Drying techniques impact functionality of soy protein isolate.

The applications of soy protein isolates generally are based upon their functional properties, such as solubility, emulsification and viscosity. These applications make the isolates a high value-added product.

Soy protein isolate is usually stored as a dry powder. The ingredient is dried to enhance its shelf life and make it easier to distribute. The commercial drying method often involves spray-drying the isolate.

The technique used at the bench scale for research purposes involves freeze-drying the isolate. Iowa State University scientists wanted to determine the functional properties of the isolates after using both drying techniques. The scientists wanted to relate them to the functionality of fresh undried isolate and frozen-thawed isolate. Four extraction temperatures--25 C, 40 C, 60 C and 80 C--were used to produce the isolates for each stabilization method.

The scientists determined the effects of the temperatures on isolate solubility, surface hydrophobicity, emulsification, foaming, thermal and rheological properties. There was a slight loss of the native state of proteins during both the freezing and freeze-drying processes. These losses were not as extensive when spray-drying was used. The emulsifying capacity was improved by all of the stabilization methods, but the emulsions obtained were less stable when the soy protein isolate was stabilized.

Viscosity decreased when spray-drying was used. But it increased with the use of freezing or freeze-drying. The foaming capacity decreased, but foam stability greatly improved with stabilizing. Solubility significantly decreased with freeze-drying, but no significant decrease was observed when spray-drying was used.

Surface hydrophobicity was significantly affected by all of the stabilization techniques that were tested. Functionality significantly changed during the drying process. Each drying technique yielded unique characteristics that were difficult to relate to one another.

In other work at Iowa State, scientists are evaluating new and improved lines of soybeans that have enhanced soy food processing characteristics. Rapid screening methods for measuring the potential end-use performance of soybean lots are also being developed. In addition, technologists are working to determine how the compounds associated with the off-flavors of soy food can be removed or inactivated. They are developing model systems that mimic the natural binding of off-flavor compounds to soy protein.

Further information. Lawrence Johnson, Center for Crops Utilization Research, Iowa State University, Department of Food Science and Human Nutrition, 1041 Food Sciences Building, Ames, IA 5001; phone: [phone]; fax: [phone]; email: [e-mail].