Peel apples if desired and chop into chunks. Process in food processor until you achieve the thin or chunky texture you desire.
Mix in whey, cinnamon, and salt if using.
Transfer to a quart jar, leaving at least an inch of headspace.
Cover the jar with a tight lid, airlock lid, or coffee filter secured with a rubber band.
Culture at room temperature (60-70°F is preferred) until desired flavor and texture are achieved, about 1-3 days. If using a tight lid, burp daily to release excess pressure.
Once the applesauce is finished, put a tight lid on the jar and move to cold storage. The flavor will continue to develop as it ages. It should keep for 1-3 months.
Please note that you must check this ferment frequently because it is made of fruit. The lactic acid bacteria can quickly convert to an alcohol ferment so only culture for a short period of time, especially if the temperature is warmer.