The Zymology 21 team provided me with a very loose recipe for making the Carrot Grapefruit Shrub. This process is best done to taste, so specific ingredient amounts really aren't needed. The finished shrub should have a nice sweet-tart balance with notes of carrot and grapefruit. Here's notes from Louis Chavez, Zymology 21's head mixologist:
Peel equal parts ripe grapefruit and orange, completely removing the pith. Cube or thinly slice all fruit, and place in a large jar or bowl. Coat fruit with sugar and let sit loosely covered for a few days. Check, taste, and stir the fruit each day (bubbles are okay, it means it's fermenting a little). Keep adding a little sugar each day. Next, make the vinegar syrup: equal parts champagne vinegar and sugar. Stir until sugar is dissolved. Add vinegar syrup to the shrub. Stir until all sugar on the fruit is dissolved. If sugar remains, add a touch of vinegar until it is liquid. Juice an equal amount of carrots, then stir in the fruit-vinegar mixture. Pour the shrub through a fine strainer, pressing on the solids to extract as much liquid as possible. Taste and bottle. Adjust the flavor by adding more sugar, vinegar, or freshly squeezed juice to taste.