Food extrusion can be used to produce pasta and other cold-formed products, ready-to eat cereals, snacks, pet food, confectionery products, modified starches for soup, baby food and instant foods, beverage bases and textured (or texturized) vegetable protein (TVP). One of the more popular food ingredients in recent years is produced via extrusion cooking; TVP can be made from high soy protein soy flour or concentrate (some other material sources are cotton seeds, wheat and oats) and can be extruded into various shapes, from chunks to flakes, nuggets to grains to strips.
As Mian Riaz, Ph.D., of Texas A&M’s Extrusion Technology Program has described it, “(S)oy protein...today is on the hot list of ingredients for its ability to contribute to two top food trends--the continued quest for high-quality, low-fat foods and the thriving field of functional and nutraceutical foods.”1 As Riaz explains, soy protein concentrates and soy protein isolates have the benefits of a neutral flavor profile as well as high functionality. He notes, “Meat extenders are produced from the extrusion processing of defatted soy flour or flakes and soy concentrates, and they represent the largest portion of textured protein.”
As he mentions, a vegetable protein source can be converted directly into simplified varieties of meat analogs with the use of one or two extruders. The resulting meat analogs, he finds, bear a “remarkable similarity” to meat’s appearance, mouthfeel and texture.
There is more to extrusion than TVP, however. In fact, one study attempted to use wet okara (the residue remaining after the production of soymilk and tofu) to “produce and enrich extruded cereal products and to study the effects of extrusion on the dietary fiber and isoflavone content.”2 The researchers combined the okara with soft wheat flour to produce a pair of formulations, one with 33.3% okara, another with 40% okara. The authors analyzed various physicochemical properties, dietary fiber by enzymatic-gravimetric method, and isoflavone content by HPLC, and found, “The radial expansion ratio decreased as fiber content increased. On the other hand, both bulk density and breaking strength increased as fiber content increased. Combining okara with soft wheat flour resulted in increased protein, dietary fiber, and isoflavone contents compared with soft wheat flour alone. Extrusion of the formulations resulted in decreased insoluble fiber (≤25.5%) and increased soluble fiber (≤150%) contents of extrudates. Extrusion decreased the total detectable isoflavones (≤20%) and altered the distribution of the six detected isoflavones.”2
Extrusion cooking of defatted chickpeas, corn and bovine lung flours has also been looked at for its potential in malnutrition programs. In 2001, Brazilian researchers found, “Highly acceptable snack products were obtained by extrusion cooking of admixed defatted chickpea, corn and bovine lung flours. These snacks had a high-quality protein content and provided 30-40% of the iron RDA for children... The final product, a chickpea/bovine lung/corn snack, reduced to acceptable levels the prevalence of anemia among the population of young children in crèches of a poor Brazilian region. Sensory analysis showed a high acceptability for these snacks that was comparable to commercial brands.”3
As J.M. Dust et al. found in a 2004 paper appearing in the Journal of Agricultural and Food Chemistry, extrusion conditions can even have an effect on the chemical composition and in vitro digestion of select food ingredients. The experiment examined the hydrolytic and fermentative digestion of barley grits, cornmeal, oat bran and soybean flour, soybean hulls and wheat bran. The researchers found “organic matter disappearance (OMD) increased for extruded versus unprocessed samples of barley grits, cornmeal and soybean flour that had been hydrolytically digested.” In summary, they found the effects of extrusion conditions on ingredient composition and digestion are “influenced by the unique chemical characteristics of individual substrates.”4
While the use of extrusion equipment for texturizing soy flour and other proteins into useful meat extenders and replacers was initially limited to a narrow range of raw materials, Riaz explains developments in “machinery, new techniques and the art of texturizing protein products has changed, and as a result, the spectrum of ingredients that can be texturized into useable end products has increased greatly.”1
1. Mian N. Riaz, Ph.D., 2001. Textured Soy Protein and Its Uses. Texas A&M University.
2. V.E.A. Rinaldi, P.K.W. Ng and M.R. Bennink, 2011. “Effects of Extrusion on Dietary Fiber and Isoflavone Contents of Wheat.” Cereal Chemistry 77(2):237–240.
3. R.A. Santiagoa, R.A., R.S.R. Moreira-Araújob, M.E.M. Pinto e Silvaa and J.A.G. Arêasa, 2001. “The Potential of Extruded Chickpea, Corn and Bovine Lung for Malnutrition Programs.” Innovative Food Science & Emerging Technologies 10.1016/S1466-8564(01)00038-8.
4. J.M. Dust, A.M. Gajda, E.A., Flickinger, T.M. Burkhalter, N.R. Merchen and G.C. Fahey Jr., 2004 “Extrusion Conditions Affect Chemical Composition and in Vitro Digestion of Select Food Ingredients.” Journal of Agricultural and Food Chemistry 52(10):2989-96.