The foam of a beverage:^[1] [Talmud/Shulchan Aruch Harav/Achronim]

Drinking foam: One who drinks the foam that is formed on the surface of a beverage can lead to Ririn which comes from the nose.

Blowing foam: One who blows with his mouth the foam that is formed on the surface of a beverage, this can lead to a difficulty in the head.

Pushing the foam: One who pushes to the side the foam that is formed on the surface of a beverage, this can lead to poverty.

The definition of foam:^[2] The definition of foam referred to in the above Talmudic statement is the foam that is created during the fermenting process of a beverage within a barrel. However, the foam that is created when one pours a beverage is not considered foam at all in this regard. Nonetheless, it is proper for one to avoid even this type of foam, being that matters of danger are very severe. [Thus, it is proper for one not to drink from a recently poured glass of beer or soda until the foam dissipates. Seemingly, based on this, one should also avoid drinking the foam that is formed when milk is shaken, or in a milk frothier such as when drinking a cappuccino beverage. However, the custom is to be lenient in this, and perhaps this is not considered foam at all and rather is considered milk cream.]

^[1] Admur Hilchos Shemiras Haguf Vinefesh 9; Chulin 105b; Shnei Luchos Habris; Shemiras Hanefesh 302; Kaf Hachaim O.C. 170:82; Y.D. 116:204; Sefer Shemiras Haguf Vihanefesh [Lerner] 41

[2] Shnei Luchos Habris p. 80; Elya Raba 170:24; Likkutei Maharich p. 112; Shemiras Hanefesh 302; Kaf Hachaim 116:204; Sefer Shemiras Haguf Vihanefesh [Lerner] 40:2