Question

What happens when milk is left open at room temperature during summers$?$

Answer 1

The milk contains Lactobacillus bacteria which produce lactic acid. These bacteria grow rapidly in the favorable temperature. These bacteria when grows in higher number, produces more lactic acid, which turns the milk sour. Also, the pH of the milk becomes more acidic and the protein of the milk become curdles. In the summer, the temperature and other atmospheric conditions favors the growth of these bacteria so, if we left the milk open at room temperature during summer it may split resulting into curdles and sour in taste.