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Acceptance Standard
Linear regression method determined (expressed by ‘D’ value) the decimal-reduction time of microorganism in challenge test, namely, the time that a 90% microorganism reduction required. This method is used by traditional microorganism measurement –APC. The evaluation speed by the linear regression method is quicker than CTFA and USP method.

The results of USP and CTFA methods indicated ‘pass’ or ‘fail’. The linear regression method result is showed by the D value that could be determined from a survivor curve by the construction method and least-squares method. The curve is prepared by plotting the number of viable organisms recovered at various times.

We could compare the test standard of microorganism death rate of USP and CTFA method with linear regression method through translating the D value. The D value for bacteria in USP method is 112; the D values for bacteria and fungi in CTFA are 56 and 168, respectively. They all are far greater than the D value 4 and 28 in linear regression method. It is clearly that the standard of linear regression method is stricter than USP and CTFA methods. It has been shown that no re-contamination of adaptation microbes have been found in products that passed the acceptance standard of linear regression method. The preservative system, if it passed the test of linear regression method, has less possibility of failure than if it only passes the test of USP or CTFA methods. It is illustrated that the linear regression method provides a more reliable means of performing preservative efficacy tests.

Experimental Time
Linear regression method is a rapid preservative evaluation method. The test of pathogens such as staphylococcus aureus and Pseudomonas aeruginosa could be compared within 3 days and the test of non-pathogens could be completed in two weeks. The test period of USP and CTFA method are both 28 days, it even needs 8 weeks if it is continued with re-challenge test. But the evaluation time is greatly shortened for linear regression method because it is unnecessary to go on for re-challenge test.

There is still not a fully standardized method by now to evaluate the preservative systems in cosmetics and personal care products. In fact, many large-scale companies have built internal criteria for evaluation according to the USP method or CTFA method. Under the same condition, it is suggested that Linear Regression method could be alternatives to evaluate the efficacy of preservative systems because it is easy, quick and reliable evaluation method.

What Makes a Good Preservative?9,10
To overcome the broad spectrum of microbes, and at the same time, not to be harmful to the skin and deleterious to other ingredients in a cosmetic product, it is critical to use the right preservative. The optimal preservative should have the following attributes:
• Broad spectrum activity (bacteriae & fungi)
• Be effective over the anticipated shelf life
• Be preferably liquid and water soluble
• Be effective over a wide pH range
• Not be deactivated by other ingredients
• Be odorless, colourless, and safe

The following table gives an overview of the most used synthetic preservatives in the cosmetic industry. They all have been tested for efficacy and safety. By far the most used preservatives are the parabens (in about 80%).

Appropriate Use of Preservatives
Every cosmetic formulation requires a tailormade preservative system to meet its specific needs. Several factors influence the choice and dosage of a preservative:
• Working conditions (clean environment and equipment reduces amount of preservatives)
• Type of product (leave-on and eye-area products require more preservatives)
• Type of formula (some preservatives work less effectively in complex emulsions)
• Combinations (by combining preservatives the amount of each agent can be reduced)
• Length of shelf life (the longer the desired shelf life the more preservatives are needed) Based on these factors there is no standard preservative system that can be utilized for all kinds of cosmetic products. Nevertheless the following table gives some clues on how the shelf life of a cream can be prolonged:

Measures to Reduce Preservatives
Although preservative-related side-effects as skin irriations and allergies occur very rarely, use of preservatives should not be excessive (do not use them at higher concentrations than allowed by the FDA or other authorities). There are additional possibilities to avoid premature spoilage of homemade cosmetics:
• Disinfect the working utensils and containers with isopropyl alcohol or by putting them in boiling water for 20 minutes.
• Use sterilized (boiled for 20 minutes) and distilled water for your products
• Make your products in small batches only and not in family sizes that last forever
• Do not dip your fingers into your products (particularly creams). Use a spatula or
• Store your products in the refrigerator and label the product with the date of production. Keep products out of the sunlight and sunheat.

Loss of Effect of Preservatives
Besides a gray-green layer of mold on the surface of a product, there are several other factors indicating that a cosmetic product is severly contaminated with microbes:
• Loss of viscosity (product becomes thinner)
• Emulsion break (separation of water and oil)
• Cloudiness of previously clear products
• Loss or change of color or malodorousness
• Drop in pH (product becomes more acid)

In conclusion, it is strongly recommended to preserve your homemade products! Do not be afraid of using synthetic preservatives. They are truly effective and safe. But do not forget to work clean! If your product has spoiled though, throw it away. Adding preservatives will not make it usable again

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9.    Adams, H.J., Mastri, A.R., Charron, D., 1977. Morphological effects of subarachnoid methyl paraben on rabbit spinal cord. Pharmacology Research Communication 9, 547–551.
10.    Aeling, J.L., Nuss, D.D., 1974. Systemic eczematous ‘‘contact-type’’ dermatitis medicamentosa caused by parabens. Archives of Dermatology 110, 640.
11.    Agrup, G., 1969. Hand eczema and other hand dermatoses in South Sweden. Acta Dermato-Venereol 49, 5–91.
12.    Alder-Hradecky, C., Kelentey, B., 1960. On the toxicity and local analgesic effects of p-hydroxybenzoic acid esters. Archives of International Pharmacodynamics and Therapeutics 128, 135–142.
13.    Ali, M.S., Chaudhary, R.S., Takieddin, M.A., 1999. Simultaneous determination of metronidazole benzoate, methylparaben, and propylparaben by high-performance liquid chromatography. Drug Development and Industrial Pharmacy 25, 1143–1147.
14.    E.J. Routledge, et al., “Some Alkyl Hydroxy Benzoate Preservates (Parabens) Are Estrogenic,” Toxicology and Applied
15.    Pharmacology Vol. 153, November 1998, pp. 12-19,
16.    Marianne Marchese, “Environmental Influences on Women’s Health: How To Avoid Endocrine Disrupting Compounds,”
17.    Townsend Letter for Doctors and Patients, July 1, 2004.
18.    Rose Marie Williams, “Think before You Pink; Breast Cancer Awareness,” Townsend Letter for Doctors and Patients, October 1, 2005.
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