The soybeans are cracked to remove the hull and rolled into full-fat flakes. The rolling process disrupts the oil cell, facilitating solvent extraction of the oil. After the oil has been extracted, the solvent is removed and the flakes are dried, creating defatted soy flakes. The defatted soy flakes can be ground to produce soy flour, sized to produce soy grits or texturized to produce textured vegetable protein (TVP). The defatted flakes can be further processed to produce soy protein concentrates and isolated soy protein.

Primary and secondary products

Soybeans are grown primarily for their meal. Meal is the primary product and oil and lecithin are secondary.

Meal and oil

The early oil mill processing of soybeans was on a small scale using a hydraulic and screw presses (Goss, 1944). Gradually the screw press replaced the less efficient hydraulic press and in 1934 the first countercurrent solvent extraction was introduced. Improvements in oil extraction are continuously evolving. Major changes during the last two decades have included reduced energy purchases by introduction of the expander, installation of heat recovery systems and co-generation (making steam and electricity on site by burning waste by-products like hulls), improved working conditions for employees (dust and sound control), reduced contamination of the environment and automation of equipment, introduction of computer control of the processes and reduction of manual labour (Lusas, 2000).

Direct solvent extraction, "full" pressing or prepress-solvent extraction can accomplish separation of oil from soybeans. Some crushing industries combine these extraction methods to maximize oil extraction and its quality. Solvent extraction is the most widely used method for oil extraction in the Western world. However, mechanical extraction is often preferred by small extraction plants throughout the world to remove the oil. A flow sheet of soybean oil solvent extraction process is shown in Figure 20. Some of the soybean oil extraction by-products are shown in Figure 21.

Soybean meal and oil can also be produced by the ExPress System, where the whole or de-hulled soybeans at field moisture are fed continuously to a dry extruder. Within the extruder barrel, the material is subjected to friction, shear and pressure whereby heat is generated. The temperature profile within the extruder barrel can be varied depending upon the intended use of the processed meal. This process does not require external heat source. Typically, the top temperature at the exit of the extruder barrel is 150 ° C. Lower temperature profiles are used when the meal is intended for use as a functional ingredient in food applications (Wijeratne, 1999).

Soy meal is mainly used as animal feed. Only a small portion is processed into soy protein ingredients including soy flour, concentrates, isolates and textured soy proteins. These ingredients have functional and nutritional applications in various types of bakery, dairy and meat products, infant formulas and the so-called new generation soyfoods. Due to this difference in soybean use, two different types of soybeans have emerged: food beans and oil beans (Liu et al. 1995, Orthoefer and Liu 1995; Wilson, 1995). Annually, the United States exports 10 to 15 percent of the soybean production estimated at 6 to 7 million metric tonnes, to Japan, Korea and Taiwan. Most of this portion is used for whole bean applications (Motoki and Seguro 1994).

 Figure 20. Flowchart of soybean oil extraction process.

 Figure 21. Some of the soybean by-products obtained during oil extraction: (1) Whole soybeans, (2) hulls, (3) meats, (4) flakes, (5) collets or pellets and (6) meal.

Lecithin

Lecithin is produced by degumming the crude oil, followed by drying and cooling. Lecithin is modified to produce speciality lecithin. Soy lecithin is a very effective emulsifier that is added in small amounts to chocolates (0.25 to 0.35 percent), cookies, peanut butter (1 to 2 percent), confectionery coating, power mixes, baked products and dietary food. Two types of lecithin are generally recognized in the trade: crude soybean lecithin and refined lecithin. Emulsifiers or emulsifying agents are surface-active substances that increase stability of an emulsion when added. They usually contain both polar and non-polar groups, with one group being slightly dominant. Among the most commonly used emulsifying agents approved for food uses are lecithin and mono- and diglycerides of edible oils (particularly those from soybean oil). At levels of 0.1 to 0.5 percent in combination with other emulsifiers, usually mono- and diglycerides, lecithin is added to the fat in manufacturing margarine as well as some shortenings. When added to margarine, lecithin prevents "sweeping" or "bleeding" of the moisture present, reduces spattering during frying, increases the shortening effect for baking and helps protect the vitamin A in fortified margarine from oxidation. When used in baked goods, lecithin helps bring about rapid and intimate mixing of the shortening in the dough, improves the fermentation, water absorption and handling of the dough, gives a more tender and richer product after baking and prevents baked goods from staling.

Refined lecithin or vegetable oil, at 0.5 to 30 percent levels, is combined with extracted soy flour to make lecithinated/refatted flours. Dustiness is reduced and the products disperse more readily in beverages, dry mixes and during food processing (Soy Protein Council, 1987).

Derived products

The most important soybean derived products are the traditional soyfoods. The most popular soyfood is tofu, followed by soymilk and soy sprouts. Other includes okara, roasted soy nuts or flour, yuba, fresh immature soybeans, sweet beans and mature whole soybeans. Among the popular fermented soyfoods are soy sauce, miso, tempeh and natto.

Soybean milk

A traditional Chinese method for preparing soymilk is shown in Figure 22. The whole soybeans are soaked in water overnight, then washed and ground with fresh water at a water: bean ratio between 8:1 and 10:1. The slurry is then filtered through a cloth. The residue, known as soy pulp or okara, is separated and the filtrated is boiled for a few minutes before serving (Liu, 1997). Several modifications to the traditional Chinese method have been made in order to increase soymilk acceptability by consumers, especially Western consumers. This has to do mainly with the characteristic flavour associated with soybean products, known as bean-like flavour. One of these modifications consists on preventing the bean-like flavour formation through heat inactivation of lipoxygenase (LOX). Two-well known methods were developed in the 60’s and 70’s. They are the so-called hot-grinding method (Cornell method) and the pre-blanch or Illinois method. In the Cornell method, unsoaked, dehulled soybeans are ground in a preheated grinder with hot water. The slurry is maintained at temperature between 80 to 100 ° C in the grinder to completely inactivate LOXs and then boiled in a steam-jacketed kettle before filtering into soymilk and okara (Wilkens et al., 1967). In the Illinois method, the process starts with blanching pre-soaked soybeans in boiling water for 10 min. The beans are then drained and ground with sufficient cold water to make soy slurry and then into soymilk (Nelson et al., 1976). Another alternative is to strip off the volatile bean-like compounds once they are formed. During commercial production of soymilk, a deodorization step is sometimes added. The process involves passing cooked soymilk through a vacuum pan at high temperature in the presence of a strong vacuum (Liu, 1999).

Tofu

Tofu (Japanese), Dan fu (Vietnamese), Teou fu or Tou fu ho (Chinese) or bean curd is a cottage cheese-like product formed into a cake, which is precipitated from soy milk by a calcium salt or, in some instances, by concentrated sea water. Tofu can be prepared for the table in many different ways; the most important are in soup and by deep fat frying, the latter called aburage in Japanese and Yu Tou Fu in Chinese. Figure 22 shows the flowchart for tofu preparation. Conditions and yield were taken from Smith and Circle (1980).

 Figure 22. Flowchart for preparation of soymilk and tofu according to the Chinese method.

Food-grade full-fat soy flours and grits

There are three types of full-fat soy flours: enzyme-active, toasted and extruder-processed (Lusas and Rhee, 1995). Typical specifications for enzyme-active flours are moisture, 10 percent maximum; and on moisture free basis (mfb): protein, 42 percent; fat, 21 percent; and ash, 4.7 percent. Enzyme-active full-fat soy flours are preferred for bleaching wheat flour (by lipoxygenase activity) in making bread in Europe and enzyme-active defatted flours are preferred in the United States (Lusas, 2000). Toasted soy flours ("heat-treated full-fat soy flours") pass U.S. No. 100 or 200 mesh screen, have a protein digestibility index (PDI) in the 20 to 35 range. Grits are available in various granulation, for example, coarse (through No. 10 screen on No. 20); medium (through No. 20 on No. 40); and fine (through No. 40 on No. 80). Full-fat soy flours have been made using extruders to inactivate lipoxygenase. This type of flour is reported to have 89 percent trypsin inhibitor inactivation, a protein efficiency ratio (PER) of 2.15, urease activity of 0.1 pH change and a product nitrogen solubility index (NSI) of 21 percent (Mustakas et al., 1970). Full-fat soy flours contain approximately 40 percent protein (N x 6.25).

Extracted flake products and flours

Extraction of soybeans for food protein uses differs from processes where the meal is used for animal feeds (Witte, 1995). A cleaner United States No. 2 soybean, or a United States No.1, is used. Splits removal is emphasized since may harbour early lipoxygenase activity that produces bean-like flavour. In processing soybean food proteins, a solvent-extracted "white flake" is made first. It may be sold as a food ingredient, ground into flour or grits, extracted to produce soy protein concentrate, or solubilized in preparation of soy protein isolate (Lusas and Rhee, 1995). NSI of white flakes before toasting are in the 85 to 90 range. The Soy Protein Council (1987) has defined flakes as "white," NSI>85, "cooked," 20 to 60 NSI and "toasted," NSI<20.

Soy protein concentrates

Soy protein concentrates are basically flour from which sugars (sucrose and the nondigestible oligosaccharides stachyose and raffinose) and other soluble materials have been removed (Lusas and Rhee, 1995). Soy concentrates have a medium level of protein (65 percent-72 percent) and a similar high level of crude fiber to soy flour (3.5 percent-5 percent).

Soy protein isolates

In making isolates, essentially all solubles, including sugars and fibre are removed from the starting flour. Preparation of soy protein isolate takes advantages of solubility characteristics of proteins at different pHs. Soy isolates have a higher level of protein (90 percent-92 percent) and lower carbohydrate content (3 percent-4 percent) than soy concentrates.

Composition of commercial soy protein products mentioned above is shown in Table 22. Fat content in all of them is similar since all are made from defatted soybean. The carbohydrate content is low in isolates, intermediate in concentrates and high in flours and grits. The opposite occur in protein content, that is, isolates contains the highest amount of protein (90 to 92 percent), followed by concentrates (65 to 72 percent), flours and grits (56 to 59 percent). On the other hand, isolates had the lowest fibre content (0.1 to 0.2 percent), followed by defatted flours and grits (2.7 to 3.8 percent) and concentrates (3.4 to 4.8 percent).

Textured soy protein products

Soy flour and concentrates may be further processed by thermoplastic extrusion (Atkinson, 1970) to impart meat like texture to these products. Similarly, soy isolates may also be textured by a spinning process that involves solubilizing soy isolates in alkali and then forcing it through a spinneret into an acid bath to coagulate the proteins. The fibres formed are stretched and combined into bundles or tow. The tows are then used to produce meat analogues.

Table 22. Chemical composition (%) of soy protein products

Source: Soy Protein Council (1987).

Other uses

Soybeans are also used in the manufacture of many industrial products such as resins (John Deere is testing it now), plastics, varnishes and paints, caulking compounds, disinfectants, insecticides, oiled fabrics, printing inks (Danbury Printing and Litho uses soy-based ink), soap, crayons, paints, graffiti remover, fire extinguisher foam, wallpaper, glycerine, lubricants, soft soaps, waterproof goods, oilcloth, rubber substitutes, artificial petroleum, gasoline, cardboard, grocery sacks, linoleum, enamel, copy toners (for use in printers), hydraulic oil (performs as well as petroleum-based), wood adhesives (replacing petroleum-based products), concrete, wax polish (outperforms other waxes, easier to apply), solvent to remove grease, oil, asphalt, paints, etc. with no damage to surfaces, SoyDiesel, an alternative mixture of diesel fuel and soybean oil used to power small boats.