Creamy, tangy kefir is very similar to yogurt in its flavor and healthy levels of probiotic bacteria, but it's in a thinner, drinkable form. It's much easier for the home enthusiast to make, because it doesn't require the finicky temperature control needed for yogurt. Instead, lumpy, curd-like colonies of yeasts and bacteria -- the kefir "grains" -- simply work their magic in a bottle of milk at room temperature. The grains are strained out and reused indefinitely. They grow as they're used, so you'll soon be able to store surplus grains as a backup in case of emergencies.
Short-Term Refrigerator Storage
Strain your kefir grains, using a fine-mesh, non-metallic strainer. Kefir grains can be very small and sand-like when the bacterial colonies are new, so the finer your strainer is, the more of these nascent, fast-growing grains you'll retain.
Transfer the grains, or the portion you wish to keep in reserve, to a fresh glass or ceramic bottle or jar. The jar should be freshly washed, and, ideally, it should be sterilized with boiling water to prevent contamination with outside yeasts or bacteria.
Cover the grains with the same quantity of fresh, full-fat milk they'd normally culture. For example, if you work with 1-quart batches and wish to store half of your grains, use a pint of milk.
Seal the bottle or jar, and refrigerate the grains in a back corner of your refrigerator, where the temperature will remain constant.
Strain the grains and replace the milk weekly.
Medium-Term Freezer Storage
Boil a kettle or pot of plain water to sterilize it, then let it cool to room temperature. Alternatively, purchase a bottle of distilled -- not spring -- water, and leave it sealed until you prepare your kefir grains.
Strain the grains from your kefir with a non-metallic, fine-mesh strainer. Separate the portion you wish to freeze, and return the rest to their bottle.
Rinse the grains well with the boiled or distilled water, using a gloved or scrupulously washed or rinsed hand to minimize the risk of cross-contamination. Blot the grains dry between layers of fresh paper towels.
Cover the grains completely with dry skim milk powder, to provide protection against the freezer's harsh environment. Vacuum-seal the grains in their protective powder, or pack them into a heavy-duty zipper-seal bag and squeeze out as much air as possible when you seal it.
Label and date the grains, and freeze them for up to 2 or 3 months.
Long-Term Dry Storage
Pour your kefir through a non-metallic, fine-mesh strainer. Divide the grains, returning some to their bottle and keeping out the rest for drying.
Rinse the grains carefully in distilled or cooled, boiled water; then let them drain. Tap them out onto a pad of fresh paper towel, and gently blot the grains dry.
Spread the grains on a sheet pan lined with parchment paper, and cover them loosely with paper towel or a light, clean fabric napkin to protect against dust and deter insects.
Dry the grains in a breezy, well-ventilated spot, turning them occasionally with a clean wooden spoon or nylon spatula, until they're uniformly dry and brittle. This can take as little as a day or two in an arid climate, or up to a week in a warm and humid one.
Toss the grains with a few tablespoons of dry skim milk powder, which provides a degree of physical protection and also acts as a desiccant.
Package the grains in an airtight container or zipper-seal plastic bag; store them in a back corner of your refrigerator where the temperature will remain constant. They should be used within 6 months, for the best quality, but they should remain perfectly viable for 18 months or more.