Preparation of intelligent indicator film based on purple cabbage anthocyanidin for monitoring milk freshness

Abstract: A pH-colorimetric film is one of the most important indicators to acquire straightforward information by the visible color changes without opening packages in the food industry. Among them, purple cabbage anthocyanins (PCA) can be expected to serve as a natural anthocyanin pigment with high pH sensitivity. Different color changes can be easily observed in the pH variation, due to the unique structure. The PCA color can be changed from dark to light red in the acid solutions (pH=2-4) as the dominant flavylium cation. At pH level of 5-6, the flavylium cation can react with the hydrogen, and then transform into the carbinol pseudobase (colorless) or chalcone (colorless) and appeared pink. The quinoidal base can also be established with the increase of pH (7-8). Subsequently, the anion quinoidal (blue) is dominant in the alkalinity pH (9-10). The color can turn greenish-blue for the anionic quinoidal base (blue) that is synthesized at pH 11. Soy Protein Isolate (SPI) (a byproduct of the soybean oil industry) can be used to characterize by its low cost, high biodegradability, biocompatible, and excellent film-forming properties. Also, the SPI, as a natural material is easy to fuse with the purple cabbage anthocyanins. Hence, the SPI can be selected as a film-forming material. However, the SPI films share relatively low mechanical properties and thermal stability, due to the hydrophilic bonds (-OH, -NH2, -COOH, and -SH) in the film matrix. The typical C6-C3-C6 carbon skeleton structure in anthocyanins can contain two benzoyl rings and oxygenated hexameric heterocycles with the cations (a typical 2-phenyl-benzopyran cation structure). Two typical and highly reactive structural in the PCA molecules can cross-link with the SPI molecules via the hydrogen bonds and electrostatic interactions, in order to form the homogeneity and low surface roughness of films, due mainly to the excellent fusibility and compatibility of PCA and SPI, indicating the improved mechanical properties and pH sensitivity of SPI/PCA indicator films. Therefore, this study aims to synthesize the SPI/PCA-based pH-indicator films with a uniform structure, excellent mechanical properties, and strong pH sensitivity, and then apply them to monitor the freshness of pasteurized milk. FTIR and SEM demonstrated that the PCA was successfully dispersed in the SPI film matrix, and then interacted with the SPI molecules under electrostatic interactions and hydrogen bonds. In SEM micro graphs, the number of small particles was observed to float on the surface, while small holes were in the section of indicator films with the overdose PCA. The highest limitation of the fusion degree of PCA with SPI was 4%. PCA significantly enhanced the mechanical properties of indicator films. The tensile strength of indicator films increased by 53.25% (from 2.46 to 3.77 MPa), and the elongation at break increased by 25.25% (from 105.36% to 131.96%), compared with the SPI/Na2SO4 film. The thermal stability of indicator films with the PCA was similar to the SPI films (167 to 194℃). The color difference (ΔE) of indicator films with the PCA from the standard white increased from 21.13 to 52.88. L value, where the indicator film was yellowish-green. The indicator films presented strong chromatic stability without the visible color difference within 5 days at room temperature. A high pH sensitivity was found with the visible color difference under different pH values. Two properties were beneficial to monitor the freshness of the indicator film. The indicator films were also applied to monitor the milk. The color was then changed from green to red, together with the freshness deterioration of pasteurized milk. Also, the color difference with the initial film was visibility (ΔE≥5) at any time. Hence, purple cabbage anthocyanins were used to promote the indicator function of pure SPI films. The indicator films can be used to monitor the freshness of pasteurized milk via color change. Therefore, the indicator films can be widely expected to apply in intelligent packaging, due mainly to excellent pH-sensitive properties and color stability. The finding can provide insightful ideas to design and produce SPI/PCA-based pH-indicator films.