Does a beverage with a cultured buttermilk like consistency break a fast?
Maybe not the letter, but probably breaks the spirit.
Let me tell you a little story...
At the seminary, there was one seminarian in the year above me who went into town one Ember Wednesday (seminarians could get permission to go into town on Wednesday afternoons, which were allocated to recreation) and bought a milkshake. He basically said "I don't care" and even joked about it.
Fast forward several years...
After this young man left the seminary, he completely apostatized from the Faith.
He also was very sarcastic about everything...like nothing was sacred or serious. One time I was sharing a sheet of funny rock group names I made up (like "Dingo Fortnight") and I commented that during my creative process, I ran my brainstorm of ideas through a mental filter to weed out any dirty or off-color ones. He responded that he preferred no such filter.
It's interesting, in a bad way, how various Trads lived BEFORE their apostasy.
Another apostate I know (same first name, oddly enough!) fell because of sins of the flesh. He openly "confessed" to Myspace things he did in his teen years, of which I had no idea. Today he's completely apostate from the Faith, and practices no religion.
Another apostate I know got into astrology and "red pill anti-Feminism" and become confused and bitter about how to reconcile it all. He suffered a lot from difficulty getting established and finding a good wife. But what destroyed his life -- both secular and spiritual -- and set him on the path away from God, was falling into adultery with a Trad married woman with many children. That totally broke his habit of weekly Mass, after the wronged husband threatened the young man's life.
All three of these apostates were Traditional Catholics. They would sing in choir and/or serve Mass, pray, and only attended the Tridentine Mass.