May 1, 2006
Soybeans, rich in protein and oil, are an important cereal crop. Although bakers are more familiar with soy oil, soy proteins offer baked foods strong health appeal benefits.
Dehulled soybeans contain 39% protein and 21% oil, plus 25% carbohydrates, 10% moisture and 5% ash.
During processing of soybeans after the oil is extracted, the meal is heated to reduce the level of anti-nutritional factors, mainly trypsin inhibitor. Heating also partially denatures soy protein, rendering it less soluble than unheated soy flour. The extent of denaturation is measured as the protein dispersibility index (PDI), which is the percentage of total protein that can be dissolved in water. This may range from 95 for enzyme-active soy flour to 10 for toasted soy flour.
SOY PROTEIN PRODUCTS
After the fat is extracted with solvent, the soy meal is heated to remove traces of solvent. The enzyme lipoxygenase and other enzymes are still largely active; grinding the meal gives enzyme-active soy flour. This lightly-heated flour is also desired for further processing into other soy protein products.
Washing defatted soy meal with aqueous ethanol at pH 4.5 removes the soluble sugars and some ash. The product, soy concentrate, contains 65% to 70% protein, and it is often used in replacers for non-fat dry milk in bakery applications. It is also widely used for making extruded textured soy proteins for other food uses.
The protein in defatted meal may be dissolved in a dilute alkali solution. After the insoluble fiber is removed, the pH of the extract is adjusted to 4.5, causing the protein to precipitate. The protein “curd” is washed; then it may be dried, giving isoelectric soy protein. For a different functionality, the washed curd may be redissolved by adjusting the pH to 7, then dried to make isolated soy protein (ISP). By modifications in the process, a wide range of functionalities can be produced.
The dehulled soybean does not have to be defatted before being ground to flour. Full fat soy flour is sold for many bakery applications. Because it receives no heat treatment, the lipoxygenase is fully active, and the soy oil present is a substrate for the enzyme.
The addition of lecithin, usually about 6% of flour weight, to a defatted soy flour gives lecithinated soy flour.
This product finds use where emulsification is important, such as in cake systems.
The insoluble residue obtained during protein isolation is made up mainly of various hemicellulose materials. After appropriate purification, this material is sold as soy fiber, containing 80% total dietary fiber.
Soy flour or ISP is included in formulations for milk replacers, used as substitutes for non-fat dry milk in many bakery products. There are numerous combinations, but a common one combines 60% defatted soy flour, with a PDI greater than 70, and 40% dried sweet whey. The analytical values are very close to those of non-fat dry milk.
White bread and rolls: Enzyme-active soy flour at no more than 0.5% (flour weight basis) forms hydroperoxides from linoleic acid in shortening. These react with gluten, increasing its strength and the volume of the baked bread. Defatted soy flour at 1% to 3% (flour weight basis) increases absorption by 1 lb water per 1 lb soy flour and improves crumb body, resilience and crust color.
Specialty bread. Defatted soy flour, soy concentrates or isolated soy protein increase the protein level and nutritional value in high-protein bread. At levels of soy flour above 8% (flour weight basis), additional oxidation is needed to maintain dough strength; 100 ppm ascorbic acid will provide the needed strengthening. Soy fiber is a good source of dietary fiber and is used in the production of highfiber bread.
Cakes. Lecithinated soy flour provides emulsification in cake batters, both from the lecithin and from soluble soy protein. Used at a level of 3% to 6% in white or yellow cakes respectively, lecithinated soy flour cuts the requirement for egg and milk solids by 25% to 50%. Water addition is increased to keep batter viscosity constant, and the resulting cake has a moister crumb and better keeping qualities.
Cake donuts. Addition of 2% to 4% defatted soy flour improves water retention and decreases fat absorption during frying. Lecithinated soy flour or isolated soy protein has the same effect, but the extra cost is not normally justified.
Yeast-raised donuts. When defatted soy flour is added at 2% (flour weight basis) to yeast-raised doughs that are fried (donuts, bismarcks and similar products), it increases the moisture retained by the product. The soluble protein improves crust browning, giving a more pleasing finished appearance.
Sweet goods. Defatted soy flour at 2% to 4% (flour weight basis) improves the water-holding capacity and sheeting properties of sweet doughs. The enhanced sheeting strength produces better layering during the fat roll-in process and a more tender finished product.
Cookies (biscuits). The addition of 2% to 5% defatted or lecithinated soy flour to a cookie dough improves dough machining, especially for sheeted biscuits such as Maria-style items, and imparts a crisp but tender texture to the cookie. High-protein cookies are sometimes included in specialty diets. By using isolated soy protein, cookies with a protein level as high as 23% can be made. For more modest protein enhancement, substituting a toasted soy flour with a low PDI for 10% to 20% of the wheat flour gives a dough that machines and bakes much like the unmodified formula. At 20% substitution, the protein level in the cookie is doubled.