Influence of protein conformation on the properties of soybean protein edible films

ADRIANA N MAURI; GABRIELA DENAVI; M.CRISTINA AÑÓN; SARA MOLINA ORTIZ; MARIA A. BOSCH; PABLO SALGADO

Soy-Based Bioplastics

UK Publisher

Año: 2017; p. 75 - 98

The aim of this work was to evaluate how the initial soybean protein conformation affects the functionality of protein films, including its possible activation with antioxidant properties. Soybean protein films were prepared using two different soybean protein isolates, a commercial one (CSPI) with proteins fully denatured, and a native one (LSPI) obtained in the laboratory from soybean defatted flour by solubilization at pH 9, isoelectric precipitation at pH 4.5, redissolution and freeze-drying. Protein films were prepared by casting from aqueous dispersions of the protein isolates (5% w/w) at pH 10.5, using glycerol as plasticizer (2.5% w/w). Protein dispersions were molded and dried during 3 h at 60°C. Then, resulting films were conditioned for 48 h at 20°C and 58%RH prior to characterization. In order to analyze their potential activation with antioxidant properties, soybean protein films added with 25 mM ascorbic acid (AA) were also prepared.Both soybean protein isolates contained the same protein fractions, mainly β-conglycinin and glycinin ?determined by SDS-PAGE?. Proteins of LSPI, that maintain their native structure ?as was confirmed by DSC?, showed higher water solubility and lower surface hydrophobicity that the CSPI ones, which were fully denatured and had a higher protein aggregation degree due to the treatment undergone during its industrial production. These structural differences were manifested in the rheological behavior of the filmogenic dispersions. LSPI dispersions showed a Newtonian flow behavior, while CSPI exhibited a pseudoplastic and thixotropic flow behavior and showed the highest apparent viscosities over the entire range of shear rate gradient. Films obtained with both soybean protein isolates did not differ significantly in their water contents, thickness, water vapor permeability and glass transition temperature. Although both film protein networks were maintained by the same type of interactions ?covalent and non-covalent bonds? the intensity of each type of interaction ?predicted from solubility tests in buffers with different chemical action? was different for LSPI and CSPI films and was reflected in their mechanical properties. CSPI films ?mainly stabilized by disulfide bonds? showed higher tensile strength and elongation at break than the LSPI ones ?mainly stabilized by hydrogen bonds and hydrophobic interactions?. Although the addition of ascorbic acid to the formulations significantly increased the antioxidant activity of the protein films ?determined by ABTS? + assay?, LSPI films had higher antioxidant capacity than those of CSPI. Noticeably, the lesser cross-linking degree of LSPI film favors the release of the active ingredient.These results show the decisive importance of protein conformation on processing and functionality of resulting protein films.