SOY PROTEINS AT OIL-WATER INTERFACE : A FLUORESCENCE STUDY

Soy proteins are one of the most attractive plant food proteins for human and animal nutrition for their nutritional and physicochemical proprieties. Glicinin and-conglycinin are the most abundant soybean storage proteins. Glycinin (11S) is an heterohexamer with two symmetric trimers stacked on top of one another while β-conglycinin (7S) is a heterogeneous trimeric glycoprotein, composed by three subunits. Soy proteins readily adsorb at the interface of an oil water emulsion with homogenization, but very little is yet understood on the details of the structural changes at the interface. The aim of this work is to study the structural changes of soy proteins in solution and compare it to those at the oil-water interface, with focus on heat-induced changes. Fluorescence spectroscopy was applied on solutions and emulsions containing -conglycinin or glycinin, and on soy protein isolate (SPI) to evaluate tertiary structural changes, along with the binding of fluorescent dyes (ANS), and accessibility of reactive cysteine thiols. Protein conformational changes after interaction with the hydrophobic oil surface were compared with those ensuing from physical (heat) or chemical denaturation (by added chaotropes). Results from solution denaturation experiments show that denaturation of -conglycinin solutions by both heat and chaotropes is reversible under appropriate conditions, leading to a rearrangement of the supramacromolecular assembly of the protein structure. On the other hand, glycinin treated under the same conditions underwent irreversible denaturation in solution. -conglycinin underwent partial denaturation after adsorption on the lipid surface. This denaturation is reversible after protein displacement from the interface. Glycinin denaturation at a lipid interface reflected its solution behaviour. Glycinin undergoes a partial denaturation at the surface of hydrophobic droplets, and gave no indication of structural recovery after displacement from the interface.