Simmer the Berries: In a large pot, combine raspberries and sugar with 2 quarts of filtered water. Simmer for 20-30 minutes to dissolve sugar and release berry flavors.

Cool and Strain: Let the mixture cool to room temperature. Strain out the berries to obtain sugary raspberry juice.

Transfer to Fermentation Vessel: Pour the juice into a clean 2-quart glass jar or demijohn.

Add the Starter Culture: Add ½ cup of whey or equivalent starter culture. Stir well.

Fermentation Process: Cover the jar with a cloth or paper towel secured with a rubber band. Place in a warm, dark place for at least 3 days until slightly fizzy. Taste periodically for desired sweetness.

Bottling: Transfer the soda into clean glass bottles using a funnel, leaving an inch of headspace. Seal tightly.

Secondary Fermentation: Leave bottles at room temperature for 1-2 days, checking daily and releasing pressure as needed.

Refrigerate and Enjoy: Transfer bottles to the refrigerator to slow fermentation. Serve chilled and enjoy your refreshing, bubbly Fermented Berry Soda!