Properties and Application of Intelligent pH Indicator Composite Film Based on Purple Sophora Flower Pigment

In this study, an intelligent pH indicator composite film was prepared using purple sophora flower pigment (PS) as a pH indicator and a mixture of pectin (P) and sodium alginate (SA) as a film-forming substate by the casting method. The effect of PS content on the surface morphology, water sensitivity, opacity, mechanical properties, coloration reversibility and thermal decomposition characteristics of the composite film was analyzed. The effect of the composite film on the preservation of cherries and its performance in monitoring the freshness of lamb meat were evaluated. The results showed that the composite film became denser with the increase in PS content. The addition of PS significantly improved the ultraviolet (UV) barrier capacity of the composite film while decreasing the water solubility and water contact angle. With the addition of 20% PS, the swelling property of the composite film decreased from (604.86 ± 83.61)% to (272.85 ± 27.84)%. The thickness of the composite film with 20% PS was (87.33 ± 8.62) μm, and its water vapor permeability was as low as (0.57 ± 0.09) × 10–11 g·cm/(cm2·s·Pa). Compared with uncoated samples, the mass loss of cherries coated with the composite film reduced by 35.42% after 8 days of storage at 25 ℃. Lamb meat with pH above 6.70 made the composite film turn from purplish red to yellow-green, showing spoilage. Moreover, changes in the total color difference of the composite film showed a strong linear relationship with the pH of lamb meat. The results from this study provide evidence supporting the application of pigments extracted from purple sophora flowers in intelligent pH indicator composite film.