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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.

  1. In fresh dairy milk, is there any relation between titratable acidity and heat coagulation?

  2. Does hydrogen ion concentration determine the coagulation?

  3. Is it the coffee power or the milk or the temperature?

  4. How to avoid this effect?

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Coagulation of milk is a result of casein micelles clumping together. This is favoured by higher temperatures and also by favour a pH of around 4.6 as this is the isoelectric point. At this isoelectric point, the amino acids within the casein micelle are electrically neutral. This means there is less electrostatic repulsion between the casein micelles, facilitating coagulation Furthermore, the addition of acid means that the kappa casein, i.e. hairs on the outside of the casein molecule which stabilise it, become protonated. This protonisation causes them to destabilise, making it easier for the micelle to fall apart and clump together with other micelles that have fallen apart. Relatively High temperatures also increase reaction rates, which overall speeds up the aforementioned processes as well.

Even if the coffee does not reduce the pH by much, any change in pH which leads to the pH being closer to the isoelectric point (4.6) of the casein micelle will increase the rate of the coagulation process.

Using old milk also increases coagulation, as milk naturally coagulates due to rennet, an enzyme, cleaving the kappa-casein, causing the casein molecules to destabilise.

To avoid coagulation during heating, perhaps try heating the milk gently. The ideal temperature at which milk coagulates is about 43ºC, so perhaps staying below that could work. This source also provides tips on how to avoid coagulation while heating milkAlternatively, you have yourself some cheese to enjoy :)

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