Effects of Different Sterilization Methods on the Stability and Casein Structure in Camel Milk

The effects of heat sterilization and microwave sterilization on the stability and casein structure of fresh camel milk were compared. The microwave sterilization conditions were optimized using single factor experiments and response surface methodology based on Box-Behnken design. Changes in the structure and properties of camel milk casein after sterilization were investigated using a differential scanning calorimeter, a Fourier transform infrared spectrometer, and a scanning electron microscope. The centrifugal sedimentation rate of protein and the total number of colonies were taken as response variables, the optimum conditions were determined as follows: pH 7.0, microwave power 560 W and microwave irradiation time 120 s. After microwave treatment under these conditions, the centrifugal sedimentation rate of protein was 6.96%, and no bacterial colonies were detected. Differential scanning calorimetry (DSC) plot showed that microwave treatment resulted in higher enthalpy of camel milk casein compared with heat sterilization, indicating a lower degree of denaturation and higher stability. The results of scanning electron microscopy showed that both sterilization methods destroyed the micellar structure of casein to varying degrees, resulting in cross-linking and aggregation. However, after microwave sterilization, the casein showed ordered aggregation, forming small aggregates that assembled into an ordered network structure. Appropriately increasing the pH of camel milk before sterilization was conducive to its stability. Different sterilization methods had a significant effect on the secondary structure of camel milk casein. Both microwave and heat sterilization reduced the relative contents of α-helix and β-turn in the casein. The relative content of β-sheet in heat-sterilized camel milk casein was the highest, indicating the most significant change in the secondary structure of the protein. In summary, this study revealed the stability of camel milk and the structural change of camel milk casein under different sterilization conditions, which can help address the poor stability of thermally sterilized camel milk at this stage.