How cheesemakers cultivated the appropriate microbes from their milk for its preservation
One of the easiest ways to ferment milk
It’s pretty easy to make kefir. All you need are kefir grains and milk.
Keep about a tablespoon of kefir grains in a jar.
Pour in some fresh milk. It can be raw, it can be pasteurized, it can be homogenized, it doesn't really matter. Kefir grains are much less picky about their milk than we are.
Leave the kefir grains to sit in the milk for about 24 hours or so, until the milk curdles, it thickens, and sours.
You have your kefir!
You can strain out the kefir grains by passing them through a sieve. You can then put the kefir grains back into more milk in order to initiate another fermentation.
You can continue caring for your kefir grains just like this. Straining them out of kefir, putting them into more kefir, over and over and over again, endlessly.
If you take good care of the grains, you can pass the kefir grains onto your grandkids who can pass that culture onto theirs. This is the longevity of this culture. This culture will reproduce true, over and over and over again without contamination, so long as they're regularly cared for.
You can find David Asher at the Black Sheep School of Cheesemaking. His cheesemaking book is The Art of Natural Cheesemaking.
Title image by Svorad, CC BY-SA 4.0, via Wikimedia Commons
I make almond milk kefir. This tends to separate between a large liquid portion and kefir critters. I use it in smoothies, so consistency is not critical; nonetheless, it would be nice.