66 PRESERVTIVES Cosmetic preservatives: problems in their use When cosmetic chemists speak about preservation. they refer to retarding or preventing product deterioration from the time the product is manufactured until the consumer completely uses it up. Usually, cosmetic deterioration caused by oxidation or rancidity is excluded in any talk of cosmetic preservation. Definitions "preservative" is a material thai prevents the growth of or reacts with and destroys microorganisms that might damage or grow on the product. "ntiseptics" are materials that prevent the growth of and/or destroy microorganisms when applied to living tissue. "Disinfectants" are materials that destroy disease-producing microorganisms on inanimate objects, and "germicides" refer to products that kill microorganisms. Two other terms are often used in connection with preservation. One is "cidal," which means killing; the other is "stasis," which means inhibition of growth. Preservatives may not make good antiseptics or disinfectants since these products require quick kill. There are two principal reasons why preservation is needed. One is to protect formulated products. Microbial contamination can cause off-odors. color changes or a change in viscosity. It also can degrade active ingredients, break down emulsions, change the feel of products or cause a health hazard. Thus, preservatives are used as insurance against these happening. second reason is that the U.S. Food and Drug dministration (FD), which is concerned with potential health hazards, sets requirements to ensure thai products are safe. FD can take legal action against cosmetics if they are misbranded or adulterated. FD considerations In the manual inspection Plans & Regulatory Policyfor Cosmetics. 0(.'[. t. /982. to Sept. 30. /983, FD announced plans to inspect 928 manufacturing sites in 1983, up from 400 in 1982. Thus, the chances a cosmetic establishment might be inspected have increased. FD has estimated there are 2,600 such locations in the United States. Under the topic "dequacy of Preservation" in this manual. FD states that cosmetics "need not be sterile; however, they must not be contaminated with microorganisms which may be pathogenic and the density of FD is concerned about adequately preserved products because of the effect microorganisms have on human health. nonpathogenic microorganisms should be low. In addition, cosmetics should remain in this condition when used by consumers" (1). FD inspectors are instructed to "determine whether each cosmetic, particularly each eye-area cosmetic, has been tested during product development for adequacy of preservation against microbial contamination which may occur under reasonable foreseeable conditions of consumer use" (I). The manual also outlines the need for adequate preservation of shampoos, rinses and conditioners, and cites recalls of these products as well as baby lotions, creams and cleansers due to microorganisms. FD inspectors also are instructed to collect cosmetic samples and to forward them for microbiological examination. ll Gram-positive bacteria, Gram-negative bacteria, yeasts and mold present in greater than 1,000 colony-fonning units are to be identified. This article on the chemistry of cosmetic preservatives ""'aswritten by David C. Steinberg, vice-president of Sutton Laboratories Inc. and chemistry professor at Fairleigh Dickinson University, at the request of INFORM ssociate Editor Marsha Hardin. Steinberg is the CUT rent national president of the Society of Cosmetic Chemists. The first two products listed under "Prohibited Ingredients and Other Hazardous Substances" are preservatives. One is hexachlorophene, which is allowed as a preservative only when alternative preservation systems have been shown to be ineffective. However, the concentration cannot exceed 0.1 %. lso, it may noi be used in cosmetics which in normal use may be applied to mucous membranes. lso listed are mercury compounds. which are limited for use in eye-area cosmetics. These may not exceed 65 parts per million of mercury calculated as the metal. Mercury compounds are permined when no other effective and safe preservative is available. Inspectors are to check and report all products containing hexachlorophene and the reasons given for not using another preservative. They also are asked to check for the use of mercury and to report the concentrations used. Thus, the use of either mercury or hexachlorophene in cosmetics is tolerated but the cosmetic manufacturer must show that these are the only alternatives to inadequate preservation. FD is concerned about adequately preserved products because of the direct effect microorganisms have on human health and the circuitous effect product contamination and spoilage, product separation or formation of harmful microbial metabolites may have (I). FD is particularly concerned with inadequately preserved cosmetics coming into contact with the eye. For instance, it cites the hazards of contamination with Pseudomonas oerugi nosa which has caused corneal ulceration and blindness. FD has further defined eyearea cosmetics to include products INFORM. Vol. 3. no. 1 (JonuolY 1992)
67 that might come into contact with the eye, such as shampoos. creme rinses, conditioners, creams, lotions and cleansers. FD regulations require inspectors to check for sanitary storage and handling of new raw materials, and for sanitary manufacture of finished products. Inspectors must determine whether each batch is tested for microbial contamination before being released for shipment, and whether each cosmetic has been tested for adequacy of preservation against microbial contamination which may occur under reasonable conditions of consumer use (2). FD has stated that products not tested for preservation adequacy might be declared misbranded if there is no warning label (3). Microorganisms that cause this concern fall into four broad categories: mold, yeast, Gram-positive bacteria and Gram-negative bacteria. Mold prefers an acidic ph (4.5-5.5) and grows most rapidly at room temperature (20--25 q. Yeast also usually prefers an acidic ph and room temperature. Gram-positive bacteria and Gram-negative bacteria prefer neutral to alkaline ph (7--8) and wanner temperatures (30--37 C). Microbes require several conditions to grow. These include water, air and nutrients. The latter may consist of gums; sugars; starches; alcohols such as glycerol, sorbitol, mannitol and fatty alcohols; fatty acids and their esters; sterols including lanolin and its derivatives; proteins; and vitamins. Many variables influence preservative effectiveness. In general, the higher the concentration of the preservative, the more effective it wih be. Often, preservatives are "cidal" at high concentrations but show only "stasis" at low concentrations. It is not wise to "over preserve" because high levels can act with toxic and irritant properties toward humans. Cosmetic formulators tend to over preserve their products because of the methods used 10 test preservation. The common way to determine if a preservative system is functioning is to run a challenge test. This usually means the formulator adds 10 6 or 10 1 microorganisms per ml 10 the product which is then checked in 72 hours to a week to see if they are still present. s a result, formulations are preserved to meet this severe test. I am not aware of any published studies comparing challenge results with consumer use data. FD requested such data in October 1976 but no company has responded (4). Thus, challenging with large quantities over a short time to find out if the system can withstand small quantities over a long exposure has not been verified as a correlation. Formulation factors The first and most important factor to consider in formulations is ph. For example, quaternary ammonium compounds (quats) are effective only above ph 7. More importantly, many preservatives have acidic hydrogens. Sorbic acid, benzoic acid, dehydroacetic acid and phenolic compounds such as parabens are all in this class. The acid fonn is active, while the salt has no preservation activity (Schemes I and 2). s demonstrated in Table I, the lower the ph, the more activity there is from the parabens and other acidic preservatives. Strong hydrogen bonding compounds inactivate these preservatives by tying up the hydroxyl group. The most common examples of inactivetors are highly ethoxylated com- / pounds such as acids, alcohols, and esters. Other hydrogen bonders include gums, cellulose products and lecithin. The second important factor is the solubility of preservatives in the aqueous phase, and their partition between the water-and-oil phase. Microorganisms grow in the water phase or at the water-oil interface. Therefore, the preservative must be in the water phase to be effective. For example, at room temperature. methylparaben is soluble in water at 0.25%. If this saturated solution comes in contact with vegetable oil in an emulsion, the paraben will migrate to the oil phase. third factor to consider is the effect of other ingredients. It was previously stated that ethoxylated compounds inactivate parabens, benzoic and sorbic acid. Some organic compounds form a coating a.round the microorganisms and give the cell pro. tection against chemical attack. Many compounds of cosmetics inactivate or lower the activity of preservatives by reacting with them, absorbing them or dissolving them. lthough the inactivation is sometimes complete, often it is not, and some residual activity remains. Thus, anionic surfactants inactivate cationics (quais), phenolics and mercurials. Proteins inactivate quare. parabens. phenolics and mercurials. Quats lose activity in the presence of,,0,,0, H' - C -C ctive,, OH 0- SCHEME 1 Inactive OH 0-0 0 ' /c~ o 0 /0 0 ctive SCHEME 2 loactiv<> H' INFORM. Vol. 3. 00. I (January 1992)
68 PRESERVTIVES TBLE I P~rcentage of Undissociated Preservenve at Different phs Preservative ph. ph 5 ph. ph 7 ph 8.5 Sorbic acid 86 37 s 0.' Benzoic acid 60 13 1.5 0.15 Dehydroacetic acid 95 65 16 2 Parnben 77 63 50 easy and safe 10 handle. If it is a solid, it should be an easy-to-weigh power or flake; if it is liquid. it should be nonflammable or nontoxic as shipped. In an lice-in-wonderland world, preservatives would be free. However, the cost of even the most expensive preservative is cheap compared with the cost of a product recall. lanolin, silicates, kaolin and others. related problem is absorption of the preservative by the packaging. Rubber and plastic. especially polyethylene containers or caps, are always suspect because lipid-soluble preservatives are capable of migrating into them. Some products enhance the activity of a preservative system. For instance, some perfume oils have antimicrobial properties. Some preservatives are made more effective by the presence of other ingredients which often have little or no antimicrobial action of their own. The best example is ethylene diaminetetraacetic acid (EDT). lternatively. enhanced antimicrobial effectiveness may result from the use of two or more preservatives. This is now common and is referred to as a preservative system. The 'ideal' preservative Before discussing the chemistry of the currently used preservatives. it is important to review the properties of the' ideal' preservative. One must remember, however, that the ideal preservative does not-and probably cannot--exist. Broad-spectrum activity, the most basic property. is the ability of the preservative to kill microorganisms. equally effective against Gram-posttive and Gram-negative bacteria and fungi (yeast and mold). In general. most preservatives are active against either bacteria or fungi, but not both. s preservatives do not add to the marketability of finished products. it is desirable for the preservative to function at the lowest possible conccntratlon level. This will also reduce the costs. minimize the toxicity effects and not change the physical properties of the cosmetic. Because microorganisms grow in the water phase. the preservative must be in the water phase to function. Therefore. the ideal preservative would be very soluble in water and completely insoluble in oil. This would also prevent migration of the preservative to the oil phase for longterm stability. The preservative should be completely stable in all extremes of conditions including ph and temperature that might be encountered during the manufacture of cosmetics. It should not add color or odor to the cosmetic nor react to change the color or fragrance of the product. It also should be compatible with all ingredients used in cosmetics and not lose any of its antimicrobial properties in their presence. It should offer protection for the cosmetic during manufacturing and maintain antimicrobial protection throughout the intended life of the cosmetic. It should be completely safe to use. easy to analyze for activity in the finished product. This is more difficult than it sounds. For example, it is easy to analyze for mercury in the finished product. However. this only indicates how much mercury is present and not whether it is available to preserve (as mercury) or is tied up by products such as proteins. Parabens are another example. We can determine their level by HPLC (high performance liquid chromatography) but this does not indicate if they are inactivated by "Tweens" or other compounds. Cosmetic chemists would like one chemical to completely inactivate the preservative. This chemical would never be found in any cosmetic. We would like this to be incorporated into our agar so that the preservative would not carry over from the formulation into the recovery medium. Common preservatives The following highlights some of the more common preservatives. Ethyl alcohol generally shows antimicrobial activity at levels above 15% in acid ph and 18% in neutral or alkaline ph. It does evaporate and is not generally used as preservative; rather, alcoholic solutions often need lesser quantities of preservatives. QuaiS look good in screening experiments (except against Gramnegative bacteria) but poor in fonnulations. Most are irritating to the skin and are inactivated by anionics, nonlonics and proteins. Quats are used most often for sanitizing equipment. Sorbic acid has limited water solubility and usually is added as the potassium salt. However, the ph must be lowered to its useful range. It discolors on storage and is a skin irritant. Sorbic acid has useful activity against mold but is poor against bacteria. Formaldehyde is cheap and has high activity. However, it is volatile. reacts with fragrances. is irritating to the skin, and smells. Fonnaldehyde is used only in rinse-off items. It should never be used at ph 8 or above because it becomes a strong reducing agent. II seems to inactivate proteins. and proteins inactivate fonnaldehyde. nother preservauve-cz-bromo-znitro-lj-prcpenedlcl (Bronopol)- has excellent water solubility and is useful at ph 5.5. It breaks down at neutral and alkaline ph to give fonnaldehyde and bromo compounds. Exposure to light causes brown color. and solutions should not be heated. Bronopol migrates into certain oils. It also has been implicated in nitrosamine formation. The new FD manual requires that all samples containing Bronopol be collected and analyzed for possible nitrosamine content. The parabens--esters of parahydroxy benzoic acid-are the most INFORM. Vol. 3, no. I (January 1992)
69 TBL.E 2 Solubility or Parabens in Water H,CQO 0 Methylparaben Ethylparabcn Propylparaben Butylparaben 10" C 0.2 0.07 0.025 0.005 25"C 0.25 0.17 0.05 0.02 80 0 C 2.0 0.86 0.3 0.15 o T- CH 3 ON, SCHEME 3 SCHEME 4 widely used preservatives for cosmetics in the world. They are principally active against fungi with limited activity against bacteria. and are especially weak against Gram-negative bacteria. s shown in Table 2. dissolving parabens in formulations presents a challenge. The best method is to dissolve them in a solvent such as propylene glycol. nother method is 10 heat the water phase to 75-80 C and add the parabens with stirring. This is superior 10 adding the parabens ar room temperature and healing to emulsification temperature. Parabens should be used at a level of 0.3% or less or irritation could occur. The FD inspection manual states that products must be preserved against pseudomonads (5). In 1977. it was reported in the Journal of Cosmetic Chemists that a CfF (Cosmetic. Toiletry and Fragrance ssociation) model lotion preserved only with 0.2% methylparaben and 0.1% propylparaben failed to kill Pseudomonas aeruginosa.(6). nother paper reported that 26 commercial emulsions preserved with parabens failed a challenge against P. aeruginosa after one day in 21 of the 26 products (7). nother preservative is dehydroacetic acid, or its sodium salt (Scheme 3). Used below ph 5.5. dehydroacetic acid turns yellow and causes skin irritation. It exhibits some activity against mold but is weak against bacteria. It also reacts with iron oxides. OMOM hydantoin (Scheme 4), known as Glydant, is sold as a 55% aqueous solution. It provides rapid release of formaldehyde and thus gives quick kill against bacteria. However, long-term stability is a problem. Its major use is in liquid soaps pack. aged in a pump dispenser. Diazolidinyl urea (Scheme 5) is Gennallll. Twice as active as Gennall L NH- SCHEMES rf\ CI~ /~ 'CH 3 S 115, it is very active against bacteria and exhibits some activity against fungi (8). Imidazolidinyl urea, known as Germall 115 (Scheme 6), is most frequently used as a preservative by the cosmetic industry after the parabens. It is the first nontoxic. nonirritating preservative that is active against bacteria. However, it is nor active against fungi. It is used with parabens (9). Meanwhile. a combination of magnesium nitrate (22.0%), magnesium chloride (1.0%), water (75.5%). methyl chloroisthiazolinone (1.15%) and methyl isothiazolinone (0.35%) SCHEME 7 (Scheme 7) is known as Kathon CG. This preservative is very active and very toxic, and has a use level of 0.03% to 0.07%. This product reacts with reducing agents such as bisulfites which are found in alpha olefin sulfonate. sulfosuccinates and sulfobetaines. Kathon CG is used in the United Slates for rinse-off products only. nother preservative-phenoxyethanol (Scheme 8)- is 1 mole of EO on phenol. This is a weaker acid than parnbens and has a pka of about 8.5. I! has some activity against bacteria and fungi. Usually combined with parabens. it is used at high levels of 2 INFORM. Vol. 3. no. 1 (January 1992)
70 PRESERVTIVES 0" OCHCHOH,, CI ~Hz ~H2COOH ~-CH2CH2 -~ HOOCCH 2 CH 2 COOH SCHEMES SCHEMED SCHEME 10 0.5 10 1.0%. This product is sold as Rosessence and is used in perfumes. Ouaremlum-f S (Scheme 9) is Dowicil 200. which is 90% cis isomer. The trans and the mixed isomers work the same. ctive against bacteria, Dowicil 200 is used with parabens for fungi. Drawbacks are (a) it must be added cold. (b) it discolors and (e) it is flammable. It is used most often in heavily pigmented products. The last chemical-edt (Scheme ID)-is not a preservative but rather enhances activity. It provides maximum benefit as an enhancer at 0.1 % free acid. References I. Inspection Operations Monuol, U.S. Food and Drug dministration. Exhibit 653. I-. Washington. D.C.. Sept. 15. 1982. p. 2. 2. Ibid.. Sept. 15. 1982. p. 3. 3. Eieiman. H. Drug & Cosmetic Industry:44 (December 1976). 4. Ibid.:46 (December 1976). 5. Inspection Operations Manuol, U.S. Food and Orug dministration. Washington. D.C.. 1982, p. 2. 6. Rosen. W., P. Berte, T. Matzen and. Peterson, J. Soc. Cosma, Chemists 28:83 (1977). 7. Rosen, W., and P. Berke, Ibid. 31:37 (1980). 8. Rosen, W. and P. Berke. Cosmetic &: Toiletries 97(6):49 (1982). 9. Berke. P.. W. Rosen and D. Steinberg. Ibid. (II ):89 (1982). 3RD CESIO INTERNTIONL SURFCTNTS CO GRESS & EXHIBITION - WORLD MRKET Is',15th June 1992, Barbican Centre, London, UK. -.. " """'''PICS. ernational Meeting is e pected to attract 1200 delegates from all surfactant fields. and there will a full trade exhibition and poster display. pplication of Surfactants Surfactants in the Environment Toxicology of Surfadants _... - For Information circle.105 INfORM, \-UI. 3. no. 1(Jonuay 1992)