Composition-dependent amorphous matrix formation controls vitamin C stability in reconstituted freeze-dried lactose-whey protein isolate systems.

This study examined how freezing history and matrix composition affected the solid-state structure and post-reconstitution stability of vitamin C in freeze-dried lactose-whey protein isolate (WPI) matrices (lactose:WPI = 4:0, 3:1, 0:4, w/w). The samples were freeze-dried after one- or two-step freezing and characterized using X-ray diffraction, differential scanning calorimetry, and Fourier-transform infrared spectroscopy. WPI suppressed lactose crystallization and produced amorphous hydrogen-bonded matrices, whereas lactose-dominant matrices remained highly crystalline. After reconstitution, vitamin C degradation under dark and light storage for 72 h followed a two-parameter Weibull model, which outperformed the first-order kinetics. Matrix composition affected stability more than freezing history: WPI-only matrices had the longest half-lives and retained most of their stability advantage under light, whereas lactose-dominant matrices degraded rapidly under light. These results indicate that protein-rich amorphous matrices improve vitamin C stability, specifically in the reconstituted aqueous state, a condition not captured by evaluating the dry powder alone.