Benzoic acid

Benzoic acid — commonly added as a salt (sodium benzoate) — is a fairly broad-spectrum anti-microbial used as a "preservative" or stabilizing agent in acidic foods and beverages.

It can be used as a stabilizing agent for wine, instead of the more popular sorbic acid (i.e. potassium sorbate). Likewise, brewers generally want to avoid using ingredients that contain a benzoate salt because it will very likely interfere with the fermentation.

Benzoic acid is most effective at low pH because it's the undissociated benzoic acid that is active against microbes. The pKa is about 4.2, which means at pH 4.2 only 50% of added benzoate is active.

Thanks to the Henderson-Hasselbalch equation and a bit of algebra, we can calculate the fraction of added benzoate that is active at a given pH:
Fraction active = 1÷(1+10^(pH−4.2))