Plain shrub is a type of cold-process simple syrup made with vinegar as the liquid base. Plain shrubs can be used to make fruit-based shrubs with the simple addition of fruit.
To make plain shrub, take one part each of rice wine vinegar and apple cider vinegar and add one part of Demerarra sugar. Place this in a jar with a screwtop lid, and shake well to dissolve the sugar -- it is possible to cook the shrub into a more typical simple syrup, but at the expense of freshness.
Fruit shrub can be made by taking two parts of the desired fruit and placing it in a large jar, then covering it with plain shrub. This mixture should be left at room temperature for a week to allow the fruit flavor to infuse, then strained and placed back in the jar.