2.2.4. The Color of BAL in Different pH Solutions

The absorbance of the BA and BAL solutions at different pH values was measured in the range of 450 nm to 700 nm using a UV-visible spectrophotometer (TU10CS, Beijing General Analytical Instrument, Beijing, China).

## *2.3. Preparation of the Bi-Layer Indicator Films*

The bi-layer films were prepared with two individual solvent casting methods. Firstly, 2 g of agar was stirred in 100 mL of distilled water for 2 h at 100 ◦C. Then, an agar hydrogel was formed as the outer layer by cooling the plastic Petri dish at room temperature. Secondly, 2 g of carrageenan was stirred in 100 mL of water with 2% glycerin for 1 h at 85 ◦C. After cooling at 65 ◦C, free anthocyanins and different groups of BALs (BAL1, BAL2, and BAL3, each containing 20 mg of the anthocyanin) were added to the above carrageenan solution. The solutions were thoroughly stirred at 65 ◦C for 30 min. Finally, the carrageenan solutions containing free anthocyanins and liposomes were dispersed onto the agar protective layer and dried in an oven for 24 h at 35 ◦C, and the bi-layer films were obtained and termed as the A-CBA, A-CBAL1, A-CBAL2, and A-CBAL3 films, respectively.
