Relationship between structural changes and functional properties of soy protein isolates–carrageenan systems

The aim of this work was to study the change in the structural and functional properties of a native soy protein isolate (I) induced by the incorporation of carrageenan (C). Two types of samples were used: samples lyophilized from mixtures of I and C dispersions (IC ratio: 20:1 to 8:1), and samples prepared in situ with I and C powders (IC ratio: 5:1 to 1:1). In both kinds of I:C samples a new endotherm (DSC), which may be attributed to the IC-complex, and an increase in the thermal stability of 7S and 11S soy globulins was observed. In in situ samples, at a high ionic strength and C content, an increase in the temperature of this new endotherm was observed. 7S fraction was thought to be further involved in the formation of the complex; result confirmed through experiments carried out with 7S-C and 11S-C mixtures. The complex formation modified the pH-solubility profile of soy proteins. As the amount of C increased, a decrease in solubility—to a greater extent in the acidic zone—was observed. The loss of solubility was accompanied by a decrease in the surface hydrophobicity of soluble fractions. Electrostatic as well as hydrophobic interactions would be involved in the complex formation, depending on environmental conditions. Concerning the rheological behavior of such systems, the addition of C allowed the formation of gels of a greater gelation velocity and higher viscoelasticity compared to I dispersions. Results showed that lyophilized I:C mixtures can be employed as gelling agents with improved properties regarding independent ingredients. On the other hand, gelation of I dispersions of very low protein concentration can be performed through the addition of carrageenan in appropriate proportions.