Cosmetic tests - Stability testing. Physico-Chemical and Microbiological characteristics. Evaluation of expiration date and/or PAO of cosmetic products (ISO/TR 18811: 2018 and EC Regulation 1223: 2009)

 The stability of a cosmetic product can be affected by various factors, classified as:

  • Extrinsic factors (external factors or environmental factors)
  • Weather.
  • Temperature.
  • Luminosity and oxygen.
  • Vibrations
  • Humidity.
  • Microorganisms.
  • Packaging material.
  • Intrinsic factors (nature of the product and interaction of its components, with each other or with the packaging material)
  • Physical incompatibility

Precipitation.

Phase separation

Crystallization.

  • Chemical incompatibility

pH

Oxide-reduction reactions.

Hydrolysis.

Reaction between ingredients.

Ingredients-container reaction.

Stability studies evaluate the evolution of a cosmetic product over time in different temperature conditions in order to predict conditions of use and preservation from the manufacturing of the cosmetic product to the 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 states that before introducing a cosmetic product to the market, the manufacturer shall ensure that a report on the safety of the cosmetic product is drawn up which includes data on the stability of the cosmetic product.

The stability of a cosmetic product is defined as the ability of a cosmetic product to preserve its physicochemical and microbiological properties within specified limits, over time and in different storage and use conditions. Depending on the results of the stability studies, the expiration date of the cosmetic product and/or the PAO (period in which the product is stable, after opening) is defined.

The different physicochemical and microbiological characteristics to be determined in cosmetic products are described below. Likewise, the stability studies to be carried out in order to determine the safety and evaluate the expiration date and/or the PAO of cosmetic products are briefly detailed.

Physicochemical characteristics

The physicochemical characteristics are quantifiable parameters and previously established by the manufacturer before commercialization. They are also 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.
  • pH.
  • Viscosity: quality parameter directly related to product stability.
  • Density: quality parameter widely used in liquid products.
  • Conductivity.
  • Phase separation by centrifugation: allows to observe phase separation.

Tests of other physicochemical parameters that may influence product stability can also be performed:

  • Water activity (aW): fraction of total moisture content that is free. If the activity of the water is high it can cause instability in the cosmetic product since the humidity available in the cosmetic can favor the growth of the microorganisms.

It is a qualitative measure that measures the state of water in a system. It is different from the humidity that corresponds to a quantitative measure of the amount of water in a sample on a wet or dry basis.

  • Content evaluation of active ingredients: The evaluation of analytical components, regardless of their activity in the cosmetic product, by selective and specific analytical techniques, such as high performance liquid chromatography (HPLC) or gas chromatography (GC) over time, are very useful for determining 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. 

Microbiological characteristics

Depending on the cosmetic form to be studied, the microbiological characteristics may be more relevant or not, since according to the cosmetic formula it may present a greater or lesser risk of contamination. With this, we can classify cosmetic products into three categories:

CATEGORY

CHARACTERISTICS

MICROBIOLOGICAL RISK

COSMETIC FORM

1

They allow the survival and growth of microorganisms.

HIGH

Mandatory to performminimum microbiological control and preservative efficacy test (Challenge Test)

  • O/W emulsions (oil/water)
  • Aqueous solutions and suspensions
  • Gels

2

Lower risk due to its own formulation, such as low water content, acidic or alkaline pH.

MODERATE

According to the formula of the cosmetic product, mandatory or recommended to perform minimum microbiological control and preservative efficacy test (Challenge Test)

  • W/O emulsions (water/oil)
  • Powders and granules
  • Hydroalcoholic solutions

3

Microorganisms survive in these products for a short time and do not multiply in them.

LOW (aW ≤ 0.75)

Microbiological control is not mandatory.

  • Oxidizers and reducers
  • Alcoholic products
  • Anhydrous products
  • Lipids

 

Stability studies

Prior to the start of stability studies, minimum physical stability tests (phase separation by centrifugation) and preservative efficacy (Challenge test) should be performed. If the results are not correct, it would be convenient to modify the formula of the cosmetic product before performing the rest of the tests.

Short term stability (intrinsic stability)

15 days.

Accelerated stability (forced) or aging test.

12 weeks. Evaluation of the expiration date of cosmetics and/or PAO (minimum duration date after opening).

Two types of stability study can be performed:

 

  • Short-term stability studies (intrinsic stability)

The short-term stability studies, also known as intrinsic stability studies, are carried out over 15 days, subjecting the product to abrupt thermal cycles alternating it at different temperatures simulating the storage and use conditions of the cosmetic product. An example of the test may be to submit the cosmetic product to be studied at a temperature of 45°C and 5°C (extreme temperatures 45°C and 5°C alternate every 24 hours), which makes it possible to accelerate the appearance of possible signs of instability.

Following the afore mentioned example, the stability study of the product to be determined begins in the temperature condition of 45°C and is alternated at 5°C 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 45°C and even days (2, 4, 6, 8, 10, 12 and 14) it is stored at 5°C. Every 24 hours (in each thermal cycle), the physicochemical parameters are measured. Likewise, the products susceptible to microbiological contamination, minimum microbiological control and study of the preservative efficacy (Challenge Test) are 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 UNE ISO/TR 18811: 2018, depending on the type of cosmetic product to be studied, other temperature conditions such as the following can be adapted:

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, so that 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, intended use and preservation. With this, the test temperatures could be adapted to those recommended by the standard or established test conditions depending on the type of cosmetic product, use and conservation planned 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 thus simulating the use and conservation planned.

  • Studies of accelerated (forced) stability or aging test

The long-term stability studies are carried out keeping the product at room temperature and at an extreme temperature between 40 and 45°C (variable parameter in real conditions) depending on the type of cosmetic product, use and conservation planned over 12 weeks, performing physicochemical measurements every 2 weeks to samples of products subjected to both conditions. Likewise, in products susceptible to microbiological contamination, the minimum microbiological control and the study of the effectiveness of the preservative (Challenge test) at the end of the study is carried out to ensure that the cosmetic product is safe for use.

This period of 12 weeks in these conditions of conservation at ambient temperature and extreme temperature (between 40 and 45°C), 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 at 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 exceeds the forced stability study at 12 weeks, it is not mandatory to indicate the expiration date, but the period after the 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 performed depending on the type of container, how to use, daily use and capacity of the container, among others to consider.

Determinations included in each type of stability test:

  • Organoleptic properties (appearance, smell, color).
  • pH.
  • Phase separation (Centrifugation).
  • Water activity (aW) (optional).
  • Viscosity.
  • Density.
  • Conductivity.
  • Microbiological tests (microbiological tests are 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 to ensure that the cosmetic product is safe).
  • Detection and count of aerobic mesophilic bacteria (UNE-EN ISO 21149: 2017)
  • Detection of yeasts and molds (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)
  • Candida albicans detection (UNE-EN ISO 18416: 2016).
  • Preservative efficacy test -Challenge test-(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 informed when the tests are requested.
  • Accelerated stability (forced) or aging test: two or more samples, depending on the volume of the container, so the volume of the container must be informed when the tests are requested

Delivery term

  • Short-term stability (intrinsic stability): 15 days, plus the time required for final tests of microbiological control and preservative efficacy.
  • Accelerated (forced) stability or aging test: 3 months, plus the time needed for final microbiological control and preservative efficacy tests.

Cost of the tests

  • Intrinsic stability, including monitoring and determination of organoleptic properties (appearance, smell, color), pH, viscosity, density, conductivity, Phase separation (Centrifugation), Water activity (aW), Microbiological tests at the beginning of the study and at the end of the follow-up period (for detection and counting of aerobic mesophilic bacteria UNE-EN ISO 21149: 2017; detection of yeasts and molds 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) and efficacy test of preservatives also performed at the beginning and at the end of each follow-up period:

-        Follow-up over 15 days (appearance, smell, color and pH): consult to ivami@ivami.com. 

-        Phase separation (Centrifugation): consult to ivami@ivami.com.

-        Water activity (aW): consult to ivami@ivami.com.

-        Viscosity: consult to ivami@ivami.com.

-        Density: consult to ivami@ivami.com.

-        Conductivity: consult to ivami@ivami.com.

-        Basic microbiological tests indicated at the beginning of the study and at the end of the follow-up period: consult to ivami@ivami.com.

-        Preservative efficacy test (Challenge) at the beginning and at the end: consult to ivami@ivami.com.

  • Accelerated stability (forced) or aging test, including monitoring and determination of organoleptic properties (appearance, smell, color), pH, viscosity, density, conductivity, Phase separation, Water activity (aW), Microbiological tests at the beginning and at the end of the follow-up period (for detection and counting of aerobic mesophilic bacteria UNE-EN ISO 21149: 2017; detection of yeasts and molds 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) and preservative efficacy test, at the beginning and at the end of the follow-up period:

-        Follow-up over 12 weeks (appearance, smell, color and pH): consult to ivami@ivami.com.

-        Phase separation (Centrifugation): consult to ivami@ivami.com.

-        Water activity (aW): consult to ivami@ivami.com.

-        Viscosity: consult to ivami@ivami.com.

-        Density: consult to ivami@ivami.com.

-        Conductivity: consult to ivami@ivami.com.

-        Basic microbiological tests indicated at the beginning of the study and at the end of the follow-up period: consult to ivami@ivami.com.

-        Preservative efficacy test (Challenge) at the beginning and at the end of the follow-up period: consult to ivami@ivami.com.