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.

496 JOURNAL OF COSMETIC SCIENCE
Formulas with a high pH (e.g., pH 9) may be achieved by the addition of sodium
thioglycolate, as is done for permanent wave products and hair relaxers. Alkaline agents
such as NaOH or triethanolamine may be used to increase the pH to pH 9, and this
makes it more difficult for bacteria to survive because they need to expend energy to
maintain their intracellular pH/homeostasis. For example, liquid soaps with an alkaline
pH (pH 9.5–10.5) present a hostile environment for growth of microorganisms due to the
chaotropic (i.e., membrane destabilizing) effect of ionized fatty acids and the free alkalinity
due to NaOH.11 High pH products such as liquid soaps may be adequately preserved
without added preservative chemicals.
HURDLE TECHNOLOGY: USE OF LOW WATER ACTIVITY
Water is a requirement for the growth of all known living organisms. Formulations that
limit the availability of water help prevent microbial growth because each type of bacteria,
yeast, and mold has an optimal a
w and a range of a
w values for growth under any given set
of environmental conditions (e.g., nutrients available, temperature, pH, etc.).39 Formulators
may lower the a
w in a formulation by adding polyols (i.e., glycerin, sorbitol, propylene
glycol, butylene glycol, and pentylene glycol) and water-soluble solutes (i.e., salts, protein
hydrolysates/ammino acids, short-chain fatty acids, sugars, etc.) to achieve the desired a
w .
Unfortunately, use of effective levels of these solutes often interferes with the aesthetics of the
product (e.g., skin feel, rub-in time, tackiness, etc.). One does not need to add enough polyols
or water-soluble solutes to reach the minimum a
w values for specific microorganisms because
just adding enough to lower the a
w to 0.97 will prevent growth of pseudomonads—a major
problem in aqueous products—and this reduced a
w hurdle will work with the other hurdles
in the preservative system to prevent growth of bacteria, yeasts, and molds. The minimum
a
w values for several types of microorganisms are presented in Table V.
Low a
w products may interfere with the ability of microorganisms to grow, and the low
water availability may slow metabolism to decrease the rate of killing action by some
preservatives. Such low a
w products include eyeliners, face powders, lip rouge, lip gloss,
lipstick, and mascaras and were termed “atypical products” because different methods of
preservative efficacy testing (e.g., the use of reduced numbers of microorganisms in the
inoculum, reduced volume of inoculum, surface sampling—as opposed to taking an aliquot
from an evenly dispersed/mixed aqueous sample—and use of different sample diluents),
and different acceptance criteria may be required.40 Even if such makeup products require
“relaxed” acceptance criteria, they should be bacteriostatic/fungistatic (e.g., they should
not allow microbial growth) or be slowly bactericidal/fungicidal, and they must have an
acceptable microbial load—an aerobic plate count (APC) 100 cfu/g (10 cfu/g preferred),
or PCPC limits of 1,000 cfu/g for topical products and an APC 500 cfu/g for eye-area
and baby products,41 and no objectionable microorganisms.
Table IV
pH Requirements for Types of Microorganisms.
Type of microorganism Typical pH range for growth
Many common bacteria pH 5–9
Lactic acid bacteria pH 3.5–8.5
Many yeasts and molds pH 4.5–9
*Table adapted from Orth.31