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Nutritional quality of soy protein is maintained after high-moisture extrusion.

Soy protein is an excellent source of high-quality protein and is readily supplied in a nutritional form, such as powders found in health food stores. Soy protein also is finding greater use in a wide variety of food products.

Soy protein may be unacceptable to consumers because of its taste and texture. Some food manufacturers have used a variety of techniques to mask undesirable bitter tastes, such as adding sugar, fat or salt to a formulation. But soy protein has characteristics that allow it to be processed through an extruder to alter the texture. Flavors may be readily applied in this process.

A highly palatable soy protein-based product, such as a fibrous meat analog processed by a high-moisture twin-screw extrusion process, could be more acceptable to consumers and open new markets for soy protein. But it is important to determine if high-moisture extrusion reduces the nutritional quality of soy protein. No systematic evaluation of the nutritional quality of soy processed in this manner has been performed. A longer-term feeding trial may be needed to further define the nutritional adequacy of this protein.

Scientists at Iowa State University and at the University of Missouri set out to compare the growth rate and markers of health in male and female mice that were fed diets for 90 days containing soy protein isolate. The isolate either had not been extruded or was processed with high-moisture extrusion. The two diets provided similar growth rates and food intake in both male and female mice. It appears that high-moisture extrusion may be a useful method to generate high-quality protein foods.

Bone parameters, including femur length and width and mineral content, were not affected by diet, although the breaking strength in the female mice was lower when they were fed the extruded diet. Levels of growth hormone, insulin-like growth factor 1 (IGF-1) and the insulin-glucose ratio were similar within gender across the diet groups. Cortisol levels and hematocrit (the proportion of blood volume that is occupied by red blood cells) were higher in male mice fed the extruded diet but not in females. Overall, the extruded soy protein was equally nutritious as the unextruded soy protein.

Further information. Ruth S. MacDonald, RD, PhD, Professor and Chair, Department of Food Science and Human Nutrition, Iowa State University, 2312 Food Sciences Building, Ames, IA 50011; phone: 515-294-5991; email:
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Author:MacDonald, Ruth S.
Publication:Emerging Food R&D Report
Date:Jun 1, 2010
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