Kojic acid-modulated anthocyanin/ hydroxypropyl methylcellulose-sodium alginate colorimetric sensors for real-time freshness monitoring of salmon

Utilizing natural colorant-based colorimetric sensors for food freshness monitoring is an effective approach to reduce food waste and foodborne illness. However, it is hard to modulate the colorimetric endpoint of the pH-responsive natural colorants to determine the food quality accurately. Herein, Kojic acid (KA)-modulated anthocyanin (ACN)/ hydroxypropyl methylcellulose (HPMC)-sodium alginate (SA) film-based colorimetric sensor was prepared using drop casting method. HPMC and SA provided a homogeneous framework to incorporate ACN. KA, a natural product derived from fermented foods, was introduced to manipulate the colorimetric endpoint of ACN to realize an accurate prediction for the freshness of salmon. Molecular dynamics (MD) simulations and density functional theory (DFT) revealed not only the molecular interactions among KA, ACN, HPMC, and SA, but also the strong interactions between the KA-ANC/HPMC-SA colorimetric sensor and trimethylamine (TMA). Results of UV-vis spectra and TMA response experiments illustrated that the KA can inhibit the deprotonation process and promote the hydration equilibrium of ACN, which makes the KA-ACN/HPMC-SA colorimetric sensors exhibit different color responses comparing to the pristine ACN/HPMC-SA. Among all colorimetric sensors, the 1KA-ACN/HPMC-SA exhibited the most sensitive response to TMA with a limit of detection (LOD) of 15.79 μM. When applied in the real-time monitoring of the freshness of salmon at 4°C, the 1KA-ACN/HPMC-SA showed the most significant total color difference (ΔE) of 31.28±0.76 and synchronous color change with the spoilage of salmon after 10 days. The degradation and seed germination experiments of the colorimetric sensors indicated that the colorimetric sensors were environmentally degradable and had good biosafety. This work proposes a novel and effective strategy to modulate the colorimetric endpoint of ACN, and the prepared colorimetric sensors show applicable potential in the quality monitoring of salmon.