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WHO Guideline

Guideline for production of edible, heat processed soy grits and flours


Guideline for production of edible, heat processed soy grits and flours

1. Introduction

This Guideline provides a description of desirable processing and quality characteristics for edible, heat-processed soy grits and flour of the defatted type (Product A) and of the full-fat type (Product B). The sanitary quality and the nutritive value of the protein component are of paramount importance because the food may be a major source of protein for young children. Good manufacturing practices should be observed in all aspects of processing, including quality control, raw material selection, sanitation, packaging, and handling. Pertinent information concerning these factors will be found in various references cited. Widely practiced technology for edible soybean meal production is discussed in papers by Horan,5 Myers,8 and Smith.9 Extrusion processing of full-fat soy flour is described by Mustakas, Bookwalter, et al.4,7 A village process has been proposed by Mustakas et al.6

Defatted soy flour (Product A) is defined as the screened, graded product obtained after pressing or solvent-extracting the oil from sound dehulled soybeans. A full-fat soy flour (Product B) is not subject to pressing or extraction and contains all of the oil originally present in the dehulled beans. The term "flour" generally signifies that the material has been ground finely enough to pass through at least a 100 mesh (USA) screen. "Soy grits" refers to particles of larger size.

2. Process

2.1 Soybeans used to produce grits or flours should be sound, clean, and free from other seeds or foreign matter.

This raw material should contain minimal quantities of split seeds, a moisture content not in excess of 13 per cent, and less than 1.5 per cent of free fatty acids in the freshly expressed seed oil.

2.2 Except as noted, manufacturing procedures are those generally used for conventional soybean meal. The cleaned beans are dehulled by cracking, followed by thorough removal of hulls. Proper dehulling is indicated by acceptably low fibre levels and freedom of the product from dark specks.

2.3 In the case of Product A, the hull-free meats are coarsely ground, lightly steamed, flaked, and then solvent-extracted in conventional apparatus with hexane of a quality acceptable for food processing. The defatted flakes are steamed at atmospheric pressure to completely remove the solvent and are additionally steam-cooked ("toasted") to produce a palatable, cream-yellow product free from objectionable flavour and anti-biological factors.

2.4 Heat processing should be carefully controlled and sufficiently thorough to secure optimum flavour and palatability, as well as to inactivate objectionable anti-biological factors (e.g., anti-trypsin and hemagglutinins). It must not be so severe as to damage the protein, as evidenced by excessive loss of available lysine, or impairment of protein nutritional value. Once optimum improvement in these factors has been attained, the product, after final heat treatment, should be cooled quickly to ambient temperatures. Similar care must be exercised in the processing and drying of extrusion-cooked products (Product B).

2.5 The fully processed flakes or granules should have a light yellow coloration that, in a more finely granulated form, assumes a creamy appearance. The flavour may be described as toasted and bland, completely free of the characteristic raw soybean flavour.

2.6 The defatted or full-fat products may be ground or otherwise processed to provide grits, flours, or other forms suitable for various food uses. Grits (fine, medium, coarse) would have particle sizes ranging between 10 and 80 (US sieve No.), equivalent to 2,000 and 177 microns, respectively. Flours are considered to have granulations finer than would pass through a No. 100 sieve (149 microns).

3. Product Analysis

3.1 Composition. The products should have the characteristics indicated in Table 1 when analyzed by appropriate procedures of the Association of Official Analytical Chemists (AOAC),3 American Oil Chemists Society (AOCS),2 the American Association of Cereal Chemists (AACC),1or equivalent recognized methodologies. Factors other than moisture are expressed on a dry weight basis.


A, defatted B. full fat
Moisture % (maximum) 12.0 10.0
Crude fat % 0.5 - 3.0 18 - 22.
Protein % (N x 6.25) 45 - 52* 38 - 44.
Crude fibre % (maximum) 3.5 3.5
Ash % (maximum) 6.0 6.0

* Range indicated is due to normal variation in seed composition,

3.2 Methods suitable for control of heat processing. One or more of the following tests will be found useful for evaluating the degree of heat treatment, which is a primary determinant of protein nutritive value. After uniform or standardized production is achieved, the Urease Activity Test can be considered as sufficient for routine checking and control. The other tests (Protein Dispersibility Index, Available Lysine, and Nutritive Value by Animal Assay) are helpful in confirming product nutritional quality.

3.2a Urease activity. AOCS method Ba-9-582 or AACC method 22-90.1 This procedure, which quantifies the loss of activity with heating of a major enzyme constituent, provides an indication as well of the degree of inactivation of anti-biological factors such as trypsin inhibitor (see paragraph 2.4 above). An increase in pH of 0.3 or less suggests that the product retains slight urease activity but has probably received sufficient heat treatment for inactivation of anti-biological factors and for development of good protein quality. A defatted soy flour showing an increase in pH greater than 0.3 would be suspected of having received insufficient heating for optimum protein nutrition. A product with an increase of 0.02 or less should be suspected of having been over-heated; however, this test is not considered adequately sensitive for determination of excessive heat treatment.

3.2b Protein Dispersibility Index (PDI). AOCS method Ba-10-652 or AACC method 46-24.1 Water-dispersible protein, expressed as per cent, is negatively correlated with extent of heat processing. Values in the range of 10 to 30 per cent probably indicate adequate heat treatment for good protein nutritional value. Higher values are associated with under-heating, and lower numbers indicate over-heating. Coarse materials, prior to determination, should be reduced to at least 100 mesh granulation.

3.2c Available lysine. AOAC 12th ed., 1st suppl.3 The minimum available lysine for an acceptable product would be 59/15 g of nitrogen in the product. Lower values indicate loss of lysine due to heat damage of the protein.

3.3 Nutritive value by animal assay. When these products are fed to rats at a protein level of 10 per cent for four weeks, the Protein Efficiency Ratio (PER) should be 2.0 higher (corrected to a standardized value of 2.5 for a control casein diet). (See AOAC methods.3) Net Protein Utilization (NPU) value should be 60 or higher.

3.4 Sanitary analyses

3.4a Microbiology. The soy grits or flours should be free of E. coli, Salmonella, or other pathogens. The total bacterial plate count should not be more than 50,000 per gram when assayed by methods of the AOAC3 or equivalent accepted methods, e.g., AACC method 42-11.1
3.4b Acid insoluble ash. AOAC method 31.0153 or AOCS tentative method Ba-5a-68.2 This analysis indicates relative contamination with extraneous minerals such as sand and dirt. The value in any event should not exceed 0.1 per cent.
3.4c Insect and rodent contamination. AOAC method 44.047.1Edible soybean grits and flours should be free of insects, insect fragments, rodent hairs, and rodent excrete, as determined by the method indicated.


1. American Association of Cereal Chemists, Cereal Laboratory Methods (AACC, St. Paul, Minn., USA, 1968 [reprinted 1976] ).

2. American Oil Chemists Society, Official and Tentative Methods, 3rd ed. IAOCS, Champaign, III., USA, 1968).

3. Association of Official Analytical Chemists, Official Methods of Analysis, 12th ed, and supplements, (AOAC, Washington, D.C, USA, 1975).

4. G.N, Bookwalter, G.C. Mustakas, W.F. Kwolet, J.E. McGhee, and W.J. Albrecht, "Full-Fat Soy Flour Extrusion Cooked: Properties and Food Uses," J. Food Sci., 36:5 (1971).

5. F.E. Horan, "Soy Protein Products and Their Production," J. Amer. Oil Chem. Soc., 51 :67A (1974).

6. G.C. Mustakas, W.J. Albrecht, G.N. Bookwalter, and E.L. Griffin, Jr., Full-Fat Soy Flour by a Simple Process for Villagers, USDA Northern Res. and Development Div., Peoria, III., USA, ARS-71-34.

7. G.C. Mustakas, W.J. Albrecht, G.N. Bookwalter, J.E. McGhee, W.F, Kwolek and E.L. Griffin, Jr, "Extruder-Processing to Improve Nutritional Quality, Flavor and Keeping Quality of Full-Fat Soy Flour," Food Technol., 24:1290 (1970).

8. N.W, Myers, "Solvent Extraction in the Soybean Industry," J. Amer. Oil Chem. Soc., 54:491A (1977)

9. K.J. Smith "Soybean Meal: Production, Composition and Utilization " Feedstuffs, 49(3):22 (1977).

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