Yogurt, or yoghurt is a milk product made by bacterial fermentation of milk. The lactose in the milk becomes lactic acid when it is fermented. Lactic acid acts on the protein in the milk to make yoghurt thick and sour. The milk is heated to about 80 °C to kill any bacteria present, and to change the milk proteins so that they set together instead of becoming curd. After it is cooled to about 45 °C, the bacteria culture is added, and the milk is kept at that temperature for 4 to 7 hours to ferment. Soy yoghurt is made from soy milk.