froth something up
to whip or aerate something until it is frothy. Froth the milk up before you add it to the sauce. Froth up the milk before you pour it in.
[for something] to build up a froth when whipped, aerated, or boiled. The mixture began to froth up as Dan beat it. The milk frothed up as the steam went through it.
1. To become frothy or foamy: The vinegar quickly frothed up when I added the baking soda.
2. To cause something to become frothy or foamy: The spinning blades of the blender frothed up the juice. You need to stir vigorously in order to froth the sauce up.