Whisk the starter culture and sea salt into your water to make the brine.
Place the beets into a quart-sized mason jar, then pour the brine over the vegetables. Seal the jar with an airlock or a tight-fitting lid. Then, allow the kvass to ferment for 4 to 7 days at room temperature.
When it’s finished, it will taste pleasantly sour. Strain and serve right away, or pour into bottles and store in the fridge up to 1 month.