Cosmetic tests - Stability studies. Physicochemical and microbiological characteristics. Evaluation of expiration date and/or ODP of cosmetic products (ISO / TR 18811: 2018 Standard and CE Regulation 1223: 2009)
The stability of a cosmetic product can be affected by various factors, classified as:
1) Extrinsic factors (external factors or environmental factors)
The weather, temperature, light and oxygen, vibrations, humidity, microorganisms, packaging material.
2) Intrinsic factors (nature of the product and interaction of its components, with each other or with the packaging material)
Physical incompatibility: precipitation, phase separation and crystallization.
Chemical incompatibility: pH, oxidation-reduction reactions, hydrolysis, reaction between ingredients and ingredient-container reaction.
Stability studies evaluate the evolution of a cosmetic product over time under different temperature conditions in order to predict conditions of use and conservation from the manufacture of the cosmetic product to its use by the consumer. The evaluation of the stability of a cosmetic product under foreseeable conditions of use and storage is an essential requirement to ensure its safety.
Regulation (EC) 1223/2009 of the European Parliament and of the Council on cosmetic products establishes that before placing a cosmetic product on the market, the manufacturer shall ensure that a report is drawn up on the safety of the cosmetic product in which they are included. data on the stability of the cosmetic product.
The stability of a cosmetic product is defined as the ability of a cosmetic product to retain its physicochemical and microbiological properties within specified limits, over time and under different conditions of storage and use. Depending on the results of the stability studies, the expiration date of the cosmetic product and / or the so-called PAO (period in which the product is stable, after opening) is defined.
* In a query made to the AEMPS about the stability tests that must be carried out on cosmetic products, the AEMPS told us: “It is the person responsible for the product, as knowledgeable about it, its properties, characteristics, type of container, etc .., it must establish the analyzes to be carried out ”.
The different physicochemical and microbiological characteristics to be determined in cosmetic products are described below. Likewise, the stability studies to be carried out are briefly detailed in order to determine the safety and evaluate the expiration date and / or the ODP of cosmetic products.
The physicochemical characteristics are quantifiable parameters and previously established by the manufacturer before marketing. Likewise, they are basic criteria to establish the stability of a cosmetic product. In general, the physicochemical tests performed on all cosmetic products are:
- Organoleptic properties: appearance, smell and color.
- Viscosity: quality parameter directly related to the stability of the product.
- Density: quality parameter widely used in liquid products.
- Phase separation by centrifugation: allows observing phase separation.
- Other physicochemical parameters that may influence the stability of the product can also be tested:
- Water activity (aW): fraction of total moisture content that is free. If the water activity is high, it can cause instability in the cosmetic product since the moisture available in the cosmetic can favor the growth of microorganisms.
It is a qualitative measure that measures the state of the water in a system. It is different from humidity which corresponds to a quantitative measure of the amount of water in a sample on a wet or dry basis.
- Assessment of active ingredient content: The assessment of analytical components, regardless of their activity in the cosmetic product, using selective and specific analytical techniques, such as high-performance liquid chromatography (HPLC) or gas chromatography (GC), over time, they are very useful to determine the stability profile of the cosmetic product. The presence of organic components in the formulation can favor the growth of microorganisms since in some cases they can affect the structure of the preservative agents, thus influencing the stability of the product (not currently carried out in our laboratory).
Depending on the cosmetic form to be studied, the microbiological characteristics may or may not be more relevant, since depending on the cosmetic formula it may present a greater or lesser risk of contamination. With this, we can classify cosmetic products into three categories:
They allow the survival and growth of microorganisms.
It is mandatory to carry out a minimum microbiological control and a preservative efficacy test (Challenge Test)
Lower risk due to its own formulation, such as low water content, acidic or alkaline pH.
According to the formula of the cosmetic product, the minimum microbiological control and the preservative efficacy test (Challenge Test) are mandatory or recommended.
Microorganisms survive in these products for a short time and do not multiply in them.
LOW (aW ≤ 0.75)
Microbiological control is not mandatory.
Before starting the stability studies, minimum tests of physical stability (phase separation by centrifugation) and the efficacy of the preservative (Challenge test) must be carried out. If the results are not correct, it would be advisable to modify the formula of the cosmetic product before carrying out the rest of the tests.
Short-term stability (intrinsic stability)
Accelerated (forced) stability or aging test.
Evaluation of the expiration date of cosmetics and / or PAO (date of minimum duration after opening).
Two types of stability studies can be performed:
- Short-term stability studies (intrinsic stability)
Short-term stability studies, also known as intrinsic stability studies, are carried out over 15 days, subjecting the product to sudden thermal cycles, alternating it at different temperatures, simulating the conditions of conservation and use of the cosmetic product. An example of a test can be subjecting the cosmetic product to be studied to temperatures of 45ºC and 5ºC (extreme temperatures of 45ºC and 5ºC alternating every 24 hours), which allows accelerating the appearance of possible signs of instability.
Following the cited example, the stability study of the product to be determined begins at a temperature condition of 45ºC and alternates at 5ºC every 24 hours, so that the odd days from the beginning of the study (1, 3, 5, 7, 9, 11, 13 and 15) the product is stored at 45ºC and on even days (2, 4, 6, 8, 10, 12 and 14) it is stored at 5ºC. Every 24 hours (in each thermal cycle), the measurement of the physicochemical parameters is carried out. Likewise, for products susceptible to microbiological contamination, a minimum microbiological control and study of the preservative efficacy (Challenge Test) is carried out both at the beginning of the study (T0) and at the end of the study (T15) in order to determine that the product is stable.
However, according to the UNE ISO / TR 18811: 2018 standard, depending on the type of cosmetic product to be studied, other temperature conditions can be adapted such as the following:
• 24 hours at 25 ± 2ºC and 24 hours at 5 ± 2ºC.
• 24 hours at 40 ± 2ºC and 24 hours at 4 ± 2ºC.
• 24 hours at 45 ± 2ºC and 24 hours at 5 ± 2ºC.
• 24 hours at 50 ± 2ºC and 24 hours at 5 ± 2ºC.
Likewise, the standard recommends an intermediate temperature of 30 ± 2ºC, with which the test would be carried out at three temperature conditions.
In summary, the standard recommends certain test conditions, but does not condition mandatory criteria. The conditions of the studies can be established according to the type of cosmetic product, its intended use and conservation. With this, the test temperatures could be adapted to those recommended by standard or establish test conditions depending on the type of cosmetic product, use and expected conservation according to the manufacturer´s instructions. For example, a sunscreen could be tested at the condition of 24 hours at 50 ± 2ºC and 24 hours at -5 ± 2ºC or 24 hours at 45 ± 2ºC and 24 hours at -5 ± 2ºC simulating in this way the use and conservation planned.
- Accelerated (forced) stability studies or aging test
Long-term stability studies are carried out by keeping the product at room temperature and at an extreme temperature between 40 and 45ºC (variable parameter in real conditions) depending on the type of cosmetic product, use and expected conservation over 12 weeks, performing physicochemical measurements every 2 weeks on product samples subjected to both conditions. Likewise, in products susceptible to microbiological contamination, the minimum microbiological control and the study of the preservative efficacy (Challenge test) is carried out at the end of the study to ensure that the cosmetic product is safe for use.
This 12-week period in these conditions of conservation at room temperature and extreme temperature (between 40 and 45ºC), can be compared to what would happen in 36 months (3 years), so that the result obtained in the first two weeks would be equivalent to after 6 months in conditions of use and conservation by the consumer; at 4 weeks to 1 year; at 6 weeks to 1.5 years; at 8 weeks to 2 years; at 10 weeks at 2.5 years and at 12 weeks at 3 years.
According to Regulation EC 1223/2009 of the European Parliament and Council, if the cosmetic product passes the forced stability study at 12 weeks, it is not mandatory to indicate the expiration date, but the period after opening (PAO) will be indicated, during which the use of the product is safe and can be used without harm to the consumer.
- Product / container stability studies
Different physicochemical and microbiological tests can be carried out depending on the type of container, how to use it, daily use and container capacity, among others to consider.
Determinations included in each type of stability test:
- Organoleptic properties (appearance, smell, color).
- Phase separation (Centrifugation).
- Water activity (aW) (optional).
- Microbiological tests (microbiological tests are performed at the beginning and end of each study, but the initial ones are common for both stability studies and are independent at the end of each study to ensure that the cosmetic product is safe).
- Detection and enumeration of mesophilic aerobic bacteria (UNE-EN ISO 21149: 2017)
- Yeast and mold detection (UNE-EN ISO 16212: 2017)
- Detection of Escherichia coli (UNE-EN ISO 21150: 2016)
- Detection of Staphylococcus aureus (UNE-EN ISO 22780: 2016)
- Detection of Pseudomonas aeruginosa (UNE-EN ISO 22717: 2016)
- Detection of Candida albicans (UNE-EN ISO 18416: 2016).
- Preservative efficacy test - Challenge- (performed at the beginning and end of each study, but the initials are common for both stability studies and are independent at the end of each study).
Samples for study:
- Short-term stability (intrinsic stability): one or more samples, depending on the volume of the container, so the volume of the container should be reported when the tests are requested.
- Accelerated (forced) stability or aging test: two or more samples, depending on the volume of the container, so the container volume must be reported when the tests are requested