κ-carrageenan-based pH-sensing films incorporated with anthocyanins or/and betacyanins extracted from purple sweet potatoes and peels of dragon fruits

Natural indicators (anthocyanins and betacyanins) were mixed in different volumetric ratios (3:1, 3:2, 1:1, 2:3 and 1:3) and used for the preparation of κ-carrageenan-based pH-sensing films. Anthocyanins and betacyanins were fixed into films by hydrogen bond, which changed the spatial structure of the κ-carrageenan films. Betacyanins reduced the crystallinity of κ-carrageenan films while anthocyanin had little effect on the crystallinity of the films. Betacyanins significantly improved the thermal stability of the film, while anthocyanins had little effect on it. In addition, anthocyanins and betacyanins significantly improved the oxidation resistance, water vapor permeability, ammonia sensitivity and UV shielding performance of the films. The complex films showed good color stability. It monitored the physical and chemical indexes of pork during storage, and measured the color change of indicator film during this process. Results showed that the color change of the film containing anthocyanins/betacyanins mixture in the volume ratio of 3:1 was the most significant with the pork spoilage. According to the correlation analysis and principal component analysis, total color differences(ΔE) can indicate the freshness of pork. Therefore, the pH sensing film prepared by us provides a reference for the freshness indication of pork.