From shrimp reddening to spoilage indication: Shrimp-shell-derived crustacyanin as a multi-trigger responsive chromoprotein.

Natural colorimetric freshness indicators are commonly based on pH-sensitive pigments, although pH changes do not always match food quality deterioration. Here, shrimp-shell-derived crustacyanin was investigated as a multi-trigger spoilage-responsive chromoprotein rather than a conventional pH indicator. The recovery process was designed to obtain sufficient protein while preserving the astaxanthin-apoprotein binding environment required for color response. Using Macrobrachium rosenbergii shells, salting-out extraction under mild conditions gave a crustacyanin yield of 1.49 ± 0.11 mg/g and a tofu-based ∆E of 57.23 ± 2.06. The recovered protein showed a major band near 75 kDa and a characteristic absorption maximum at 580 nm, indicating retention of the blue bound-chromophore state. pH fluctuation, microbial growth, protease treatment, and thermal perturbation all induced visible blue-to-red transitions. These responses were accompanied by protein structural changes and spectral movement toward approximately 480 nm, suggesting that spoilage-related perturbations weakened the protein constraints that maintain the blue astaxanthin-binding state. In tofu storage, crustacyanin color change was associated with microbial growth, pH variation, and sensory deterioration. This study establishes shrimp-shell-derived crustacyanin as a conformation-sensitive chromoprotein that converts multiple spoilage-related microenvironmental changes into a visible color transition.