It doesn't have to be... it's actually best when used with raw goat or cow milk.. that is unpasteurized and not homogenized... but if you have to go to the supermarket to get your milk... get ONLY pasteurized... and NOT Homogenized... and also don't get the Ultra Pasteurized... just regular pasteurized... probably need to go to the healthfood store or healthfood sections...
The reason is because raw and pasteurized milks still have some life left in them. The others are considered "dead milk" because all the good stuff has been killed. For REALLY good kefir... you're going to need to feed them something good... you are what you eat... and in this case... you are what they eat.