Problem:

Kefir is a fermented milk drink made with kefir grains. It usually is prepared by inoculating cow, goat or sheep milk with kefir grains.

I would like to prepare the drink with soy milk, which worked fine for some months. Then the kefir grain stagnated in its growth. I use bottled soy milk from the supermarket. I do not produce the soy milk by myself.

In "real" milk the kefir grains never stop growing (and dividing), but in soy milk they apparently do.

Required:

Question: Why do the kefir grains stop growing in soy milk?

Question: Are they lacking a specific nutrient that is only contained in milk from animals?

Any help would be greatly appreciated!