494 JOURNAL OF COSMETIC SCIENCE
closures (i.e., screwcap, flip top, pump, etc.) allow different amounts of human contact with
the product and allow different amounts of contamination/water intrusion during use of
shampoos, conditioners, and body washes when bathing/showering. In addition, packaging
for low a
w -based preservative systems must be designed to prevent moisture entry that
could increase the a
w to a level that may allow microbial growth. Stability studies must
be done under humid conditions to demonstrate that the packaging is capable of resisting
moisture so that the formula remains at the desired low a
w .34 Additional information on
cosmetic packaging may be found in the book chapter on “The role of packaging in product
preservation” by Brannan.35
PRESERVATIVE-FREE PRODUCTS
The cosmetic industry has been transitioning from traditional preservative systems to
preservative-free formulations for several decades. The term “preservative-free” means
without preservative chemicals. It is possible to make preservative-free cosmetics and
drugs if they are sterilized, packaged, and stored properly. Examples include a sterilized
Table III
Formula Ingredients That may be Part of the Preservative System of a Product
Ingredient Function
Preservatives Kill susceptible MO rapidly when used at adequate levels.
Acids and Alkalis Reduce growth of many microorganisms at pH 5 or pH 9.
Alcohols (ethyl alcohol, isopropyl alcohol) May increase membrane permeability, precipitate proteins and
inactivate enzymes 25% will prevent growth of bacteria,
yeasts, and molds.
Quaternary Ammonium Compounds
(benzalkonium chloride, benzethonium
chloride, cetylpyridinium chloride)
Effective against many Gram-positive bacteria at 0.1%
Gram-negative bacteria are more resistant than Gram
positive bacteria.
Anionic Surfactants (ammonium lauryl
sulfate, sodium lauryl ether sulfate)
Many Gram-positive bacteria are more susceptible than
Gram-negative bacteria. Micellular solubilization lowers
effective concentration of lipophilic preservatives.
Fatty Acids and Glyceryl Esters (glyceryl
monolaurate)
Antibacterial at 0.5%–2.0%, potentiated by chelating agents.
Polyols (glycerin, propylene glycol, butylene
glycol, sorbitol, pentylene glycol)
Lower aw to make it difficult for microorganisms to maintain
homeostasis.
Water-soluble solutes (sugars, salts, amino
acids, small peptides)
Lower aw to make it difficult for microorganisms to maintain
homeostasis.
Chelating agents (tetrasodium EDTA,
Citric acid)
0.1%–0.3% EDTA potentiates antimicrobial action against
Gram-negative bacteria by permeabilization synergy.
Phenolic antioxidants MIC values of 70 ppm for Staphylococcus aureus, 150 ppm for
Escherichia coli and Candida albicans. Antibacterial action
may be eliminated by 1.5%–3% vegetable oil.
Fragrances and aroma chemicals 0.1%–1% may have additive or synergistic killing effect with
other preservative system ingredients.
Botanicals (green tea, rosemary, willow
bark)
Flavonoids, terpenoids, and phenolic compounds effective
against many microorganisms.
Enzymes and Microbial Ferments Glucose oxidase and lactoperoxidase enzymes. Lactobacillus,
Leuconostoc, and Saccharomyces ferments.
Combinations of ingredients Experimentation needed to show which combinations work
best in a formulation.
*Table adapted from Orth.14

495 Evolution and Challenges of Sustainability
preservative-free lotion in a unit-dose package and a 500 mL bottle of sterile dextrose for
infusion. Preservative-free cosmetics and drugs are not new and have been sold for many
years, often without the manufacturer knowing why they did not require use of traditional
preservatives. Antidandruff shampoos with salicylic acid around pH 3.5, sterile eye-drop
solutions in protective packaging, antibiotic ointments, and anhydrous/low a
w products,
including powders (i.e., baby powder and pressed powder), oils, and waxes (i.e., lipsticks and
stick deodorants) may not require preservatives. These products are satisfactorily preserved
because of their physicochemical composition and/or packaging.31
Although cosmetics and many topical drug products are not intended to be sterile, adequately
preserved aqueous products in multiple-use containers have a preservative system that
makes them self-sterilizing36 or bacteriostatic/fungistatic for low a
w formulations in which
the physicochemical environment (e.g., lack of water availability) will not allow growth.
It has not been easy to replace parabens with alternative preservatives or multifunctional
ingredients and retain the same level of preservative efficacy in some formulations. Self-
preserving products may be developed by understanding the principles of preservation and
the use of hurdle technology37 to meet adequate preservative efficacy test acceptance criteria.
HURDLE TECHNOLOGY
“Hurdle technology” involves the application of the principles of preservation to reduce or
eliminate the use of traditional preservatives to achieve “mild preservation” of products.37
Hurdle technology is commonly used to reduce preservative requirements or to create
preservative-free cosmetic products by use of low or high pH, low a
w ,surfactants, phenolic
antioxidants, chelating agents, aroma chemicals, alcohols, lack of readily fermentable
carbohydrates (e.g., nutrients), multifunctional ingredients that have antimicrobial activity,
and combinations of these elements to achieve a mild preservative system along with
protective packaging, as follows.
HURDLE TECHNOLOGY: USE OF LOW OR HIGH PH
Microorganisms that can grow in aqueous ingredients and cosmetic products and which
cause infections generally grow best around neutrality (i.e., pH 7). The rate of growth of
microorganisms usually decreases as the pH departs from the optimum pH for growth for
each type of microorganism because they must expend energy to maintain homeostasis as
the pH becomes more acid or more alkaline. Many yeasts and molds and some bacteria (e.g.,
lactic acid bacteria) can tolerate acidic pH conditions, with a pH 4.0.38 The microorganisms
that often cause problems in consumer products may be metabolically injured (stressed) by
extreme pH conditions in which the pH is less than pH 4 or greater than pH 10. The pH
requirements of some types of microorganisms are listed in Table IV.
Formulas with a low pH may be achieved by addition of naturally occurring organic
acids as is done with alpha-hydroxy acid exfoliants (i.e., glycolic acid), facial washes/acne
treatment products with beta-hydroxy acids (i.e., salicylic acid), and athlete’s foot products
(i.e., undecylenic acid), as well as with products containing microbial ferments (i.e., lactic
acid), acidulants (i.e., gluconolactone), and pH lowering chemicals (i.e., citric acid). pH
values around pH 3.5–4.0 may be obtained with aluminum salts used in antiperspirants.
Low pH products may be adequately preserved without traditional preservative chemicals.