Kefir bought at the store (commercial kefir) is quite different in composition, flavor and cost to the kefir made at home from genuine kefir grains. Authentic homemade kefir is made from kefir grains that ferment the milk at room temperature for about 24 hours. Commercial kefir is not made directly from kefir grains, but rather they are an imitation that mimics some of the strains, flavor and properties of authentic kefir that’s been made at home for hundreds of years.
One of the reasons why companies cannot use authentic grains is that they are limited by the bottling process. Companies need to suppress or halt yeast activities in order to stop the continued carbonation found in genuine kefir or the bottles would likely explode. So ultimately, you have a suppressed product that is not as active, not as balanced and has much less variety of bacteria and yeast.
One of the leading commercial brands, Lifeway kefir, state that they have 10 strains of friendly microorganism. In comparison, kefir made at home with kefir grains can have up to around 40-60 different strains. Store bought or commercial kefir typically has more strains and more probiotics than yogurt, but still a far cry from what the genuine thing.
There are also kefir starter packets that are popular such as Body Ecology. Body Ecology claims 7 different strains of bacteria and can be used a limited number of times (around 7 times compared to kefir grains that never die). Kefir starter packets are also an imitation lacking the same probiotic contents, acid, carbonation, the kefiran (a health-promoting special polysaccharide formed by the grains), and many other natural healthy by-products that occur specifically during fermentation of the live grains.
One of the biggest advantages of doing kefir at home is the enormous savings. The cost of going through 2-3 store bought 32-oz kefir bottles a week is approximately $415-622 a year. The only cost associated with home-made kefir is the cost of the milk which comes out to around $72-$108 for that same amount of kefir. The grains themselves are self-sustainable and will continue to grow and never die if well taken care of.
Another advantage of homemade kefir is that you get to choose the type of milk and flavorings (if any). You can try raw, non-fat, low-fat, whole or goats milk. You even experiment with soy, rice, coconut and almond milk. You can flavor it however you like or just drink it plain which is delicious by itself (without the added sugars many commercial companies put in).