Because lemon zest contains flavorful natural oils, it adds more lemon taste to foods than lemon juice alone. You need two or three times as much lemon juice to make up for the lack of the zest. But you can substitute lemon juice for lemon zest in most recipes, with lemon marmalade as one obvious exception, because it relies on the lemon peel for both flavor and texture.
Use real lemon juice for the best flavor substitute for lemon zest. Bottled lemon juice typically contains reconstituted juice along with preservatives that give it an artificial taste.
You need anywhere from 1 to 3 tablespoons of lemon juice to substitute for 1 teaspoon of lemon zest. If your recipe for cakes or cookies already calls for lemon juice, just add 1 more tablespoon. With sauces for fish, puddings or glazes to pour over cakes, use up to 3 tablespoons of juice to boost the lemon flavor that you lose by not having zest.
Lemon extract also works as a good substitute for lemon zest. Use 1/2 teaspoon of lemon extract for each 1 teaspoon of zest called for in a recipe.
Differences in Juice and Zest
Beyond the more intense lemon flavor of lemon zest compared to the juice, lemon juice is highly acidic. If you add too much additional lemon juice beyond what a recipe calls for, you could change the texture of your cakes and cookies, making them less dense and more airy. The acids in lemon juice cause leaveners such as baking powder and baking soda to work overtime and create more air bubbles than usual.
Lemon Storage Life
Using the juice from a lemon you zested a few days or a week earlier saves you from simply throwing the lemon away. Lemons stay fresh for one week at room temperature, an additional three weeks in the refrigerator and three to four months in the freezer, so there's little chance of them going bad. Keep two or three lemons on hand so you'll always be able to have both zest and juice available, and use any leftover juice for lemonade.