust and so on to protect the processing equipment and enable the production of high-quality soy products.
Drying A moisture content of about 10% is needed for effective removal of the soy hull and hence drying of soybean before dehulling.
Cracking Cracking is performed to break soybean into small pieces for dehulling and flaking. It produces 4-6 cotyledon fragments/meat per bean and the hulls are separated from the cotyledon fractions by aspiration. Fines produced in the process are included in the meat for oil extraction to maximize the extraction yield.
Conditioning The cracked soybean grits/meats are conditioned using heat and moisture to obtain the optimum plasticity necessary for soy flake production prior to oil extraction. Steam heating raises the moisture to about 11%.
Flaking The conditioned soy grits are flaked to a thickness of 0.25-0.37 cm. This enables better flow of solvent through the bed and improves solvent penetration to oil bodies. It also reduces the diffusion distance to which the solvent or miscella (oil and solvent mixture) moves to extract oil.
enhances the nutritional value of soy protein by inactivating trypsin inhibitors and other naturally occurring toxicants. Steam comes into contact with the flakes and the heat of vaporization released from the condensing steam vaporizes the hexane, which is subsequently condensed and recovered.
The solvent extraction process is favoured over mechanical expellers because it leaves less oil in the meal. However, the possible escape of solvent from the system is a constant air pollution and explosion hazard. Economic and social factors have revived interest in searching for cheaper and safer solvents such as ethanol and isopropanol. However, it is SC fluid technology that may be a viable alternative to the current extraction methods. SCFE is a developing technology. It is a way to strip soluble extractives from prepared plant materials without physical damage or chemical change. SCFE is the substitution of a fluid in its SC state for hexane in the conventional solvent extraction process.
Commercial uses of SCFE include decaffeination of coffee and the production of spice extracts and a few costly food components and pharmaceuticals from plant materials. Ammonia, ethylene, toluene and CO2 all show promise for SCFE. Of these, CO2 offers unique advantages. It is abundant, non-reactive, non-toxic and environmentally harmless. Minor leaks or accidental losses would be of little consequence. All vegetable oils are soluble in SC-CO2 and the optimal performance of SC-CO2 has been found to be in an easy temperature range of 35-70°C.
Full-fat soy flakes are readily extracted with SC-CO2 at pressure of 200-700 kg per cm2 at 50°C. Soybean oil thus extracted is lighter in colour and contains less iron and about one-tenth the phosphorus of hexane-extracted crude oil from the same beans. The successful use of SC-CO2 could free >20 million gallons of costly hexane per year for essential energy uses. It therefore shows that a technology for oil extraction with SC-CO2 needs to be adapted or developed. This may replace the existing technology of oil extraction in coming years.
In mechanical extraction, the oil seed is subjected to extreme heat and pressure with oil mechanically forced from the oil cell. As the material is subjected to great heat during this operation, naturally occurring urease is inactivated and protein is denatured, making the product suitable for feed purposes. The quality of mechanically pressed and filtered oil is higher than that obtained from solvent extraction as less oil-soluble impurities (e.g. phosphatides) are removed, and is suitable for direct consumption. Efficiently pressed cake will retain 4-6% residual oil. Solvent-extracted meal has <1% residual oil. However, the main disadvantage of solvent extraction is high equipment cost, and it may not be economically feasible unless the capacity is >50 t day-1.
Mechanical extraction of soybean oil is often preferred by small enterprises. A screw press is generally used. The advantages are low initial costs and no requirement for solvents. However, a disadvantage of this extraction method is the low oil yields. Dry extrusion cooking of whole or dehulled soybean disrupts the cell structure of cotyledons. Consequently, the oil is released from the spherosomes into the matrix. When such soy-extrudate is passed through a screw-press, about 70% of the total soybean oil is recovered in a single pass. This technology could be used for processing soybean into oil and edible cake in low soybean-production catchment areas. It does not use any chemicals and, therefore, it is an environment- and worker-friendly soybean oil expression technology.
Soybean requires careful processing using heat treatment to make it edible. One such treatment is dry extrusion. The adoption of dry extrusion as a pre-treatment to soybean helps in expelling about 70% of the total oil in a single pass and results in an edible-grade soybean cake with about 50% protein and 4-6% oil. The cake can be converted into medium-fat soy flour -a good source of protein and calories for human consumption.
The cleaned soybean is dehulled and converted into grits and fed to the extruder after conditioning. During the extrusion of soy grits, the cell walls of oil globules are ruptured and oil is exposed on the surface of feed particles. This helps with quick and easy expression of oil with a relatively lower pressure application on the semi-fluid extrudate fed to the expeller.
Medium-fat soy flour may be used for the fortification of flour prepared from cereals or pulses (10-15% level of soy flour). The blended flour (soy and cereal or pulse) is excellent for baking bread, biscuits and so on. Both soy products - oil and flour - have a distinct market segment that requires better functional properties of traditional recipes in addition to high nutritional value. Oil has a ready market among edible oil consumers and can be sold to refining units. The medium-fat flour could find ready acceptability in army canteens, hostels, hotels, railway catering services and various other community kitchens, as well as in individual households through retail stores.
The extrusion-expelling plant has a great potential in low soybean-production catchment areas (3000-15,000 t year-1) or where soybean is likely to be cultivated in the near future. In addition to the technical and economical advantages, the system is pollution-free and avoids the use of chemicals for the extraction of oil. It produces a natural oil with a good shelf life and nutritionally good-quality protein, as it retains most of the lysine amino acid, which, upon blending (5-15% of soy flour) with cereals or pulses, improves the protein efficiency ratio.
A 1 t h-1 extrusion-expelling soybean plant consisting of buildings, transport facilities and equipment such as a grain cleaner, destoner, dehul-ler, dry extrusion cooker and accessories, oil expeller, oil filter and grinding mill may need a capital investment of about Rs 6 million (Rs 4 million fixed investment and Rs 2 million working capital). Such a plant may produce 3600 t of medium-fat soy flour, 600 t of oil and 600 t of by-products from 4800 t of soybean by working for 16 h day-1 for 200 days in a year (US$1 = Rs 50). The techno-economic feasibility analysis of producing medium-fat (4-6%) soy flour and oil using extrusion-expelling technology appears to be viable from a financial perspective, in addition to providing high-quality and low-cost food items to society.
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