I make a poor-man’s latte. I pour a couple of ounces of skim milk into my coffee cup, microwave it to heat it, then put the mug in my Keurig machine to make a cup. When the milk is fresh this works great. If the milk is starting to turn, even just barely, after this process I get a congealed glob at the bottom of my mug.
What about this process of heating then mixing with coffee causes the still-drinkable-but-starting-to-sour milk to congeal?
Heat and acid is how you make a lot of cheeses. I expect the “almost but not quite turned” milk is closer to the edge, so the heat and the slightly acidic coffee is enough to start separating the curds. Perhaps the spoilage bacteria have already turned it a little acidic, perhaps they affect the suspended particles in the milk colloid.
Yes, naita is correct. The acid in the coffee is causing the curdling, and this is more likely to happen as the milk gets closer to being spoiled.
…This is because coffee and tea contain just enough acidity to tip the pH of milk to the point of curdling. The effect is most often seen in milk that is close to going sour or when adding milk to very hot coffee or tea, since the high temperature can coagulate casein.
Side note: you’re making a café au lait. That sounds much more sophisticated than a “poor man’s latte.”