Animal Milk. The ideal environment for Kefir Grains is animal milk. Varieties include cow, goat, sheep, and other similar species. Kefir Grains thrive in animal milk due to the chemical makeup of the liquid. The lactose in animal milk provides the most efficient food source for the Kefir Grains and therefore the most efficient and effective culturing process.
Alternative Milks. While animal-based milks are the healthiest for the Kefir Grains, alternative types of “milk” (coconut, soy, rice, nut, etc.) can also be used under certain conditions. Generally speaking, Coconut Milk, Soy Milk, and Rice Milk can be cultured using Kefir Grains as long as a revitalization period is observed (see below). While some people report success culturing Kefir Grains in seed and nut milks (e.g. hemp milk, almond milk, etc.), these varieties tend to yield more inconsistent results.
Revitalization Period. When using a non-animal variety of milk, it is important to occasionally allow the Kefir Grains to revitalize in animal milk for 24 hours.
We recommend allowing the Kefir Grains a revitalization period at least once every few weeks and ideally more often. To revitalize the Kefir Grains, simply place the Kefir Grains in 1-2 cups of animal milk for 24 hours. Once the process is complete, the Kefir Grains can be returned to use with an alternative variety of milk.
This may be technically why...
''Kefiran produced by L. kefiranofaciens is a branched, water-soluble polysaccharide, containing equal amounts of D-glucose and D-galactose. The production of this polysaccharide is stimulated when L. kefiranofaciens grows in co-culture with S. cerevisiae (Cheirsilp et al., 2003).''
Without galactose, one of the monosaccharides present in lactose (a disaccharide glucose-galactose), the structural protein that forms the grains can not be done. When taking apart from the grain, the microorganism complex don't behave the same way and are not able to ferment the substrate properly.
I'm just starting and I will try with honey, which have a 10% galactose. Although not vegan, could work and try with other sources of galactose like beets... (Only they have little content but may be enough for a persistent growth and avoiding decay). Unless the lactose itself is needed in the process, it should work...