Properties of Biodegradable Films Made with Proteins Extracted from Castor Bean (Ricinus communis) Cake: Effect of Protein Extraction pH

Abstract

Aiming to increase the value of the castor bean cake through new technological applications, the objective of this study was the development of a biodegradable film using proteins extracted from castor bean cake. Specifically, the target of this work was to study the effect of protein extraction pH (10, 11 and 12) on some physical properties (mechanical properties, water solubility and moisture) of the films. The protein extraction was carried out in a reactor with 20 % castor bean cake dispersed in NaOH solutions (pH=10, 11 and 12), with mixing speed of 400 rpm and at 50 °C. The separation of the protein extract was made through centrifugation (4,000 rpm). The extracts were freeze-dried and analyzed to determine their chemical compositions. Sorption isotherms of these proteins were determined at 25 °C. The films were produced by casting, i.e., by dehydration of film-forming solutions prepared with constant concentrations of protein (7.5 g protein/100g film-forming solution); cross linking agent (0.8 g glutaraldehyde/100 g protein), and plasticizer (25 g of glycerol/100 g protein). The mechanical properties were determined at 25 °C by tensile and puncture tests and solubility in water and moisture were determined gravimetrically. The freeze-dried protein extracts constituted protein-rich (66-69 %) material at all studied pHs, and also mineral-rich (12-24 %) ones, the latter possibly due to the use of NaOH. No effect of the protein extraction pH was observed on the protein hygroscopicity. The films produced in this study had brownish color and homogeneous aspect, independently of the protein extraction pH. The pH influenced the mechanical properties of the films. Higher pH values provoked higher tensile strength and puncture force values: 4.5±0.1 MPa and 6.5±0.2 N, respectively, for the films made using the proteins extracted at pH 12. However, independently of the observed differences, those films were little extensible, with elongation at break and puncture deformation around 1.5 % and 3.2 %, respectively. Also, the film solubility in water was affected by the protein extraction pH. While films produced using the proteins extracted at pH=10 were totally water soluble, films of the proteins extracted at pH=12 had 58.5±0.2 % solubility. The moisture of the films was not affected by the pH, remaining around 13 %. In conclusion, the pH of the castor bean protein extraction has influence on the properties of the resulting films, without necessarily affecting the protein content of the freeze-dried extract.