Stability enhancement of shellac by formation of composite film: Effect of gelatin and plasticizers

The objective of the present study was to reduce the polymerization of shellac by the formation of composite films with gelatin. The 6% w/w of composite film based on shellac and gelatin was prepared by the incorporation of different concentrations of gelatin and was prepared by film casting method. The stability of the composite films was then studied at 40 °C, 75% RH for 180 days in a stability chamber and the physicochemical properties were investigated. The results demonstrated that the higher concentrations of gelatin (30%, 40% and 50%) contributed to slight change in the acid value and the percentage of insoluble solids upon 180 days of storage. These results were due to the protection at the active sites of the shellac, carboxyl and hydroxyl groups, by the electrostatic interaction between the negative charge of shellac and the positive charge of gelatin, resulting in the stabilized shellac. However, the mechanical properties could not withstand the storage time; further investigation was required to improve the stability in terms of mechanical properties of the composite film by the addition of a plasticizer. Two types of plasticizers were used: polyethylene glycol (PEG) and diethyl phthalate (DEP) represented hydrophilic and hydrophobic plasticizers at the concentrations of 5% and 10%, respectively. The addition of both types and concentrations of plasticizers could improve the poor mechanical properties and brittleness of the composite film for a longer storage time. However, only PEG 400 at 5% and 10% w/w acted as a good plasticizer capable of improving the stability of the composite film under the longer periods of storage. Therefore, the attempt to improve the stability in terms of polymerization and mechanical properties of shellac by the formation of the composite film and the addition of the plasticizer could be achieved. The result could be used to develop a new material for the various applications of edible films for coating in food and pharmaceutical industries.

► Instability of shellac could be improved by composite film with gelatin.
► Electrostatic interaction of shellac (COO−) and gelatin (NH3+) could be formed.
► Plasticizers could improve brittleness of composite film for longer storage time.
► PEG 400 at 5% and 10% showed the plasticizer that improved the stability of film.
► Composite film could contribute to the application of edible film and coating.