If using the starter culture, stir together the culture and water. Let the mixture sit while you prepare the ingredients—around 10 minutes. If using kefir whey, add it when the recipe calls for culture.
Remove and discard the outer leaves of the cabbage.
Finely shred or chop the cabbage into bite-sized pieces using a food processor or a hand shredder. Place the shredded cabbage in a large bowl.
Mix in the salt, cranberries, and cranberry juice and pack the mixture into a jar.
Add the starter culture or the whey and fill the jar with filtered water, leaving 2 to 3 inches of headspace to let the cabbage mixture bubble and expand as it ferments.
Seal the container and let it sit on your kitchen counter, out of direct sunlight, for 6 days.
Check the kraut every day to make sure it is fully submerged. If it has risen above the water, simply push it down so it is fully covered again. If white spots of yeast have formed on any unsubmerged kraut, do not worry. Remember, this isn’t harmful. Just scoop out the yeast and kraut it’s on and push the rest back under the water.
When the kraut is done fermenting, place it in the refrigerator.