Region-Specific Spoilage Dynamics in Rainbow Trout Fillets: A Dual Approach Integrating Intelligent Label Response and Sensory Evaluation

Rainbow trout (Oncorhynchus mykiss) fillets are highly perishable due to microbial and biochemical activity; therefore, reliable non-invasive tools are required for real-time freshness monitoring. This study developed and evaluated intelligent labels derived from purple onion peel anthocyanins (POPAS) to monitor the freshness of rainbow trout fillets stored at 4 +/- 1 degrees C. To assess region-specific spoilage dynamics, fillets were divided into head (H), center (C), and tail (T) sections, packaged with intelligent labels, and stored for 15 days. Label responses, including colorimetric indices such as total color difference (Delta E), redness index (RI), whiteness index (WI), and yellowness index (YI) indices, were monitored in relation to pH, in-package CO2, total volatile basic nitrogen (TVB-N), microbial growth, and sensory quality of both raw and cooked samples. The labels exhibited distinct pH-dependent color transitions: from red (pH 2-3) to pink (pH 4-5), followed by a loss of intensity at pH 6, and finally shifting to green/brownish-green (pH >= 7). Color changes were first visually detectable on day 6 in the T and H groups, corresponding to sensory odor scores falling below the acceptability threshold of 3. By day 12, panelists rejected samples due to odor and chewiness, which coincided with a Delta E > 5 and a label color shift to brownish-green. In-package CO2 increased from 5.3% on day 1 to approximately 30% on day 15, demonstrating a strong correlation with label color change. Furthermore, Delta E was closely associated with TVB-N and pH variations, highlighting region-specific spoilage dynamics. Intelligent labels developed from POPAS provided a sensitive, non-invasive, and consumer-friendly tool for real-time spoilage detection, showing strong predictive alignment with microbial, chemical, and sensory spoilage limits. Overall, the study highlights the potential of these natural, sustainable colorimetric indicators for seafood safety while introducing a novel approach that reveals region-specific spoilage patterns within a single fillet.