Ultrasonic phase velocity as a non-destructive predictor of protein aggregation in retort-processed milk-coffee beverages.

Protein sedimentation during retort sterilization is a persistent quality problem in milk-coffee beverages, yet current accelerated shelf-life tests (2-4 weeks at 55 °C) are far too slow for same-shift production decisions. Ultrasonic phase velocity (UPV) measurement offers a 30-min, non-destructive alternative. Using a 1.5 MHz FFT phase-detection system (mean RSD < 0.005% across 27 experimental groups), the heat-induced velocity change Δc = c(after retort) - c(before retort) was measured across a sodium caseinate (NaCas, 0-0.20 wt%) series in a model milk-coffee matrix (6% coffee, 13% milk, 121 °C/30 min). The resulting U-shaped Δc-NaCas profile-minimum 11.8-12.3 m/s at 0.05-0.07 wt% NaCas, rising to 19.2 m/s at 0.20 wt%-maps directly onto the bridging-to-depletion flocculation transition. Laplace decomposition confirmed that adiabatic compressibility drives >80% of the signal; density varied by less than 0.002 g/cm3 across all conditions. In a coffee-free LBG-milk validation system, SDS-PAGE revealed preferential depletion of α- and β-caseins (retention: 31.5%, 33.0%, and 25.1% at 0%, 0.4%, and 0.8% LBG respectively) while κ-casein remained near-intact (∼103%); β-Lg retention correlated inversely with Δc (r = -0.97). CLSM and laser diffraction confirmed a three-stage aggregation progression. The method reduces testing time by 96% relative to conventional sedimentation assays and provides a thermodynamic framework for rapid, non-destructive quality monitoring of thermally processed protein beverages.