Fizzy, sweet-tart refreshing drink, naturally carbonated from fermentation in the bottle. 

Read How to Brew Your Own Naturally Carbonated Sodas for complete detailed instructions for making homemade carbonated sodas.

Makes 6-8 bottles of soda

Ingredients 

2 1/2 cups sugar

3 lemons, cut into chunks

2 limes, cut into chunks

1 1/2 gallons bottled water (see notes)

1/4 teaspoon Champagne yeast (see notes)

Instructions

Wash all bottles and equipment in hot, soapy water before using.  

1. Make lemon-lime simple syrup:

Combine the sugar, lemons and limes with 2 1/2 cups bottled water in a saucepan and stir to combine everything well. Bring to a brief boil over high heat, then set the lid askew and simmer over medium heat for 1 minute. Turn the heat off, cover completely and allow to steep for another 5 minutes.

2. Initial fermentation:

See this post for more detailed instructions and special equipment needed for this step.

Transfer about half the lemons and limes from the saucepan into a 1-gallon jug (discard the rest). Use a funnel to add the lemon-lime syrup into the jug, then fill the jug with bottled water up to the shoulder, leaving 3-4 inches of headroom. Cover the top and shake to stir everything together and aerate the mixture. Uncover and add the yeast, then cover and shake again to combine.

Plug the top of the jug with a drilled stopper. Add a little water to an airlock (below the "max" line) and insert the end of the airlock into the stopper.

Put the jug in a dark place to ferment at room temperature for 12-48 hours. In warmer weather, the lower end of the range will be sufficient, but in cooler weather it can take longer.

Initial fermentation is complete when you can see 2 bubbles pass through the water in the airlock within a minute.

3. Bottle fermentation:

See this post for more detailed instructions and special equipment needed for this step.

Strain the solids from the fermented mixture and discard them. Use a funnel to transfer the strained liquid to flip-top bottles, where it will finish fermenting. Important: bottles must be purchased from a reputable homebrew supplier like this one. Flip-top bottles at craft stores are for decorative use only and may explode from the pressure of fermentation.

Secure the bottle tops and put the bottles back in your dark place to continue fermenting at room temperature for 24-48 hours. Again, in warmer weather you'll need less time than in cooler weather.

After 24 hours, you can test by opening one of the bottles and tasting it. The top should "pop" audibly when you open it, and there should be noticeable fizziness when you pour and taste the soda. Note that the bubbles in homemade soda will taste softer than commercial soda bubbles (where a different process is used). More details about this step here.

4. Refrigeration:

Once your soda is ready, transfer the bottles to the refrigerator for 24 hours. This will slow additional fermentation to a crawl, and prepare your soda for drinking!

To open the bottles: lay a towel over the top of the bottle before opening, just in case there is more pressure than you expect. (Unlikely, but it could overflow like Champagne!) To open, hold the bottle with the round neck piece of the wire baling facing away from you. Use your thumbs to press on the back of the vertical wire baling. This will pop the round piece forward and release the top.

After the first 24 hours in the fridge, continue to store your soda in the fridge for up to 4 weeks (but note that the taste may begin to change after the first couple of weeks due to ongoing fermentation). 

Notes:

Homemade soda is a fun project to make a refreshing, fizzy drink. It's a hand-crafted drink, and because fermentation is a natural process, each batch may turn out a bit differently. 

This recipe isn't intended to imitate commercial soda, which uses a different process. In my experience, the carbonation from home-fermented soda produces softer bubbles than commercial soda. Also, I didn't consciously attempt to recreate the flavor of Sprite or 7 Up.

You'll want to use good bottled water for this, since chlorine and other chemicals in tap water can kill the yeast and prevent fermentation. I used Poland Spring, but any good bottled water should be fine.

Champagne yeast is available at brewing supply stores and online. It's preferred for home soda-making, because it produces a bubblier brew than brewer's yeast or baking yeast. These other types of yeast would work, but the bubbles won't be as vigorous (which is the point when you're making soda, right?).

Be sure to read this post for all the how-to details, equipment suggestions, and everything else you need for this project.