Thermal degradation of edible films based on milk proteins and gelatin in inert atmosphere

The thermal degradation of edible films based on pure sodium caseinate, whey and gelatin, and for these proteins in the presence of sorbitol as a plasticizer, was studied by thermogravimetry and infrared spectroscopy in nitrogen atmosphere. The presence of sorbitol significantly reduced the activation energy of the degradation of the edible protein films. This behaviour was in agreement with the decrease of the initial and maximum temperatures of degradation observed by thermogravimetry. The decrease in the thermal stability is apparently associated with the effect of sorbitol on the inter- and intramolecular hydrogen bonds of the proteins. The FTIR spectra showed that the effective degradation began at ca. 300 degreesC with the formation of gas products such as CO2 and NH3, 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 proteins. The suggested mechanism of reaction was supported by the high values of the activation energy (E > 100 kJ mol(-1)) which are probably associated with a process that occurred by random scission of the chain. (C) 2002 Elsevier Science Ltd. All rights reserved.