Non – EDIBLE SOYBEAN FLOURS AND GRITS
Flours and grits are the simplest of all edible soybean protein products. The extent of processing which goes into their production is minimal. The cost of extra processing, starting with the dehulled clean beans (for full fat flour) or with dehulled white flakes (for defatted flour), has been estimated at 60 to 100 U.S.$ per ton. The total cost of the product, in the bag, at the production site, would then be less than $ 400 per ton. Recently (January 1991) a leading supplier in the U.S. has quoted soybean flour at $14.00/cwt. ( approximately $ 308 per metric ton), ex factory. This makes soybean flour one of the most economical sources of edible protein. Specialty flours, produced in smaller quantities, may be more expensive.
The annual production of edible soybean flours and grits increased from some 60,000 tons in 1960 to about 2,000,000 tons today.
The production of edible soybean flours and grits may take place either as an independent industrial activity or as a natural sequel of oil-mill operations. In fact, many oil-mills, recently erected in various parts of the world, feature production lines or departments for edible products, in addition to the usual oil and meal lines. The principal differences between processing for meal and processing for edible flour are in the quality of the raw material, the need for dehulling and the more rigorous control of the sanitary conditions of the plant and the process. Frequently, oil-mill operators prefer to produce only edible products or only meal, in alternate fashion rather than simultaneously.
4.2 Defintions, composition and quality parameters
4-2-1 Definition and classification of edible soy flours and grits
Soy flours are products obtained by finely grinding full-fat dehulled soybeans or defatted flakes made from dehulled soybeans. To be called soy flour, at least 97% of the product must pass through a 100-mesh standard screen.
Soy grits have essentially the same composition as flour, but coarser granulation. They are usually classified into three groups, according to particle size:
Coarse 10 to 20 mesh
Medium 20 to 40 mesh
Fine 40 to 80 mesh
Circle and Smith (1972) have pointed out that the name soy flour may be misleading, since its composition is totally different from that of the popular product commonly known as flour, i.e. wheat flour. They suggested alternative names such as “defatted soy solids” (as non-fat milk solids) or “soy powder” or “soy pulverate”.
Edible soy flours are made from dehulled beans, hence their relatively low crude fibre and high protein content.
Soy flours (or grits) are classified according to their lipid content as follows:
* Defatted soy flour, obtained from solvent extracted flakes, contains less than 1% oil.
* Full-fat soy flour, made from unextracted,dehulled beans, contains about 18% to 20% oil.
* Low fat soy flour, made by adding back some oil to defatted soy flour. Lipid content varies according to specifications, usually between 4.5% and 9%.The most common range is between 5% and 6%.
* High fat soy flour, produced by adding back soybean oil to defatted flour, usually at the level of 15%.
* Lecithinated soy flour, made by adding soybean lecithin to defatted, low fat or high fat soy flours in order to increase their dispersibility and impart emulsifying properties.. Lecithin content varies according to specifications, usually up to 15%.
Commercial soy flours and grits are further classified according to their Nitrogen Solubility Index (NSI), or their Nitrogen Dispersibility Index (NDI). It will be recalled that these parameters indicate the extent of protein denaturation and hence the intensity of heat treatment which has been applied to the starting material. Flours made from “white flakes” have NSI values of about 80%, while those made from toasted flakes show NSI levels of 10 to 20%. Other grades are available over the entire range of intermediate NSI values.The specification of a specific value of NSI reflects , in fact, a compromise between the need to maintain the functional properties of the soy proteins or some enzyme activity, and the desire to inactivate anti-nutritional factors and eliminate the beany taste, all in function of the end use.
The typical composition of different types of soy flours is given in Table 4-1. The basic composition of soybeans is added for comparison. Since the moisture content of the products may vary during storage, the percentage figures for protein, fat, fibre and ash are given on moisture free basis. A typical level of moisture content is also shown.
Table 4.1 Typical analyses of soybeans and soy flour
( Protein, fat, fibre and ash given on moisture-free basis; Protein % = N% x 6.25)
Material Protein Moisture Fat Fibre Ash
% % % % %
Full-fat soy flour
Defatted soy flour
Lecithinated soy flour 42.6 11.0 20.0 5.3 5.0
46.6 5.0 22.1 2.1 5.2
59.0 7.0 0.9 2.6 6.4
48.6 5.5 16.4 2.2 5.3
Source: Circle and Smith (1972)
4-2-3 Quality standards
In addition to the identity standards and definitions mentioned above, quality standards have been formulated by official agencies (e.g. FAO/WHO/UNICEF Protein Advisory Group). Trade specifications usually exceed the official standards. The quality parameters which constitute a specification usually include:
Protein a minimum value
Fat a maximum value for defatted flour a range for others
a range for lecithinated flours
Crude fibre a maximum value
a maximum value
Moisture a maximum value
b- Physical parameters:
Granulation as mesh number or particle size distribution.
Total plate count a maximum value
Coliforms a maximum value
Salmonella a maximum value (usually 0)
d- Heat treatment history:
as NSI, NDI, PSI or PDI
Tryps ininhibitor activity
Lipoxidase activity for enzyme-active flour.
e- Sensory parameters:
Insect parts a maximum value or total absence
Foreign material ” ” ”
Black specks ” ” ”
Packaging, delivery etc.
4.3 Full fat soy flour and grits
4-3-1 Producti on processes
a- Oil-mill related industrial production process: The process for the production of full fat soy flour and grits as a side line of large scale oil-mill operation is relatively simple. It consists of three major steps: dehulling, heat treatment and milling.
Cleaned, grade A yellow soybeans are dried, tempered, classified to separate split beans, cracked and dehulled by aspiration. These operations are essentially similar to the seed preparation steps of an oil-mill, from raw material silos up to the obtention of dehulled meats, and have been discussed in detail in the previous chapter. (Section 3-2-3, a to e ).
The dehulled meats coming out of the vibrating screen are now subjected to humid heat, to achieve the specified product NSI value. This is conveniently done in a vertical conditioner with direct and indirect steam heating sections.
This step is obviously omitted if the final product is to be unheated (enzyme active) flour. The last sections of the conditioner are used to dry the meats to a moisture content below 10%.
The properly conditioned and dried meats are cooled and then finely ground. Hammer mills, pin mills, impact turbo mills and similar pulverizers are used to grind the meats so that not more than 3% of the product will be retained by a 100-mesh screen. In practice , full fat soy flour is difficult to screen on such fine sieves, due to particle agglomeration. Air classification systems which separate the fine product and recirculate the coarse fraction through the mill are more adequate than screen sifters.
b- Alternative processes: In the framework of the efforts to promote direct consumption of soybeans in the less industrialized parts of the world, methods for the preparation of full-fat soybean flour with a minimal amount of processing have been developed. These methods permit production of flours independently of the oil industry.
One such process has been described by Mustakas et al.(1967) In this village scale production method, the soybeans are soaked in water, then cooked in boiling water, air dried, cracked by hand, winnowed to separate the hulls and finally hand ground in a mortar or any other grinding device available.
A more industrialized version of the process (Mustakas et al.(1970) is similar in most aspects to the large scale production process described above, except for the step of heat treatment. In this process the flour is submitted to a continuous high temperature-short time humid heat treatment , using an extruder-cooker. The dehulled meats are first equilibrated with moisture in a direct steam fed conditioner/ tempering bin, then cooked under pressure in a continuous extruder/cooker. The extrudate is cooled and ground as usual. The HTST treatment eliminates the beany flavour and produces a light, open structured flour. A slightly different process, also centred around extrusion cooking, known as the WENGER PROCESS , is available from the Wenger Mixer Manufacturing Co. More recently, low-cost extruders have been made available for the less sophisticated extrusion-cooking applications. “Low-cost” may mean $ 5,000, compared to $ 100,000 for a regular extrusion system. Such low-cost extruders have been used for the preparation of full-fat soy flour. According to Lorenz et al.(1980), the total investment needed for a 550 kg./hr plant was (1980) about $120,000 including building and land. The cost of production, including raw materials, packaging materials and overhead was $ 223 per ton. Extruded full-fat soybean flour was being produced with a low-cost extruder in Mexico in 1980.
In extrusion cooking, the material reaches temperatures in the order of 150oC. At such high temperatures, destruction of urease activity is no longer a credible indicator for the inactivation of trypsin inhibitor, which must be monitored directly.
The BUHLER PROCESS developed by Buhler Co. in Switzerland, is based on very fine grinding and fast heating. Since the resulting powder has been suggested as an alternative for soymilk solids, the Buhler Process will be described in the section dealing with soymilk (Chapter 8).
A process, based on pre-germinated beans has been described by Suberbie et al.(1981). The beans are soaked in water for 3 hours and allowed to germinate. At the end of the germination period, the soybeans are steamed, dried to 6% moisture, dehulled and ground in a cooled hammer mill. Germination resulted in flavour and odour improvement. Milling capacity was impaired by germination. Pre-germinated full-fat soybean flour has been produced commercially in Mexico.
The principal use of full-fat soybean flour, as well as re-fatted and lecithinated flours, is in the bakery industry. Two types of flour are used: enzyme-active and enzyme-inactive.
Enzyme-active full-fat soybean flour is prepared without heat treatment and has a high NSI value around 80%. It is used in bakery products (white bread and rolls), mainly for its lipoxidase activity. Lipoxidase catalyses oxidative bleaching of the carotenoid pigments in wheat flour. Enzyme-active soybean flour is a valuable “natural” flour bleaching agent, especially where the use of chemical bleaching agents has been prohibited. Lipoxidase activity is also beneficial to the mechanical properties of the dough. Since the soybean product is added in relatively small quantities ( up to 0.5% on flour basis in bread and buns in the U.S.A. ) the beany flavour of unheated soybeans is not a limiting factor. Usually, enzyme-active full-fat soy flour is not sold as such, but rather in mixtures containing other ingredients such as corn flour.
With the development of successful flash desolventizing systems which permit desolventizing without appreciable enzyme inactivation, defatted enzyme-active flours have largely replaced the full-fat product, especially in the U.S.A.
Enzyme-inactivated (heated) full-fat soybean flours, alone or with re-fatted and lecithinated soy flours, is mainly used in the heavier types of cake batters, such as sponge cake and pound cake. It contributes to the richness of the cake while increasing the proportion of water that can be added to the mix. Due to their oil and phospholipid content, these flours exert egg and shortening sparing effects and act as emulsifiers. In these formulae, soybean flours are used at the level of 3-5%, based on flour weight. Full-fat or lecithinated soy flour with high nitrogen solubility (NSI of 80%) has been found to improve eating quality and reduce fat absorption in doughnuts.
4.4 Defatted soy flours and grits
4-4-1 Production processes
The processes for the manufacture of raw or heated dehulled solvent extracted flakes have been described in the previous sections (Chapter 3).
Usually, all the flakes made for edible products are flash-desolventized, then carefully steam-heated to the desired NSI value.
The final milling is critical and energy consuming. Although identity standards require milling to 97% minus 100-mesh, specialty flours (such as those used as milk solids replacement in infant formulae) are ground to a finer particle size.
At such levels of fineness, the conventional hammer mill is practically useless. Impact turbo mills or high-speed pin mills have to be used.
a- Use in bakery and other cereal products:
Nutritionally, soybean protein is an excellent complement to lysine-limited cereal protein, hence the basis for the use of soy flour as an economical protein supplement in bread, tortillas, pasta and other cereal products.Supplementation of bread and other cereal staples with defatted soy flour has been promoted in a number of countries, and even enforced in some. The use of defatted soy flour in bread does not create any appreciable technological or quality problems, as long as less than 10% of the wheat flour has been replaced by soy flour. At higher replacement levels, up to 15%, loaf volume and crumb texture may be impaired. Baking quality can be recovered, however, by means of some adjustments such as higher yeast level, use of lecithin and other emulsifying agents etc.
Another bakery related potential use of soy flour in combination with cereals is in the production of the so-called “composite flours.” These are mixtures of flours, starches and other ingredients, supposed to replace wheat flour, totally or partially, in bakery products. Extensive research projects aimed at the development of such flours have been sponsored by international and national development agencies in the last 20 years or so. The main reason for developing composite flours is to relieve the economy of countries where wheat is not grown, from the burden of importing this commodity. Other reasons include the production of alternative baking flours for people who cannot tolerate wheat products (e.g. coeliac disease patients).
Considerable quantities of soy flour ( 1.5 to 2% on flour weight basis) are used in bakery products, particularly in white bread, as a replacement for nonfat milk solids. In this application, soy flour (and sometimes soy protein concentrate) is used in combination with whey solids. Milk replacer blends, consisting mainly of defatted soy flour, whey solids, caseinates and other nutritional or functional ingredients are available at protein content levels of 20% to 40%.
In many applications, especially in the U.S.A. and Europe, the largest quantity of soy flours is used in bakery products, not for nutritional reasons but rather for their functional characteristics.
Enzyme-active defatted flour is used as a bleaching and dough improving agent as discussed in the previous section dealing with full-fat flours.The characteristics of such a flour ( SOYBAR, made by Solbar Hatzor Ltd.), as reported by the manufacturer, are given below, as an example:
Product description: Enzyme active defatted soy flour, derived from high quality, dehulled soybeans. Has a mild flavour and aroma profile and a light cream colour.
Characteristics: Highly dispersible in water. Has excellent water binding properties.
Protein (as is) 50% min.
Moisture 10% max.
Crude fibre 4% max.
Ash 6.5% max.
Fat 1.0% max.
Particle size 95% less than 74 microns
Standard plate count 50,000/g. max.
Salmonella in 200g. Negative
E. Coli in 1g. Negative
Packaging: 20 kgs. net weight, in multi-ply, valve-pack, kraft paper bags with polyliner.
Defatted soy flours with 50-75% protein dispersibility are extensively used in bakery products. They increase the water absorption capacity of flours in bread dough and cake batters. In cakes,they improve film forming and even distribution of air cells. As a result, even cake texture and more tender crumb structure are achieved. In hard cookies, soy flour improves machining. In all these products, soy flour is used at the level of 2-5%.
More thoroughly toasted flours and grits are used to impart a pleasant nutty flavour to whole-grain and multi-grain specialty breads.
An important application of defatted soybean flour and grits in combination with cereals is in the production of nutritionally balanced all-purpose food blends, distributed to under-nourished populations or in cases of food shortage emergencies. The best known of these blends are: CMS (corn-milk-soy), developed in the U.S.A. by the Northern Regional Research Centre in cooperation with the American Corn Millers Federation, National Institute of Health and AID, CS (corn-soy) and WS (wheat-soy). More than 1.5 million tons of CMS have been distributed between 1966 and 1979. An “instant” CMS has been also developed. CMS can be used in soups, gruels, porridges etc. typically, CMS contains 17.5% defatted soy flour, 15% non-fat milk solids, about 60% corn. CS contains 22% soy flour and 71% corn. WS has 20% soy, 53% wheat bulgur and 20% wheat protein concentrate. Another well-known blend is INCAPARINA, developed by the Instituto de Nutricion de Centro America y Panama (INCAP), to fight children malnutrition. The oilseed protein source in the original formula of INCAP was cottonseed, but it has been replaced by soybean flour.
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