Availability and activity of preservatives in emulsified systems

Availability and activity of methyl-p-hydroxybenzoate, phenoxyethanol and chlorocresol in emulsions were studied by equilibrium dialysis and microbiological methods. Equation proposed by Bean and coworkers for calculating the availability of free preservatives in the aqueous phase of simple emulsion was also found to be applicable to modern emulsions composed of more than one oil and surfactant. The antimicrobial activity varied with the type of preservative and microorganism. Among the four test organisms, Pseudomonas aeruginosa and Staphylococcus aureus were more resistant to the various preservatives than Candida albicans and Aspergillus niger. The concentration of free preservative in the aqueous phase of the emulsion required to reduce the microbial population to the level specified in BP was found to be less than the concentration needed in distilled water indicating that the antimicrobial activity of the preservatives in emulsions is not entirely dependent on the concentration of their free form in the aqueous phase of emulsions.