According to Dominic Anfiteatro, whose kefir enthusiast website is mentioned in Sandor's The Art of Fermentation
, they are quite tough. He reports that a friend once let his kefir grains sit in contaminated water in a refrigerator for seven months, and he was able to bring them back from green-colored and foul-smelling with daily changes of milk.http://users.sa.chariot.net.au/~dna/kef ... resilience
A professor of agriculture at Cornell University, Frank V. Kosikowski, in Cheese and Fermented Milk Foods
, says that kefir grains can be refrigerated in milk for up to 10 days without losing vitality.
Sources on the Internet seem to be divided. Some say that making weekly batches of kefir, storing the grains in milk in a refrigerator between cultures, will damage the grains, and one must commit instead to culturing fresh milk daily in order to avoid damaging the grains.