Preparation and characterization of pH-sensitive intelligent oak tree acorn starch/halloysite nanotube/quercetin composite films

In this study, new bioplastic composite films based on oak tree acorn starch were prepared by using starch obtained from oak tree acorns widely grown in our country. In addition, natural quercetin was extracted from purple onion skin and the different amounts of it was added to the film matrix to make the film pH sensitive. Halloysite nanotube (HNT) was doped into the starch film matrix to improve the mechanical and thermal properties of the bioplastic film. Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), opacity, color, water contact angle, mechanical strength, thermal gravimetric analysis (TGA) and water vapor permeability analyzes were performed on the prepared bioplastic film samples. It was understood by the SEM analysis performed that the film surface became rough with the addition of HNT into the starch film matrix. With the addition of quercetin into the rough starch films containing HNT, it was observed that the roughness in the film structure was dispersed, and the film became more homogeneous. In the opacity and transmittance analyzes performed, it was determined that the increase in the opacity values of the films and the decrease in the % transmittance values of the films with the addition of quercetin occurred in parallel with the amount of quercetin. Compared to the starch film, the addition of 3% HNT into the film matrix caused an increase of 31.2% in the ΔE value of the Starch-3% HNT film, while the addition of quercetin caused an increase of 146.1% in the ΔE vallue of the Starch-Quercetin 1 film. The addition of quercetin into the starch film matrix decreased the water contact angle value of the starch film from 37° to 27°. When compared with the Starch-Quercetin composite films, it was found that the water contact angle value exhibited an increase up to 40° in Starch-HNT-Quercetin films. The addition of quercetin into the starch film matrix has an opposite effect compared to HNT on the mechanical properties of the starch film. Compared with the mechanical properties of the starch film; the addition of quercetin to the film matrix caused a decrease in the maximum tensile stress and Young's modulus values of the film and an increase in the maximum tensile strain value in parallel with the amount of added quercetin. Among the prepared films, the highest maximum tensile stres (14.53 MPa) value for the Starch-HNT-Quercetin 1 film, the highest maximum Young's modulus (1413.21 MPa) value for Starch-%3 HNT film and the highest maximum tensile strain (10.73%) for Starch-Quercetin 10 film were recorded. The addition of quercetin and HNT into the starch film matrix increased the thermal stability of the films. It was found that the film with the highest thermal stability was Starch-HNT-Quercetin 10 film. Compared with starch film; the addition of quercetin and HNT into the starch film matrix caused a slight decrease in the water vapor transmission rate (WVTR), permeance and water vapor permeability (WVP) values of the starch film. As a result of the application of Starch-HNT-Quercetin 5 film to fresh chicken meat, a change in the color of Starch-HNT-Quercetin 5 film over time was detected. The detection of this change has revealed that low-cost and eco-friendly Starch-Quercetin and Starch-HNT-Quercetin films are pH sensitive and have a great potential to be used as intelligent food packaging.