You don't! I do think fresh grapefruit juice is worth the (minimal) extra effort. The flavor and color of fresh grapefruit juice is simply more intense, but bottled grapefruit juice will definitely get the job done. If you buy bottled grapefruit juice, be sure to select 100% juice that's sugar free for best flavor. Similarly, opt for fresh lime juice. Use the lime juice in the bottles if you must (the kind that can be found in the produce section), but please, no Rose's syrup.