Like all the other dairy products, milk kefir can also spoil. While the shelf life may be a bit longer than the other products which are milk-based, it is not that long enough.  The first sign is mold or fuzzy growth on the surface. There can be a change in color as well. If that happens, do not consume and throw it out. Even though kefir smells sour, if it is really sour or too smelly it is better not to consume.