Homemade sour mix means that having this in my fridge is the start to some delicious cocktails. Add your favorite spirit and shake up something tasty in minutes.
Remove the zest from the grapefruit, limes and 3 lemons using a vegetable peeler. Being careful to avoid the white part or pith.
Combine zest, sugar, pinch of salt and 1/2 cup water in a small saucepan. Bring to a simmer over medium heat. Stirring until sugar is dissolved, about 4 minutes. Cool syrup in a bowl of ice or set aside and let cool.
Squeeze all the lemons to yield 1 cup of juice, limes to yield 1/2 cup juice and grapefruit to yield 1/2 cup. Strain juice through a fine sieve into a medium bowl.
Strain syrup through sieve into bowl with juice. Pressing to extract as much liquid as possible. Discard zest. Stir to combine and store in a mason jar in the fridge.