This review is about recent developments in the application of natural pigments as pH-sensitive food freshness indicators in biopolymer-based smart packaging. However, there are many reviews about the topic, such as ACS Food Science & Technology, 2021, 1(2): 124-138, Critical Reviews in Food Science and Nutrition, 2023: 1-20, Food Control, 2022, 134: 108769, Critical Reviews in Food Science and Nutrition, 2021, 61(14): 2297-2325. Comparing with them, the innovation of this review is pale. Some suggestions are given as follows:
1. The logic of the review needs enhancing.
2. Some important recent work on the topic were not cited and analyzed.
3. The review just demonstrated the results and issues from the cited references, which is lack of the special critical issues of the authors.
4. More valuable suggestions on the pH-sensitive food freshness indicators in biopolymer-based smart packaging should be given to the readers and researchers in this filed.
5. The authors are lack of accumulation of the work on this topic.