Microwave-induced thermal response and protein variation of oil-water biphasic systems in foods: A Case study of anhydrous butter and skim milk

Oil and water are crucial constituents of liquid foods; however, the effect of their varying dielectric properties and distribution forms on microwave heating remains unclear. The study investigated microwave absorption and subsequent quality consequences in oil-water biphasic systems (two-layer liquid and emulsion). Food-derived skim milk and anhydrous butter were selected for research. The developed numerical model demonstrated that the distribution configuration and oil content affected the microwave heating pattern. Vertical temperature gradients were observed in both systems. The temperature in the two-layer liquid was higher than that in the emulsion with 20% oil content for the strong-intensity electric field in the oil and the high dielectric permittivity of water. Moreover, the upper thermal zone during microwave heating triggered milk protein unfolding, thereby exposing more free sulfhydryl groups. Protein composition analysis showed partial degradation of milk proteins. This study provides a basic understanding of microwave heating of liquid food biphasic systems.