Have a look at the Industrial fibre optic sensor. You get quite a
variety for diffirent applications. I would use a light beam in this
case. I am sure froth will break the beam. You set the sensitivity to
what you need.
Most are 1mm Red light (Tx=LED) but you get laser versions as well
(pricy).
Omron.com
Keyence.com
And a whole lot of others.

Quentin

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