Intrinsic milk photoluminescence (PL), though empirically observed, remains insufficiently explored in terms of mechanism and application. This work illustrates the general dual-emission characteristics of milk and elucidates their distinct origin: blue emission at 390–460 nm from casein and whey protein aggregates via clustering-triggered emission and yellow-green emission at around 530 nm from riboflavin. Crucially, microbial metabolism during spoilage induces pronounced physicochemical transformations: lactic acid accumulation that drops the pH from 6.69 to 4.79 within 72 h, extensive protein degradation with a 200-fold increase in free proline, and colloidal reorganization from uniform particles to polydisperse aggregates. These changes dynamically modulate PL signatures: early-stage (<12 h) riboflavin decay induces blueshifted emission, while advanced spoilage (24–72 h) disrupts protein aggregation, reducing quantum yield (Φ) by >80% and further blueshifting the emission toward the blue-violet region. Exploiting this correlation, we establish a dual-mode milk freshness assessment strategy: (1) visual colorimetry under 365 nm UV excitation, where fresh milk appears bright yellow-green and spoiled milk turns dim blue, and (2) quantitative (Φ/%) scaling that differentiates fresh samples above 3.5 from spoiled samples below 2.4. Validated against physicochemical benchmarks, this noncontact strategy enables real-time, field-deployable milk quality visualization for supply chain and consumer applications. This study not only reveals the underlying mechanism of milk luminescence but also provides a facile dual-mode approach for rapid quality assessment.
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2026 The Author(s). Aggregate published by SCUT, AIEI, and John Wiley & Sons Australia, Ltd.
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