Combine the shredded sweet potato and sugar in a half-gallon (2-L) vessel. Fill the rest of the vessel with water, leaving 1 inch (25 mm) of headspace. Cover with a permeable lid such as a coffee filter or clean cloth secured with a rubber band or canning ring.
Set at room temperature to ferment for 3 to 5 days or until it is bubbly and beginning to get tangy. During this time, yeast may accumulate on the surface of the ferment. If it does, you can simply scoop it out. It will abate during the second fermentation.
Once the initial fermentation is underway, strain the sweet potato from the kvass and transfer the liquid to airtight bottles, leaving a bit of headspace, because the carbonation should be strong. Once bottled, leave to ferment at room temperature for at least 2 to 4 days, or until the beverage has a bit of tang and is carbonated.
Consume within a few days if storing at room temperature. Or, refrigerate for 1 to 2 weeks, checking the carbonation frequently to avoid the buildup of too much pressure.
Notes
This recipe was reprinted with permission from Traditionally Fermented Foods by Shannon Stonger, Page Street Publishing Co. 2017Makes: Approximately 3 pints (1.4 L)Fermentation Time: 5 to 9 daysStorage Time: Several days at room temperature, 1 to 2 weeks in the refrigerator