The model milk solution was prepared by dispersing 12 wt% UHT sterilized milk, 6 wt% sugar, and 0.05 wt% emulsifier in water. Sodium citrate (as an organic acid) or caffeine was added to the model milk solution. The concentration of sodium citrate was in the range of 0.1–1.0% and that of caffeine was 1.0–1.5. After stirring, we observed the milk separation.