I was making coffee with a cup of boiling milk which was 2 days old. I used instant coffee powder and some sugar for the making of it. Tried to make a very hot coffee but usually prefer medium hotness. When I mixed the hot milk with coffee powder, I immediately observed coagulation. Then, I decided to use an unopened milk packet and boiled it the same way. But this time I used a relatively more roasted coffee powder with same amount of common sugar. Again I observed coagulation but the particles were smaller.
I know that milk is a colloid and can be coagulated by using an acidic compound. The casein starts coagulating at pH 4.6, But I don't think that coffee is acidic enough to lower the pH. As in my second experiment, I've used a lesser acidic coffee powder (more roasted) but still observe coagulation.
In fresh dairy milk, is there any relation between titratable acidity and heat coagulation?
Does hydrogen ion concentration determine the coagulation?
Is it the coffee power or the milk or the temperature?
How to avoid this effect?