Step 1

For a soda with deep red color, look for ruby-colored rhubarb stalks. Like unfiltered beer, this soda is not completely clarified, so it will foam more than traditional soda and features a creamier head. Save the strawberries in step 2 for topping yogurt, oatmeal, or ice cream.

Step 2

Line fine-mesh strainer with 4 layers of cheesecloth, leaving 2 inches of overhang, and set over 8-cup liquid measuring cup or large bowl. Place rhubarb in blender and blend on high speed until mostly smooth, about 1 minute. Transfer rhubarb to prepared strainer and press with back of ladle to force juice through. When mixture looks dry, gather edges of cheesecloth together to form ball. Using your hands, squeeze ball to extract as much juice as possible; discard rhubarb pulp. (Rinse and hang cheesecloth to dry—you will use it again in step 2.) Transfer rhubarb juice to lidded container. Toss strawberries and sugar together in medium bowl; cover. Transfer both covered containers to refrigerator and let sit for at least 12 hours or up to 24 hours.

Step 3

Line fine-mesh strainer with reserved cheesecloth, leaving 2 inches of overhang, and set over 8-cup liquid measuring cup or large bowl. Transfer strawberry mixture to prepared strainer and press with back of ladle to force juice through. When strawberries look dry, gather edges of cheesecloth and join in middle to form ball. Using your hands, squeeze ball to extract as much juice as possible. Transfer strawberries to airtight container and save for another use. Gently pour rhubarb juice into strawberry syrup, being very careful to leave sediment behind; discard sediment (some juice will be left with sediment). Stir in cold water, transfer to airtight container, and store in refrigerator until ready to make soda. (Juice can be refrigerated for up to 2 days.)

Step 4

Transfer cream whipper to freezer and chill until very cold, at least 15 minutes. Pour 1 cup cold strawberry-rhubarb juice into cream whipper and add 1 ounce (about 3 cubes) ice. Stir until ice is dissolved. Screw cap on cream whipper and charge with 1 CO₂ cartridge. Shake cream whipper for 20 seconds. With nozzle facing up, cover nozzle with paper towel (to catch any small amount of spray), and very slowly vent the cream whipper completely. (This step removes air from the cream whipper, which is important for two reasons. First, getting the air out means we have more space to add carbon dioxide, which is what will make our drink nice and bubbly. Second, small pockets of air trapped in the soda act as unwanted nucleation sites, causing the dissolved CO₂ to turn into bubbles in the chamber instead of in our glass.) Charge with 1 more CO₂ cartridge and shake cream whipper for 20 seconds. Let cream whipper sit for 5 minutes. With nozzle facing up, cover nozzle with paper towel (to catch any small amount of spray), and very slowly vent cream whipper completely. Unscrew cap, pour soda gently down side of serving glass, and serve (with ice, if desired).