People have been fermenting milk for a very long time. This process can be traced back to around 10.000 BC! Probably around the time villages started planting different crops to survive. The most popular product is Yogurt, but there are a lot more!
Milk fermentation is done mainly by one type of bacteria, lactic acid bacteria. This kind of microorganism eats carbohydrates present in the milk and produces lactic acid, which gives its characteristic taste to fermented milk products.
Different products are made using different combinations of bacteria, but the process is almost the same. We will see that there are lots of cultured products, each one with its own taste.
Kefir is a drink made by using this grains (made of the lactic acid bacteria, yeasts and a matrix of organic compounds) on cow, sheep or goat milk. It is quite easy to make at home, you just need to find someone to give some kefir grains to you.
Because of the yeasts, it also has some alcohol content (around 0,5%). It has sour taste and effervescent texture. I would recommend you to try it before asking for some grains!
This one comes from the Nordic countries. It is made in a similar way to Yogurt, a small part becomes the starter of the new culture. It uses a different combination of bacteria than the other beverages. It is thick, sour-flavoured with some diacetyl, a molecule that gives butter its characteristic taste.
Famous in the Central Asian steppes, Kumis is made from mare's milk. Because of mare's milk being hard to get, it is commonly made using cow's milk. This drink is made with bacteria and yeast, similar to Kefir (but this one doesn't have grains). However, mare's milk has almost 40% more lactose, making the alcohol content higher. It can be considered the region's equal to beer.