Sparkling water lighten doughs and give batters more lift, but not for the reasons you might think. Carbon dioxide in sparkling water does two things: It creates bubbles and lowers the pH to around 4, making it slightly acidic. The bubbles in sparkling water start to dissipate when exposed to oxygen and disappear when mixed with flour, but water's pH stays the same. This extra acid slows the yeast's leavening action because the pH doesn't suddenly increase when flour is added, which softens the dough's "yeasty" taste. Acidulating bottled water with lemon juice is the simplest way to recreate the effects of sparkling water in baking.
Measure an amount of bottled water equal to the amount of sparkling water called for in the recipe. Most bread recipes call for about 1 to 1 1/2 cups of sparkling water.
Mix in 1 tablespoon of lemon juice in 1 cup of bottled water for every cup of sparkling water called for in the recipe.
Add the acidulated water when you would add the sparkling water in the recipe. Mix the dough and finish the recipe as usual.
Use 1 cup of water mixed with 1 tablespoon of lemon juice to substitute for the sparkling water in tempura batter.