Optimization of the potato starch film preparation process and release kinetics of thymol in active packaging

Essential oils extracted from natural plants can serve as safe food additives, compared with conventional chemical or synthetic additives. Essential oils have also been incorporated into food packaging materials to impart their bacteriostatic and antioxidant properties. Among them, thymol (THY), a monoterpene phenolic compound extracted from the thyme plant, has been used as a bacteriostatic agent in active packaging materials with functional properties, due to its significant bacteriostatic properties. However, the activity of highly volatile THY is easily affected by the packaging environment. Therefore, appropriate techniques are needed to incorporate the THY into polymer matrices, in order to improve its stability and utilization for controlled release. This study aims to prepare the potato starch films of active packaging, taking the mesoporous nano-silica (mobil composition of matter No. 41, MCM-41) as the controlled-release carrier of THY (THY-MCM-41), potato starch as the film-forming substrate, glycerol as the plasticizer, and calcium chloride as the cross-linking agent. Single-factor experiments were carried out to determine the suitable additive ranges of potato starch, glycerol, calcium chloride, and THY-MCM-41 mass concentration in the packaging film. The L9 (3⁴) orthogonal test was also implemented to investigate the interactive effects of potato starch, glycerol, calcium chloride, and THY-MCM-41 on the thickness, opacity, moisture absorption (M_A), water vapor permeability (W_VP), tensile strength (TS), and elongation at break (EB) of the packaging film. The optimal conditions were obtained to prepare the potato starch films of active packaging. A systematic investigation was made to clarify the THY release kinetics in the optimal combination of packaging film at different temperatures and relative humidity (RH) environments. A prediction model of THY release was constructed to optimize the THY. The results showed that the optimal properties of the prepared packaging film A were achieved in the TS, MA, WVP, oxygen permeability (OP) and opacity were 7.16 MPa, 89.23%, 1.42 × 10^-10 g/(m·s·Pa), 1.02 × 10^-15 cm²/s·Pa, and 1.16 mm^-1, respectively, when the mass concentrations of potato starch, glycerol, calcium chloride and THY-MCM-41 were 0.04, 0.015, 0.005, and 0.005 g/mL, respectively. Scanning electron microscope (SEM) and Fourier-transformed infrared (FTIR) spectroscopy confirmed that the denser and more uniform microstructure of packaging film was observed, where THY-MCM-41 was uniformly dispersed in the potato starch film, indicating better compatibility with potato starch. Thermogravimetric (TG) analysis showed that the encapsulation of MCM-41 improved the stability of THY in the potato starch film. In addition, the release kinetics of THY in the potato starch film of active packaging demonstrated that the initial explosive release was shifted to a subsequent sustained release under different temperature-RH environments. The active packaging film was prepared to effectively control the release rate of THY for the effective time of THY up to 10 d. The release pattern of THY was in accordance with the First-order release model (R² > 0.980), and the release index “n” was less than 0.5, indicating that the release behavior of THY followed Fickian diffusion law. This finding can also provide the theoretical foundation for the precise release of active substances in the development of active packaging films.