497 Evolution and Challenges of Sustainability
HURDLE TECHNOLOGY: USE OF CHELATING AGENTS
Chelating agents such as tetrasodium ethylenediaminetetraacetate (EDTA) and citric acid
are used to sequester divalent metal ions that may serve as pro-oxidants that accelerate
autooxidation, cause rancidity, and color changes. Chelating agents enhance the preservative
system because they sequester divalent metal ions in the outer membrane of the Gram-
negative bacterial cell envelope, which destabilizes the envelope to make it less of a barrier
to antibiotics, preservatives, and antimicrobial chemicals. The potentiation of antibacterial
action by chelating agents is known as permeabilization synergy.31 MacGregor and Elliker
found that P. aeruginosa readily adapted to become resistant to QACs.42 However, EDTA
eliminated this acquired resistance. Orth et al. reported antimicrobial synergy against
fluorescent pseudomonads (i.e., P. aeruginosa, P. putida, P. fluorescens and P. stutzeri) in
nonionic lotions with 0.2% MP and 0.2% acrylic acid homopolymer/copolymers, which
acted as a chelating agent, and in tap water with 0.2% MP and 0.01% Na
2 EDTA.43 The
antibacterial action against P. aeruginosa depends on the strain and growth conditions.
Higher concentrations of EDTA may be needed for coliforms (i.e., E. coli) and B. cepacia, and
Kabara recommended that EDTA concentration should be up to 0.3% in some systems.44
Kabara prepared a mild preservative system that contained MP, glyceryl monolaurate, and
EDTA (1:1:1 ratio) that was effective against P. aeruginosa and E. coli when used at 0.3% in a
formulation.45 Denyer, Hugo and Harding studied synergy in preservative combinations and
found that MP plus EDTA had more than additive increases in preservative efficacy.46 Many
workers have reported potentiation/synergism of antibacterial action when EDTA is added
to chemical preservatives, and it is likely that a similar potentiation of preservative efficacy
will be obtained when EDTA is added to cosmetic formulations with multifunctional
ingredients that have antimicrobial action.
HURDLE TECHNOLOGY: USE OF SURFACTANTS
Many surfactants are multifunctional ingredients because they reduce the surface tension
of aqueous solutions in which they are used and have antimicrobial action, in part due
to destabilization of cell membranes. Cozzoli gave an extensive discussion of the role of
Table V
Minimum aw Required for Growth of Selected Bacteria, Yeasts, and Molds
Microorganism Minimum aw required for growth
Bacillus subtilis 0.90
Clostridium botulinum type A 0.95
Escherichia coli 0.95
Pseudomonas aeruginosa 0.97
Pseudomonas fluorescens 0.97
Salmonella spp. 0.95
Staphylococcus aureus 0.86
Aspergillus flavus 0.78
Aspergillus niger 0.77
Penicillium chrysogenum 0.79
Saccharomyces cerevisiae 0.90
Zygosaccharomyces rouxii 0.62
*Adapted from Enigl and Sorrels.39

498 JOURNAL OF COSMETIC SCIENCE
surfactants in self-preserving cosmetic formulas.47 Anionic surfactants including sodium
lauryl sulfate, ammonium lauryl sulfate, and sodium laureth sulfate are commonly used
in shampoos, cleansers, and body washes. These agents kill Gram-positive cocci, such
as S. aureus and micrococci, relatively quickly, but Gram-negative bacteria including
coliforms (i.e., E. coli), pseudomonads (i.e., P. aeruginosa), and some Bacillus spp. may be
tolerant to anionic surfactants. Although nonionic surfactants generally are thought to have
negligible antimicrobial activity at use levels, polysorbate 80 may increase the preservative
system of a product at concentrations below the critical micelle concentration (CMC).
Lipophilic preservatives, such as the parabens, may partition into polysorbate 80 micelles
at concentrations above the CMC, with a reduction of antimicrobial activity. This effect
is known as “micellar solubilization” and is a primary reason why formulators who use
polysorbate 80 to solubilize ingredients often need higher than expected amounts of the
parabens to achieve adequate preservation of their formulas.33
QACs including benzalkonium chloride (BAC), benzethonium chloride (BEC), and
cetylpyridinium chloride (CPC) have appreciable antimicrobial activity at concentrations
around 0.1%. Scott and Gorman reported that Gram-negative bacteria (especially the
pseudomonads) are more resistant to QACs than Gram positive bacteria.48Polyquaternium-10
is believed to have antibacterial activity.33 QACs such as CPC, BAC, and chlorhexidine
gluconate (CHG) have been used as components of the preservative systems of contact
lens wetting solutions, emulsions, and conditioners.47 Chelating agents often potentiate
preservative action in formulations that contain QACs.
HURDLE TECHNOLOGY: USE OF FATTY ACIDS AND GLYCERYL ESTERS
Fatty acids are generally used in cosmetics as acidulants, co-emulsifiers, and super-
fatting agents, and stearic acid has been the fatty acid most often used. In the extensive
review of fatty acids and esters as multifunctional ingredients, Kabara noted that fatty
acids and their corresponding esters have a long history of use as antimicrobials.49 In
1973, Freese, Sheu, and Galliers reported that lipophilic acids, such as lauric acid and
myristic acid, are antimicrobial because they inhibit membrane transport of oxidizable
substrates into cells.50 The antibacterial activity of fatty acids is affected by pH because
the pH determines the degree of dissociation. Antimicrobial action is increased by
decreasing the pH, which increases the concentration of unionized fatty acid that can
partition into the cell membrane. Lauric acid and palmitoleic acid are the most active
antimicrobial saturated and monounsaturated fatty acids, respectively. Fatty acids
generally are more effective against Gram-positive bacteria than against Gram-negative
bacteria.49
Esters of monoglycerides frequently have been used in cosmetics, with the most
important ester being glyceryl monolaurate because it is a more active antimicrobial
than lower (i.e., C
8 –C
10 )or higher (i.e., C
14 –C
18 )chain length esters.49 Kabara stated that
monoesters of glycerol and diesters of sucrose had higher antimicrobial activity than the
corresponding free fatty acids. Kabara advocated the use of antimicrobial systems with
food grade chemicals, and glyceryl monolaurate is recognized as a generally recognized
as safe ingredient by the FDA. The antimicrobial spectra of activity of fatty acids
and glyceryl esters are increased by decreasing the pH and using a chelating agent or
antioxidant.49