Why very cold or very hot milk cannot be converted into curd?

Very cold milk cannot be converted into curd because the low temperature slows down or inhibits the activity of the beneficial bacteria needed for fermentation. Conversely, very hot milk kills these bacteria, preventing the fermentation process altogether. For optimal curd formation, milk should be at a warm temperature, typically around 110°F (43°C), which promotes the growth of the cultures necessary for curdling.