The parabens are effective over a wide pH range and have a
broad spectrum of antimicrobial activity, although they are most
effective against yeasts and molds. Antimicrobial activity increases
as the chain length of the alkyl moiety is increased, but aqueous
solubility decreases; therefore a mixture of parabens is frequently
used to provide effective preservation. Preservative efficacy is also
improved by the addition of propylene glycol (2–5%), or by using
parabens in combination with other antimicrobial agents such as
imidurea; see Section 10.
Owing to the poor solubility