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Phrase(s): 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., Phrase(s): froth up
[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.
McGrawhill's American Idioms And Phrasal Verbs