Kefir grains are the starter culture used to ferment fresh milk and create a sweet or sour kefir. When kefir grains are used to ferment milk, the process is similar to making yogurt. Kefir, or fermented milk, contains more probiotic cultures than yogurt, according to the KefirOnline website. It is possible to ferment fresh milk at home, but you need to be very careful to keep your hands and equipment clean. As you store and reuse the starter culture, its quantity grows just a little.

Pour the kefir culture into the glass container.

Pour the fresh milk into the container until it is about two-thirds full. Cover the jar with a clean lid or cloth.

Leave the milk-kefir culture mixture on the counter for 24 hours. If you leave it out for only 12 hours, it will be thinner and have a sweeter taste. If you leave it out for closer to 48 hours, it will be thicker and taste more sour.

Mix the liquid kefir and milk curds that have formed in the jar with a wooden or plastic spoon.

Place a non-metallic sieve on top of a second jar, and pour the kefir into it, allowing the liquid to seep into the jar.

Place the kefir grains in the sieve into a clean, small jar with a lid. Do not wash the grains. Save them for the next batch of kefir.