Exploration of interaction between α-lactalbumin and β-lactoglobulin under dUHT treatment and storage: Experimental and molecular dynamics study

Direct ultra-high temperature (dUHT) milk is considered as the next generation of liquid milk product because it keeps more nutritional ingredients and better flavor than UHT milk. However, sedimentation and age gelation are urgent problems to develop dUHT milk. The role of whey protein interaction in the destabilization of dUHT milk is rarely reported. In this study, the interaction between α-lactalbumin (α-La) and β-lactoglobulin (β-Lg) was investigated by thermodynamics, spectroscopy and silico approach at 75 °C (preheat), 145 °C (dUHT) and 37 °C (storage). The results showed that α-La could interact with β-Lg via hydrophobicity along with exothermic reaction. During dUHT and storage, the average molecular weight (MW) distribution of α-La/β-Lg complexes increased from 20.3 kDa to 274.4 kDa. The particle size increased from 135.53 ± 5.26 nm to 153.50 ± 3.83 nm and the zeta potential decreased from −27.80 ± 5.90 mV to −7.73 ± 2.65 mV. Fibrous cluster-like protein bridges promoted the formation of α-La/β-Lg aggregates. The interaction mechanism between α-La and β-Lg was explored by molecular docking and dynamics simulations. It was found that heat treatment reduced the binding free energy of α-La and β-Lg and promoted the formation of α-La/β-Lg complexes. Electrostatic and van der Waals were the main energy contributors to the binding. UHT treatment increased the number of hydrogen bonds and weakened the hydrophobic interaction. These findings indicate that the interaction between α-La and β-Lg forms aggregates during heat treatment and storage, which help to optimize process and storage of dUHT milk.