The cultures need either fat or protein to be able to produce lactic acid. In many types of plant milk there is hardly any of either. Lactic acid can often be produced anyway, but if there is no protein to form the typical structure of yoghurt, it will not become firm. Therefore, a thickener is necessary in these cases. We recommend 2.5 teaspoons of tapioca starch to 1 l of vegetable milk or 2 teaspoons of agar-agar.