Comparison of cellulose nano-fibrils (CNF)-based heat-treated (HT) and non-heat-treated (NHT) colorimetric indicator film sensors for monitoring pH and ammonia changes during pork storage

This study investigated the potential of a safe and eco-friendly colorimetric indicator film (CIF) for monitoring the spoilage of pork during storage. The film was developed using neutral red covalently immobilized onto TEMPO-oxidized cellulose nanofibrils (NR@TOCNFs), and fabricated in two forms: non-heat-treated (NHT) and heat-treated (HT). To evaluate the stability of the indicator, both CIF types were stored under fluorescent light at room temperature for 6 weeks. During this period, they maintained colorimetric stability despite exposure to light, temperature, and oxygen. Subsequently, the CIFs were applied to pork samples stored under the same conditions. During storage, pH, total volatile basic nitrogen (TVB-N), and total viable count (TVC) were monitored. Both CIF types initially exhibited a red color with fresh pork, which gradually shifted to orange as spoilage progressed, particularly along the film edges. By day 6, the pork was fully spoiled, and both NHT-CIF and HT-CIF had completely turned orange. These findings demonstrate that the Polyvinyl alcohol/Polyacrylic acid/NR@TOCNF-based CIF can serve as an effective smart indicator for visual spoilage detection. Furthermore, HT-CIF exhibited superior mechanical stability and dye retention. This study provides foundational insights into the development of intelligent indicator films, and suggests that the adoption of more sensitive dye systems could enhance stage-specific spoilage detection in future applications.