Naked-eye monitoring of pasteurized milk freshness via highly stable, pH-responsive resazurin/cationic guar gum/κ-carrageenan intelligent labels

Progressive acidification during pasteurized milk spoilage provides a chemical signal for visual freshness assessment. Here, we developed a pH-responsive colorimetric label by confining resazurin (Res) within a cationic guar gum/κ-carrageenan (CGG–CAR) polyelectrolyte network. Nonlinear fitting of the pH-dependent UV–Vis absorbance ratio (A₅₉₈/A₅₃₅) to a monoprotic acid–base model yielded an apparent pKₐ of 5.78, with the steepest response occurring within the pH 5.5–6.0 interval associated with declining milk freshness. After 3 h in selected food simulants, the optimized label released 3.66–7.18% of its Res; the CGG-free control released 3.1–4.3 times as much. After 60 days at room temperature in the dark, stored labels showed only a small dry-state color difference from freshly prepared labels (ΔE = 2.56 ± 0.32) and retained distinguishable responses to pH 5.0 and 5.5. Two commercial pasteurized milk products were evaluated in laboratory storage trials at 25℃ and, as an accelerated chilled condition, at 10℃. Within each trial, ΔE₀ correlated strongly with pH (r = -0.969 at 25℃ and −0.988 at 10℃) and with counts of total mesophilic aerobic bacteria (r = 0.942 and 0.964, respectively). At 25℃, ΔE₀ increased by more than 27 units from day 1 to day 2 as milk crossed the spoilage threshold, accompanied by a dark-blue-to-purple transition. These findings support the potential of the Res-loaded CGG–CAR label for visual monitoring of pasteurized milk freshness under the tested laboratory conditions.