In this study, poly (ethylene glycol)-poly(caprolactone)(MPEG-PCL) diblock copolymers were synthesized by ring-opening polymerization of ε-caprolactone using monomethoxy PEG as initiator. Alizarin-loaded MPEG-PCL micelles were fabricated using a solvent evaporation method. After freeze-drying, pH-responsive colorimetric tablets were fabricated by compression to monitor the freshness of pork. The composition, molecular weight, structure and thermal properties of the polymer were studied by proton nuclear magnetic resonance (1H NMR), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR). Morphology and size of the micelles were observed by transmission electron microscope (TEM). Ultraviolet visible spectroscopy (UV-vis) showed that the light absorption peak of alizarin solution shifted to a longer wavelength as the pH increased and the colorimetric tablet showed a distinct color change of alizarin solution from yellow to purple when pH value in the range of 2-12. The colorimetric tablet was found to be sensitive to different concentrations of volatile ammonia. The colorimetric tablet was applied to explore the freshness of pork and the brown tablet changed to purple as total volatile basic nitrogen (TVB-N) of the pork reached 23.1 mg/100g, which exceeds the threshold value of freshness in the Chinese standard. The results showed a high correlation between the ΔE of the color change and the TVB-N, pH, total viable count (TVC), thiobarbituric acid reactive substances (TBARS) change of the sample. Therefore, the colorimetric tablet could effectively be used to monitor pork freshness.
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