Answer:
When adding lemon juice or vinegar to hot milk, it will curdle almost immediately, but adding it to cold milk will not produce a reaction for quite some time. This is the very same reaction used to create fresh cheeses like ricotta or paneer.
When lemon juice is added, it increases milk''s acidity as lemon contains citric acid. Due to this lowering of pH, calcium causes precipitation of the casein and thus milk gets curdled.
When milk becomes too acidic, like when we add lemon juice or when it goes sour, the negative charge on the casein groupings becomes neutralized. Now instead of pushing each other apart, the casein starts to clump together. Eventually large enough clumps are formed that we can actually see the separation, and then we have curdled milk.
Explanation:
