Acidification: Lactose (milk sugar) is converted to Lactic Acid in a process called Acidification. When a Starter Culture (beneficial bacteria) is added to milk, the acidity of the milk is raised and the liquid begins to change into solid. Coagulation is the process used to further develop the milk into a solid - this process can also be achieved by introducing an acid such as vinegar or lemon juice, a combination of acid and heat, or by the addition of Rennet (an enzyme derived from animals, plants or from microbes in fungus or yeast).