The presence of preservatives in the milk
Milk with preservatives in its content must to be avoided.
The preservatives are designed to kill bacteria that may live in the milk, so that it can be preserved for longer.
However, they will also kill the yogurt and kefir lacto bacteria and yeast, preventing the fermentation process from starting.
When choosing milk from the supermarket, always go for the cheapest whole dairy milk you can find in the shop refrigerator, which usually has the shortest shelf live. This will guarantee that the milk did not go through any special treatment with additional ingredients and will increase the probability of a successful fermentation.
What about the long-lived milks, homogenized, and so
on? Well, they will work as long as other components are not added and the preservation is obtained through pasteurisation or in other mechanical ways.
However, “chemically” treated milks are never a good option, so avoid them.