*3.5. Exudation Rate and Color Response of Indicator Films to pH Solution*

The issue of anthocyanin leaking out from films causes the failure of the indicator function. The pH behavior of the films differed noticeably, as seen in Figure 4. The exudation rates of bi-layer film with free anthocyanins (A-CBA) exuded rapidly, reaching 80% at pH 2 after 70 min. In addition, the exudation rate of the A-CBA film reached 100% at pH 7 and pH 9 after 50 min, due to the higher degradation of anthocyanins under the alkaline environment [30]. Thus, the films with anthocyanin-loaded liposomes (A-CBAL) slowed down the exudation rate by no more than 45%. However, there was no correlation between the anthocyanin exudation rate and the ratio of lecithin in liposomes. In conclusion, the liposomes enhanced anthocyanin encapsulation, which can improve the stability of an indicator film in a high-humidity environment.

**Figure 4.** The exudation rates of bi-layer films at pH 3 (**A**), pH 5 (**B**), pH 7 (**C**), and pH 9 (**D**), and the color of the films at pH 2-10 (**E**).

Figure 4E shows the color responses of the indicator films at different pH values. With pH increases, the color of the A-CBAL films changed from pink to purple and then gradually tended toward yellowish green. It can be verified that the color changes of the indicator films were consistent with anthocyanin-loaded liposome solutions, but with different degrees of coloration. However, compared with the A-CBA film, the response chrominance of the A-CBAL films decreased, which corresponded to the color appearance results in the encapsulation of liposomes. The encapsulation hindered the coloration of the butterfly bean flower anthocyanin. With the addition of lecithin, the coloration of the indicator films decreased, but they still presented visible color changes. As a result, in high-humidity food packaging, our bi-layer film with anthocyanin-load liposomes can be used as a pH indicator.

#### *3.6. Color Stability of the Bi-Layer Films*

The storage stability of the indicator films was determined under 4 ◦C and 25 ◦C, respectively. Generally, when the Δ*E* value of an indicator is no more than five, it will be difficult to notice with the naked eye [31]. As can be seen from Figure 5A, each of the bi-layer films presented higher stability with a lower Δ*E* value at 4 ◦C within 14 days. Thus, the film with free anthocyanins was not stable on the 4th day at 25 ◦C with an Δ*E* value of 5.35. The values of the A-CBAL1 and A-CBAL2 films were greater than 5 on the 10th day. At 25 ◦C, the films were more easily able to form a ring-opened chalcone structure with color changes [32]. The Δ*E* value of the A-CBAL3 film was 4.48 on the 14th day. This was because more radio lecithin with high encapsulation could protect free anthocyanins from external intrusion.

**Figure 5.** Color changes of the films stored at 4 ◦C (**A**) and 20 ◦C (**B**) for 20 d.
