J. Soc. Cosmet. Chem., 24, 663-675 (September 16, 1973) Modern Concepts of Cosmetic Preservation WILLIAM E. ROSEN, Ph.D., and PHILIP A. BERKE, Ph.D.* Presented September 6, 1972, be[ore the New York Chapter, Saddlebrook, N.J. Synopsis-The maior variables associated •vith control of MICROBIAL GROWTH and antimicrobial action in COSMETICS are considered. Some factors of special importance for preservative effectiveness are pH, solubility of the preservative in the aqueous phase and its partition between water and oil phases, interference with antimicrobial action by other components of the formulation, and enhancement and synergism. The advantages and disadvantages of approximately a dozen PRESERVATIVES generally accepted for use in cosmetic preparations are briefly reviewed. INTRODUCTION There are several factors that determine whether microorganisms find a cosmetic favorable for growth. First, the composition of the cosmetic deter- mines what types of organisms can grow. Modern cosmetic ingredients pro- vide many of the nutrients needed by microorganisms, but they do not usually support growth to the same extent as do media used in a microbiological lab- oratory for the enumeration or production of organisms. This limits the rate of growth and can render the contaminant in the cosmeti.c more susceptible to the effect of preservatives than it would be when growing in laboratory media. Other important factors are the physical form of the cosmetic and the water content. Aqueous solutions and oil-in-water emulsions are especially prone to rapid growth of microorganisms. Bacteria prefer a high water content, usually above 15%, and the continuous water phase of an oil-in-water emulsion permits their spread more readily than .does the discontinuous water phase of a water-in-oil emulsion. In partially aqueous products, higher water contents encourage more vigorous microbial growth. Dry powders can become sus- ceptible to microbial growth when moisture collects on the surface of the powder during exposure to air. *Sutton Laboratories, Inc., Roselie, N.J. 07203. 663

664 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The effect of pH will be considered later in connection with antimicrobial effectiveness of preservatives. In general, bacteria prefer neutral or slightly al- kaline conditions, whereas molds prefer neutral to slightly acidic conditions. Because this generalization has many exceptions, it is prudent to protect cos- roetic products of all pH's against bacteria, yeast, and molds. The presence of other microorganisms is a critical factor. The batfie for growth and survival goes on in a cosmetic just as it does in nature. There is competition for nutrients, including trace elements, and typically one orga- nism outgroxvs the others. One practical consequence of this competition is that a preservative system with antimicrobial activity against only a few types of organisms, i.e., with a limited spectrum of antimicrobial activity, may ren- der a product susceptible to unexpected microbial contamination. Another factor which influences microbial growth is temperature. Typically, molds and yeast grow well at temperatures of 20ø-25øC, whereas bacteria prefer temperatures of 30ø-37øC. A cosmetic held at usual room tempera- tures, therefore, may have a different susceptibility to microbial attack than a product left in a hot car or kept on a sunny beach. VABIABLES WHICH INFLUENCE PBESEBVATIVE EFFECTIVENESS There are several important factors which regulate the effectiveness of preservatives in combating microbial growth. In general, the higher the con- centration of a preservative, the more effeetlve it will be. Often, a preservative has a eidal (killing) effect at high concentrations and stasis (inhibition of growth) at 1oxv concentrations. It is not wise to "over-preserve" since high concentration or high levels of antimicrobial activity can coincide with toxic and irritant properties toward animal tissues. On the other hand, too low a concentration may be ineffective or may even stimulate microbial growth. The longer the contact time at a fixed preservative concentration, the great- er the number of organisms killed. In theory, microorganisms are killed at a logarithmic rate (first-order reaction kinetics). Under a specific set of condi- tions the same percentage of a microbial population is supposed to be killed with each unit of time. For example, if 90% are killed the first 3 hours, 90% of the remaining 10% (i.e., 9%) are killed the second 3 hours, and 90% of the remaining 1% (i.e., 0.9%) are killed the third 3 hours. In cosmetics, the micro- bial death rate from preservative action is generally not a regular logarithmic function. A third factor regulating preservative activity is the number of microorga-- hisres challenging the preservative system. The greater the number of micro- organisms, the longer it takes for the preservative to drop the count to some low arbitrary level, e.g., less than 100 microorganisms per gram. Since there is typically a chemical and/or physical reaction between preservative molecules and microorganisms, a preservative can be exhausted by excessive microbial