Comparative Stability of Carotenoids in Grass-Fed and Grain-Fed Milk: Impact of Thermal Processing and the Protective Role of the Milk Matrix.

Carotenoids in milk are essential micronutrients, yet their stability remains a challenge during industrial processing. This study systematically evaluated the thermal and storage stability of lutein, zeaxanthin, and β-carotene in grass-fed and grain-fed milk. The results demonstrated that grass-fed milk tested in this study provided significantly superior protection for endogenous carotenoids during both pasteurization (65°C for 30 min) and accelerated storage (45°C) compared to grain-fed milk. Zeaxanthin showed the highest rate of loss over time, whereas β-carotene exhibited the highest stability. Evaluation of different milk processing technologies revealed that advanced methods, such as ceramic membrane filtration and direct steam injection, preserved carotenoid integrity more effectively than traditional high-temperature treatment. The enhanced stability observed in the tested grass-fed milk sample is hypothesized to be associated with its unique matrix composition, particularly the combined protective effects of high concentrations of lipid and protein, which may potentially form protective hydrophobic environments and barriers. These findings highlight the critical role of milk matrix composition in preserving fat-soluble bioactive compounds during thermal processing and storage.