The phenomenon of something being named for a part of the whole is known as 'synecdoche'. The ballad 'Gil Brenton' (Child #5, version A) begins like this:

"Gil Brenton has sent oer the fame,
He's woo'd a wife an brought her hame"

...where, when anglicised, 'fame' is 'foam' (and, of course, 'hame' is 'home'). This is the earliest synecdochic use of 'foam' of which I am aware.

AR