Thermal stability of films formed by soy protein isolate-sodium dodecyl sulfate

The thermal degradation of films formed by soy protein isolate (SPI)-sodium dodecyl sulfate (SDS) complex was studied by thermogravimetry and infrared spectroscopy under nitrogen. The presence of SDS markedly reduced the activation energy in the degradation of SPI based films. This behaviour was in agreement with the decrease of the initial temperature of degradation observed by thermogravimetry. The SPI/SDS interaction (electrostatic and hydrophobic) apparently decreased the thermal stability of the films by disrupting low-energy inter-molecular bonds that maintain the protein conformation. The FTIR spectra of gas products evolved during the thermal degradation indicated the formation of CO2, CO, NH3 and other unsaturated compounds, suggesting that the reaction mechanism included at the same time the scission of the C-N, C(O)-NH, C(O)-NH2, -NH2 and C(O)-OH bonds of the protein. The suggested reaction mechanism was supported by the high values of the activation energy (E > 100 kJ mol−1) which are probably associated with a process that occurs by random scission of the chain.