Shrubs, switchels and drinking vinegars
Updated
The easiest category in this property to make safely, because the thing that preserves them is also the flavor.
Acid as the product
A shrub is fruit, sugar and vinegar. A switchel adds ginger. A drinking vinegar is the same idea without the fruit.
The vinegar is not a preservative added to a drink; it is the character of the drink, which means the safety and the flavor point in the same direction for once.
Why they keep
Very low pH, frequently below 3, which is well under every threshold that matters.
High sugar, which lowers water activity.
Two strong hurdles together, which is why shrubs have a long history as a preservation method rather than as a drink.
Note the consequence for the two lines: the acid is already there, so there is no acidification question in the way there is for a low-acid product, and there is no fermentation continuing in the bottle unless you put a live culture in one.
Making one
Cold process: macerate fruit in sugar, draw out the juice, strain, add vinegar. Better flavor, and the maceration is a wet ambient step to keep clean.
Hot process: cook the fruit and sugar, then add vinegar off the heat. More stable, less bright.
Either way the vinegar goes in last, because heating it drives off the acetic acid you want.
Measure the finished pH rather than trusting the recipe, because fruit varies.
Selling it
As a concentrate to dilute, which is how they are used, and which gives the shipping advantages of a syrup.
To bars, where the acid and sugar structure is exactly what a cocktail wants, and where a distinctive shrub is a genuine product rather than a commodity.
And note the labeling: if there is fruit juice in it, the juice questions apply, however acidic it is.