Intrinsic Milk Luminescence: Underlying Mechanism and Application for Quality Visualization

ABSTRACT 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 (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 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.