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Why does milk clot when it becomes yogurt? Is it only because of the decrease in the $\mathrm{pH}$ of milk? If it does so, what is happening when I increase the $\mathrm{pH}$ level? And by decreasing the $\mathrm{pH}$ level can I clot any type of liquid like milk?

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The clotting in a yoghurt is indeed caused by the lactic bacteria producing lactic acid and therefore lowering the pH of the milk. However in this particular case (yoghurt making) this is not due to the denaturation of the milk protein, they are in fact intact. When the pH drop in the milk, it get near the isoelectric point (IEP) of the casein (milk protein) therefore making them partialy neutral (or completly if the pH reach it). It weaken the electrostatic repulsion between the casein. Consequently when the pH get closer to the IEP, casein solubility decrease and often precipitate.

And this is reversible, if you add caustic soda or another base, the yoghurt or any sour milk will go back to being liquid (well not perfect in some case because the bacteria often produce exopolysaccharide which thicken the milk).

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  • $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. $\endgroup$ – Martin - マーチン May 11 '16 at 8:50
  • $\begingroup$ Do casin milk protein repulse to each other? $\endgroup$ – Abu sayed May 11 '16 at 10:08
  • $\begingroup$ Yes when the milk is at its normal pH (around 6.7) casein are charged negatively and repulse each other. $\endgroup$ – Matt R. F May 11 '16 at 15:58
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The clotting that you see is caused by denatured proteins. You can denature proteins physically and chemically. You can use acids or bases (chemical). Also, you can heat it up shake the protein (physical). The $\mathrm{pH}$ of yogurt is different from milk because of fermentation. Fermentation causes production of lactic acid in milk. Which is responsible for the change in $\mathrm{pH}$ (yogurt $\mathrm{pH}$ 4.0 vs milk pH 6.5).

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