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The Important Action of Preservatives on Cosmetics

The Influence of Microorganism 3
There is no doubt that all kinds of microorganisms are widely spread around us. Although we have killed successfully an amount of microorganism in cosmetics products through sterilization in the manufacturing procedure, there are some still alive more or less. After choosing suitable preservative system and packaging, cosmetic products can be preserved well. But maybe they would be re-contaminated during consumption. Some ingredients in cosmetics, such as esters, hydrocarbon and soluble polymers tend to provide not only abundant nutrition with carbon and nitrogen sources that are necessary for microbes to grow but also chemical energy with its adduction. The microorganisms in cosmetics would grow, reproduce, and even cause the products to be spoiled once they live under appropriate conditions. The following cosmetic deteriorations are caused by microorganisms, opacities, deposition and color change by decomposing the substance; changing the product’s pH value through the catastate and the appearance through the gases created. The microorganisms may even secrete toxins, which can cause harm and skin anaphylaxis.

The microorganisms that exist in used or unused cosmetics normally include bacteria (either gram positive or gram negative), mold, and yeast. Some microbes are harmless, but some, such as Staphylococcus aureus, Pseudomonas aeruginosa, are greatly harmful to human beings.

At present, there are still not universal criteria for microbiological evaluation of drugs and cosmetics. In general, there are strict standards for baby care and eye care products. In China, according to cosmetics health standard requirement, the colonies of bacteria in cosmetics should not be more than 1000 cfu/mL (g), the colonies in baby care or mucosa products should not be more than 500 cfu/mL (g), the colonies of mold and yeast should not be more than 10 cfu/mL (g), harmful microbes such as Escherichia coli, Pseudomonas aeruginosa and staphylococcus aureus and so on should not be detected.

The Mechanism of Action of Preservative 3
A preservative actually is a protective agent to keep cosmetics unspoiled through killing or inhibiting microorganisms in cosmetic products. It doesn’t show strong instant killing effect until the preservative contacted the microbes’ cells directly at enough concentration.

Some preservatives can destroy the cleavage of cells to inhibit the microbes grow by acting on the target nucleus of microbes, such as cell membranes, cell wall, enzymes of cell. Actually, it affected the activity of enzymes or the structure of heredity particles in bioplasm. For example, the acting target nucleus of phenoxy ethanol or alcohol was cell membrane, Bronopol (2-Bromo-2-nitropropane-1, 3-diol) acted on the SH enzymes, the preservative with formaldehyde donator, such as EverguardTM series, acted on the COOH and NH2 enzymes in the cells, and that, the phenols and aldehydes could cause the protein denaturation. In short, the preservatives interfere cells growth through inhibiting synthesis of enzymes, proteins and nucleic acids.

Factors of Influencing Activity of Preservatives4
The preservatives’ efficacy depends on many factors. The preservative may be highly effective in a special system but it may be of no efficacy in others. Many factors can cause the preservative inactivation in a formula. The following are several major factors that affect preservatives’ efficacy.

Influence of pH
The pH of the system can affect the preservative’s activity through influencing the dissociation of organic acid. For example, Bronopol (2-Bromo-2-nitropropane-1, 3-diol) is very stable when its pH is 4; it also could keep active for a year when its pH is 6; but its activity only lasts several months when its pH is 7.

Influence of Particles and Gels
Some particles like Kaolin or Magnesium Aluminum silicate in cosmetics could reduce the preservative activity because of its adsorption of preservatives. However, the particles could sometimes strengthen the bacteriostatic effects through adsorbing the microbes as well.

The preservatives’ performance may be affected by combination with water-soluble high molecular weight polymer, which can bring down the concentration of free preservatives in the formulation.

Influence of Nonionic Surfactant
All kinds of surfactants in cosmetics, especially the nonionic surfactants, can interfere the activity of preservatives by solubilization and complexation. The oil soluble nonionic surfactant (HLB value is 3-6) has more de-active effect to preservatives than the water soluble one with higher HLB value. The preservative’s activity decreases with its free concentration reduction. So, the use level of preservative should be increased in the nonionic surfactants system in order to keep the same preservation power.

Influence of Preservative Decomposition
Some factors may reduce the preservation efficacy through the preservatives’ decomposition. For example, light or heat cause preservative decomposition; chemical reaction may lead to preservation inactivation or radiation sterilization may reduce preservatives’ activity.

Other Influencing Factors
In addition, there are many other factors, such as preservative distribution in oil/water phase, packaging, fragrance, chelating agents, may all interfere with preservative activity.

Ideal Preservatives5
No single preservative can provide a broad-spectrum anti-microbial activity at very low use levels. But, it is important for us to assume “ideal” preservative criteria so that we can develop a broad –spectrum preservative system successfully. We think an excellent preservative should at least have the following characteristics:
1) It should have broad-spectrum anti-microbial activity against both bacteria and fungi.
2) It should have excellent anti-microbial activity at low levels in cosmetic products.
3) It should be effective over a wide pH range.
4) It should be safe, non-toxic and non-irritating.
5) It should be inert, non-reactive with other ingredients in formula or container materials.
6) It should have suitable distribution coefficient of oil-water phase in order to get effective concentration in water phase.
7) It should be compatible with essentially all cosmetic materials and not to affect the color and fragrance in the finished products.
8) It should be cost effective and easy to obtain.

It is the trend that several preservatives are obtained to form an effective, broad-spectrum mixture system. An effective, broad-spectrum antisepsis performance also depends on the final formula. How to make a preservative system resist microorganisms successfully? The first thing is to avoid being contaminated in manufacturing process; on the other hand, we should avoid contamination in packaging and use procedure.

Preservative Efficacy Test Methods6
There are several different preservative efficacy test methods of evaluating the preservative performance in different countries, including USP method, BP method and EP method. In 1995, BP method was revised in accordance with EP method. The CTFA method is the internal method for Cosmetic, toiletries and fragrance Association in USA. Besides standard USP method and CTFA method, there are some rapid evaluation techniques, such as linear regression method, presumptive challenge test and accelerated preservation test methods. The test process of above three methods are almost the same, but there were some differences in the time needed for analysis and performance criteria. These differences could lead to different results and whether a sample passes the challenge test. So, the three evaluation methods of USP, CTFA and Linear Regression methods are discussed below, and selection of microorganisms, inoculums level, measuring intervals and acceptance criteria are compared. Table 1 gives the details for these three methods.

Table1 The Comparison of Three Evaluation Methods



USP Method

CTFA Method

Linear Regression Method


Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Candida albicans and Aspergillus niger

Gram-positive and gram-negative bacteria, mold, yeast and aerobic spore-forming bacillus some strains separated from anti-preservative samples

Staphylococcus aureus, Escherichia coli, sporeforming bacillus (separated strain), Aspergillus niger and Aspergillus flavus

Inoculums level Unit: cfu/ml (g)


1.0×106 cfu/ml (g) the level of yeast and mold in eye-cosmetic is 1.0×104 cfu/ml (g)


Measuring Process of Sampling

0, 7, 14, 21 and 28 days after original inoculation

0, 7, 14, 21 and 28 days after original inoculated. Take sample additional after 28 days if it was re-challenged.

0, 2, 4, 24, 28 and 168 hours after original inoculated. Or up to the last APC is less than 10 cfu/ml (g).

Discussion of Preservative Efficacy Test Methods 7,8
How to choose microorganism in challenge test

From the above, we can find that the same microorganisms in the challenge test can be adopted in the three evaluation methods of USP, CTFA and Linear Regression: staphylococcus aureus, Pseudomonas aeruginosa and Aspergillus niger. The test procedure shows the organisms changes during the products manufacture and consumption. Although these recommended strains are representative, the seudomonas cepacia is recommended mightily to get the broad-spectrum preservative system. Seudomonas cepacia is a non-fluorescent monad, such as Pseudomonas aeruginosa, which is more sensitive to preservatives than fluorescent monad. Furthermore, the organisms separated from environment or housing microbes recommended by CTFA shall be considered as well, because these microbes are more likely to be present in the products. It is the most ideal if the challenge test can be performed with both the standard trains and the separated organisms partly if the condition permits.

Rationality of Re-challenge Test 11,12
The aim of re-challenge test is to reassess contamination during product use period. It shows the number of challenge test that the preservative system could hold before the system breads down by repeated inoculation with special microorganisms. It is un-necessary to go on re-challenge test for linear regression method, because the microorganisms’ death rate of tested formula that contained enough preservatives really has nothing to do with the amount of microorganisms during the test time.



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