World Protein Resources

8 Full-Fat Soybean Flours

Downloaded by UNIV OF TENNESSEE KNOXVILLE on June 18, 2016 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0057.ch008

by Continuous Extrusion Cooking GUS C. M U S T A K A S , E D W A R D L . G R I F F I N , JR., and V I R G I L E . SOHNS Northern Regional Research Laboratory, Peoria, Ill. 61604

Extrusion processing, a method continuing to find new and wider applications in the food industry, is adaptable to the production of full-fat soybean flour, and can produce an edible flour of good flavor, oxidative stability, and high nutritional value. Experimentally produced flours were evaluated by chemical analyses, biological assays, available lysine content, vitamin assays, organoleptic tests, oxidative-stability storage tests, and clinical testing with infants up to 12 months old. The future potential of this process for soybeans seems promising, owing to unique characteristics which lend themselves to modern high-production, low-cost techniques that can yield a high-quality product.

A

deficiency in protein is one of the important dietary problems en countered in many of the developing countries of the world. Although animal products are a desirable protein source, their high cost and limited availability restrict their use. Consequently, there is a large and growing need for high-quality, low-cost plant proteins. The protein-deficiency problem is particularly serious with infants and preschool children, es pecially in tropical and subtropical areas of the world. Although powdered milk has been of major importance in feeding programs, its supply is limited, and there is a great need for additional sources of food protein. U N I C E F nutritionists have observed that deficiencies in the diet of both children and adults in newly emerging countries are not just from a lack of protein; they arise from a shortage of both proteins and calories. If calories are limited, proteins are consumed for the necessary energy and, therefore, are not available for body building. Full-fat soybean flours would serve both purposes because soybeans contain an oil high i n calories and because flour from them can be formulated easily with i n digenous foods. Since these flours would contain all the dehulled soybean, 101 Altschul; World Protein Resources Advances in Chemistry; American Chemical Society: Washington, DC, 1966.


102

WORLD PROTEIN RESOURCES

they represent a concentrated form of food and a most economical source of protein as well. For use of the flour in beverages, the natural lecithin in soybeans lends emulsifying properties to give a powder easily and rapidly mixed with water to produce an instant dispersion. Introduction of foods of an unfamiliar type can be expected to take time for acceptance. W e cannot expect people in need of protein supple mentation to like" the kind of foods we do. Instead, the protein must be incorporated into the traditional foods of these populations. For the broadest use of soybean flours, the mild, but characteristic, flavor of soy beans must become acceptable to those not familiar with it. Extrusion cooking has good potential for making desirable forms of soybeans econom ically available to developing countries. Extrusion is now widely applied by the American food industry for shaping new products and simplifying production of older ones. Some new food areas where extrusion processing is being used are pet foods, breakfast cereals, confections, bakery goods, dough processing, cheese, macaroni, sausage, and the many crispy snack terns.

Extrusion Cooking of Soybeans In developing a system for cooking whole-fat soybeans, the objectives are to achieve a cooked soybean flour product with a high nutritional value, to cook the beans adequately to control the growth inhibitors in soybeans, and to produce a bland and palatable product substantially free from unpleasant bitterness or "beany" taste and with adequate shelf life without refrigeration. Studies were previously carried out and reported (1, 2) jointly by our laboratory, U N I C E F , and the Wenger Mixer Manufacturing Co. on a new method for producing full-fat soybean flour based on extrusion cook ing. Flours of high nutritive value, stability, and flavor acceptance were produced during the experimental program. The process for full-fat extrusion cooking is carried out as shown in Figure 1. The soybeans are first cracked, dehulled, and flaked, although the flaking step is optional. The material is then preconditioned in two stages with both direct and indirect steam to add water and heat, mixed, extrusion-cooked, cooled, dried, and milled to a flour. Flakes or grits initially have an average of 9 to 1 2 % moisture and the preconditioner adds additional moisture as steam, bringing the product up to approximately 2 0 % moisture on extrusion. Retention time in the preconditioner and mixer is approximately 3 minutes. The extruder-cooker has the configuration shown i n Figure 1. It consists of at least three sections separated by back-pressure dies (perforated plates). The unit shown is a production model of that used i n our cooperative work with Wenger. Throughput is approximately 70 pounds per minute.

Altschul; World Protein Resources Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

8.

MUSTAKAS E T YL.

103

FuU-Fat Soybean Flours

\

Whole Soybeans

/

Τ —~


^Cracking - Dehulling^ τ """" Ί \ Flaking / Gyts

Figure 1. Continuous extrusion-cooking process for producing full-fat soybean flour During passage through the various sections, the material is subjected to increased pressure and temperature. Pressure is increased by pitched screws separated by the perforated plates that create back-pressure and prevent feedback of oil through the feed inlet, which is at atmospheric pressure. Finally, the maximum pressure is created i n the last taperedscrew or cone-nose section. The first two segments are steam-jacketed, but heating is accomplished primarily by mechanical friction of the screw on the material, particularly i n the final segment. Retention time i n the extruder is 1 minute. During extrusion, oil is expelled and freed from the cells, but as the material extrudes from the final die plate, the oil is immediately reabsorbed i n the meal. Freeing the oil from the cells should make i t readily available for digestion. Figure 2 shows a time-temperature profile of the cooking process. Extrusion-cooking should offer many unique features that will enable it to grow i n importance as a process for converting soybeans to food products. B y this process, the beans are held at elevated temperatures for only a few minutes, i n contrast to commercial soybean meal toasting at about 212° F . for 30 to 45 minutes. Thus, almost instant cooking is achieved under continuous operation, which results i n a minimum of damage to heat-sensitive, nutritional factors. Temperature of the meal in the final high-pressure section reaches values above atmospheric boiling;



104

WORLD PROTEIN RESOURCES

however, moisture is not lost, and the extruder section acts as a continuous pressure cooker. Diversified applications, flexibility, and simple control are valuable features of the extrusion process that appeal to the food industry. The character of a product can be controlled by altering extrusion temperatures, pressures, or both, to change product viscosity, taste, moisture content, water absorption, and protein solubility. Extrusion-cooking of cereal products can pregelatinize starches. B y incorporating other chemicals one can modify the product chemically on a continuous basis. Physical shapes and bulk densities of the product can be controlled by using dies in various forms, shapes, and thicknesses. These and other possible varia tions i n processing make extrusion-cooking a highly flexible operation. J

Preconditioning

I

Mixing Extruder Cooking

2

Drying Cooling

3

Time, minutes Time-temperature profile of extrusion-cooking process

Discussion Removal of Growth Inhibitors. The short cook time with extrusion minimizes damage to nutritional properties but still adequately removes growth inhibitors. This removal is demonstrated by low assays for trypsin inhibitor and urease activity. Representative products analyze 9 5 % or better destruction of trypsin inhibitor and have low to zero urease activity. Proximate Analyses of Flours The soybean protein contains a good balance of the amino acids essential to human nutrition and, in this respect,



8.

MUSTAKAS ET A L .

Full-Fat Soybean Flours

105

tends to be comparable to animal proteins. It is a good source of lysine, more or less lacking in cereal proteins. The extrusion-cooked products average 5.3% available lysine. Table I gives a proximate analysis of extrusion-cooked flours; these values compare favorably with commercial flours. In most cooking processes, the percentage of water-soluble protein, as measured by the nitrogen solubility index (NSI), generally reflects the degree of heat treatment received by the product. Extruded flours are cooked to values ranging from 13 to 21. Protein solubility correlates inversely with processing temperature. The higher the maximum tem perature, the lower the protein solubility. In commercial processing of soybeans, the heat treatment generally reduces their vitamin content. Soybeans are outstanding as a source of thiamine, but this vitamin is heat-sensitive and generally heat treatments so necessary to prepare soybeans for animal and human consumption result in destroying thiamine. Assays for thiamine, riboflavin, and niacin on the extruded products indicate no significant loss of original vitamin content during processing. Table I. Proximate Analysis of Extruded Soybean Flours Protein, % Crude fat, % Crude fiber, % Ash, % Acid-insoluble ash, % Available lysine, % of protein Peroxide value, meq./1000 g. extracted oil Free fatty acid, % in extracted oil Granulation

44 20

World Protein Resources

Recommend Documents