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

499 Evolution and Challenges of Sustainability
HURDLE TECHNOLOGY: USE OF PHENOLIC ANTIOXIDANTS
Many phenolic antioxidants have antimicrobial action. Branen and Davidson reported that
systems containing multiple antioxidants or antioxidants used in combination with metal
chelators often demonstrate synergistic antimicrobial action.51 Butylated hydroxyanisole
(BHA), butylated hydroxytoluene (BHT), tertiary butylhydroquinone (TBHQ), and propyl
gallate are the most common phenolic antioxidants used in food and cosmetic products.
BHA is the most effective antimicrobial of the phenolic antioxidants, and it is active against
bacteria, yeasts, and molds. Generally, Gram-positive bacteria are more susceptible to BHA
than Gram-negative bacteria, with growth inhibition being obtained with 25–50 mg/mL
for Clostridium botulinum, 50–400 mg/mL for S. aureus, 400 mg/mL for P. aeruginosa, and
10,000 mg/mL for Salmonella senftenberg.51 Davidson, Brekke, and Branen found that all
combinations of BHA, TBHQ, and potassium sorbate had synergistic antibacterial activity
against S. aureus and Salmonella typhimurium in Trypticase Soy Broth.52 However, Rico-Munoz
and Davidson found that as little as 1.5–3% corn oil virtually eliminated the antimicrobial
activity against five test organisms in microbiological media.53 These results illustrated that
lipids may decrease the effectiveness of phenolic antioxidants. Branen and Davidson noted that
the effect of pH on the antimicrobial activity of BHA was dependent on the microorganism
tested and they concluded that the greatest potential for use of phenolic antioxidants would
most likely be in combination with other antimicrobials as part of a preservative system.51
HURDLE TECHNOLOGY: USE OF FRAGRANCES AND AROMA CHEMICALS
Kabara noted that spices and essential oils have been used for product preservation.54
The active components in essential oils generally include alcohols, phenols, esters, acids,
aldehydes, and/or terpenes. Essential oils including caraway, cinnamon, eucalyptus, lemon,
and sandalwood have substantial antimicrobial activity, based on zone of inhibition assays.
In 1979, Strum formulated deosafe fragrances using combinations of aromatic chemicals
(e.g., decyl alcohol, citral, eugenol and cinnamaldehyde) and essential oils with antimicrobial
activity (e.g., thyme, clove, and spice oil).55 Deodorancy may be obtained by use of
fragrances that mask offensive odors (e.g., axillary odor), such as natural oils that may have
antimicrobial/deodorant activity (e.g., citronellol, citral, hexyl cinnamal, limonene, linalool,
hydroxycitronellal, geraniol, eugenol, etc.),56,57 or by use of multifunctional ingredients that
have (or enhance) antimicrobial action (e.g., propylene glycol, dipropylene glycol, butylene
glycol, propanediol, BHT, methylheptylglycerin, C12-15 alkyl benzoate, tetrasodium EDTA,
etc.). Houstein et al. developed a deosafe fragrance containing farnesol, glyceryl monolaurate,
and phenoxyethanol that killed corynebacteria associated with axillary malodor.58 Aroma
chemicals have varying degrees of antimicrobial activity for different microorganisms, so
the most prudent use of these multifunctional ingredients may be as part of the preservative
system of a product. A comprehensive discussion of the use fragrances and aroma chemicals
for preservation may be found in the review by Kabara.54
HURDLE TECHNOLOGY: USE OF BOTANICALS
There is a growing interest in using natural extracts from plants as alternatives for synthetic
ingredients because they have antimicrobial synergy with other formula ingredients, they
are considered safe, they often have antioxidant and antibacterial properties, the emerging