Lactobacillus bacteria converts the sugar (lactose) into an acid (lactic acid) by means of fermentation. When milk is heated to a temperature of 30 - 400C and a small amount of old curd added to it, the Lactobacillus in that curd sample starts to grow. These convert the lactose into lactic acid, which imparts the sour taste to curd as the optimum temperature for bacterial growth is 370C, at this temperature they grow luxuriously and convert the pH of the milk.