A stability test is an analytical procedure designed to evaluate a product's ability to retain its physicochemical , microbiological, and sensory characteristics over time, under defined environmental conditions.
These tests are essential in many industrial sectors, including the pharmaceutical, cosmetic and food industries, to guarantee the quality and safety of products until their expiry date.
By monitoring the evolution of a product exposed to controlled parameters (temperature, humidity, light), stability tests allow for the anticipation of potential degradation and the adjustment of formulations if necessary. Certain specific tests, such as the salt spray for packaging corrosion or the Rancimat for the oxidative stability of oils , effectively complement this qualification process. But what exactly is a stability test, and how is it performed?
What is a stability test?
A stability test is an analysis that verifies that a product maintains its essential characteristics throughout its shelf life. It applies to many products such as pharmaceuticals, cosmetics , and processed foods.
The test analyzes the product's ability to withstand physical changes (such as texture or color) and chemical changes (degradation, reactions) that could alter its quality or safety.
In the cosmetics industry, for example, a product must maintain the same texture, color, and effectiveness from its first day of production until the end of its shelf life. In the pharmaceutical industry, stability ensures that the drug retains its efficacy and safety throughout the shelf life indicated on the packaging. Stability testing is therefore fundamental to preventing any alteration that could affect product quality or even pose risks to consumer health.
Types of stability tests
Long-term stability test
Long-term stability testing is a procedure that simulates normal storage conditions for a product over an extended period. It assesses how the product reacts when exposed to constant temperatures and humidity levels, often representative of real-world storage conditions. The results obtained during these tests determine the product's shelf life, that is, the length of time it remains safe and effective for consumers.
For example, in the pharmaceutical industry, batches of products are stored in climate-controlled chambers where temperature and humidity are carefully monitored, typically for a period of 12 to 24 months. During this time, samples are taken regularly to analyze any changes in the product's characteristics (pH, odor, appearance, solubility).
Accelerated stability test
The accelerated stability test, meanwhile, exposes the product to more extreme conditions than those encountered under normal circumstances, such as high temperatures and humidity levels, to accelerate the aging of the product.
The goal is to quickly predict the effects of aging over a shorter period (generally 6 months).
This type of test is particularly useful for manufacturers seeking to obtain data on a product's lifespan in a short period of time.
For example, a pharmaceutical product stored at 40°C with 75% relative humidity for 6 months can be estimated to have a shelf life of 24 months under normal storage conditions. This allows for verification of any chemical or physical changes, such as accelerated degradation of the active molecule, and for validation of the optimal storage conditions for the product.
These two types of tests are complementary and allow for a better understanding of the product's behavior in real and extreme conditions, thus guaranteeing the reliability of the information provided on the packaging regarding the use-by date or expiry date.
How to conduct a stability study?
The essential steps of a stability study
To ensure a complete and accurate stability study, several steps must be followed. Here are the main phases to respect:
1. Sample selection:
The first step is to select representative samples of the product to be tested. These samples must come from the same production batches as those intended for sale, thus ensuring that the results obtained accurately reflect reality. Generally, at least three production batches are selected to ensure statistical validity.
2. Definition of storage conditions:
Storage conditions vary depending on the product and target markets. They are often defined by regulatory guidelines, such as those issued by the ICH (International Council for Harmonisation) for pharmaceutical products. Typical climatic conditions to test include:
- Controlled temperature (e.g., 25°C for normal conditions or 40°C for accelerated testing).
- Relative humidity (often 60% for long-term tests, 75% for accelerated tests).
- Light or darkness (depending on the product's sensitivity to light).
3. Frequency of tests and sampling:
Tests must be carried out at regular intervals to monitor the product's evolution over time. For example, in a long-term stability study, samples might be tested every 3 months during the first year, then every 6 months for subsequent years. Each sample is then analyzed to check for changes in physical, chemical, or microbiological parameters.
Parameters to be evaluated during stability tests
During stability testing, several key parameters are monitored to assess whether a product retains its properties over time. These parameters are defined according to the type of product and regulatory requirements. They allow for the detection of any changes that could affect the product's quality, safety, or efficacy.
Physical parameters
Physical parameters are often the first indicators of product degradation. They include:
1. pH:
pH is a crucial indicator in cosmetic, pharmaceutical, and food products. It measures the acidity or alkalinity of the product, and a change in this parameter can signal chemical degradation or contamination. For example, in a cosmetic product, a pH variation can affect the effectiveness of preservatives or cause skin irritation.
2. Appearance (color, texture):
Any visible change, such as a shift in color or texture, can indicate that the product is undergoing chemical reactions. A cosmetic cream that yellows or a medication that changes color may indicate the formation of degradation products. These alterations are often the first to be observed and require further analysis.
3. Viscosity:
In some products, such as creams and gels, viscosity plays a key role. Changes in viscosity can signal interactions between components, a loss of effectiveness, or an alteration in texture. For example, a cream that becomes too runny might lose its moisturizing properties or become unpleasant to apply.
Chemical parameters
Chemical testing is essential to ensure that a product's components remain intact throughout its lifespan. These parameters include:
1. Oxidation:
Oxidation is a common chemical reaction, especially in products containing oils or air-sensitive ingredients. Oxidation can lead to a loss of activity in active ingredients, as well as the formation of free radicals, which can be harmful. To measure this phenomenon, chemical tests are performed to detect the levels of oxidants or oxidation byproducts.
2. Chemical degradation:
Some chemical components of a product can degrade over time, which can affect the product's efficacy and safety. For example, in medications, the degradation of the active molecule can reduce the treatment's effectiveness. In stability testing, monitoring these degradation products is crucial for assessing long-term safety.
Microbiological parameters
Microbiological safety is a key element in stability testing, particularly for food and cosmetic products, which are susceptible to contamination.
1. Microbial load:
The growth of bacteria, mold, or yeast in a product can make it unsafe for the consumer. Microbial load testing aims to ensure that the product remains free of harmful microorganisms throughout its shelf life. In the cosmetics industry, preservatives are often added to inhibit microbial growth, but their effectiveness must be validated through stability testing.
2. Contamination test:
This test is performed to check for the possible presence of microbial contaminants. It is essential to monitor products that come into contact with the skin, such as creams or lotions, to ensure they do not contain pathogenic bacteria. If contamination is detected, it may indicate that the preservatives or storage conditions are unsuitable.
Sensory parameters
Sensory parameters refer to the perceptible characteristics of a product through the senses, such as smell, taste, or texture. These characteristics are particularly important for cosmetics and food products.
1. Odor:
A change in odor can indicate chemical degradation or microbial contamination. For example, a cosmetic cream that develops an unpleasant odor may signal that its ingredients have oxidized or that bacteria have grown.
2. Taste:
In food products or dietary supplements, any alteration in taste must be monitored, as it can affect the product's acceptability to the consumer. An alteration in taste may indicate the presence of spoilage products or contamination.
Points to check during stability testing
- Appearance: Detection of any change in color or texture.
- Odor: Monitoring for odor changes indicating storage problems.
- Viscosity: Evaluation of viscosity variations, particularly in creams or gels.
- Microbial load: Monitoring for the absence of microbial contamination.
- pH: pH measurement to ensure that the product remains stable and compliant.
The results of these assessments help determine whether the product remains safe and compliant with specifications throughout its intended shelf life.
Frequency of checks and interpretation of results
The frequency of inspections depends on regulatory guidelines and the estimated lifespan of the product. Periodic analyses allow for monitoring gradual changes and identifying long-term trends. At each stage, the results are compared to the product's initial specifications to verify continued compliance.
The data collected during these tests is then analyzed to determine if adjustments need to be made to the product formulation, its packaging or its storage conditions.
Scientific data and molecular analysis in stability testing
During stability testing, it is essential to examine the stability of the molecules present in a product. This allows us to anticipate potential chemical reactions or molecular interactions that could compromise the quality and safety of the product over time.
Analysis of volatile compounds with HS-GC-MS
A key analytical method for studying volatile compounds in products is HS-GC-MS (Headspace Gas Chromatography-Mass Spectrometry). This technique allows for the detection and quantification of volatile molecules that could evaporate or degrade over time or under storage conditions. The analysis is performed by subjecting the sample to a gas-phase separation of the compounds, followed by identification using mass spectrometry. The results of this analysis are crucial for detecting substances such as residual solvents, aromatic compounds, or impurities that could affect product stability.
Samples must be collected in sealed, inert containers to prevent any unwanted interaction with the external environment. The limit of quantification (LOQ) varies between 0.1 and 10 mg/L, depending on the nature of the compounds being analyzed. This technique allows for the study of chemical degradation at a molecular level, thus ensuring that the product does not develop new, potentially harmful substances during its shelf life.
Impact of molecular changes on product stability
A product's stability can be influenced by molecular changes over time, particularly by chemical reactions such as oxidation, hydrolysis, or photodegradation. A photostability study of sunscreens allows for the validation of their resistance to simulated UV radiation.
These reactions can lead to the formation of new molecules, called degradation products, which can be harmful or reduce the effectiveness of the product.
For example, in medications, an active molecule can lose its effectiveness if exposed to unsuitable conditions such as high humidity or high temperature. In cosmetics, certain preservatives or fragrances can break down and affect the product's scent or texture.
Stability tests aim to identify these degradation products and analyze their potential impacts. Techniques such as high-performance liquid chromatography (HPLC) or mass spectrometry (MS) are used to detect even the slightest changes in chemical composition, thus ensuring that the product remains compliant with defined quality standards.
Analysis of container-content interactions
One aspect often overlooked in stability testing is the interaction between the product and its packaging. Packaging materials, such as plastics, metals, or papers, can release chemicals upon contact with the product, thus affecting its stability. This migration of compounds from the packaging to the product can pose safety problems, especially if toxic components are detected.
Specific tests, such as migration testing, are performed to verify that the packaging does not alter the product's composition. Furthermore, rheological analyses assess the compatibility of the packaging materials by measuring the product's texture and viscosity over time.
These analyses are essential to ensure consumer safety and to comply with current regulations, such as those imposed by European legislation (EC Regulation No. 1935/2004) or the American FDA for materials in contact with food or cosmetic products.
Our stability and packaging-product compatibility tests allow us to evaluate these interactions in depth.
The analytical methods used
Analytical methods play a key role in stability studies. They allow for the detection of even the slightest variations in the composition or characteristics of the product. Laboratories must use methods that are validated and recognized by regulatory authorities.
Among these methods, we often find:
1. HS-GC-MS (Headspace Gas Chromatography-Mass Spectrometry):
This method is frequently used to analyze volatile compounds present in products. For example, in a stability study, the laboratory may use HS-GC-MS to measure substances that evaporate or degrade under the influence of heat or time. Samples are collected in sealed, inert vials to ensure accurate analysis, with quantification limits ranging from 0.1 to 10 mg/L depending on the compound.
2. HPLC (High Performance Liquid Chromatography):
Used to analyze the chemical composition of products, HPLC can detect impurities or degradation in the product at very low levels. This method is particularly useful for pharmaceuticals and cosmetics, as it ensures that the product's active components do not deteriorate over time.
3. Analysis of container-content interactions:
Studying the interactions between a product and its container is crucial. Some packaging materials can react with the product's active ingredients, thus altering its stability. Therefore, tests are conducted to verify whether components of the container migrate into the product (migration test) or whether the product affects the integrity of the packaging (compatibility test).
These analytical methods not only guarantee product stability but also ensure its compliance with international quality standards. For bio-based packaging, compostability validation tests are also essential.
Are you looking for an analysis?
Regulatory requirements surrounding stability testing
Stability testing is not only a quality imperative, but it is also subject to strict regulations. Every industry sector, whether pharmaceutical, cosmetic, or food, must follow specific guidelines to ensure compliance with legal standards. These requirements are in place to protect consumers and ensure that marketed products meet safety and efficacy standards.
Compliance with ISO standards and regulations
Stability testing must comply with several international standards and regulations. ISO 17025, for example, is a benchmark for analytical laboratories, ensuring that they conduct tests reliably and accurately. This standard applies to all laboratories performing stability testing to guarantee that the methods used are validated and the results are accurate.
Food products, meanwhile, must comply with regulations such as the European INCO regulation (1169/2011), which imposes strict rules on food labeling and storage. For pharmaceutical products, the ICH (International Council for Harmonisation) guidelines govern stability studies with precise guidelines (ICH Q1A-Q1F) regarding storage conditions, test duration, and sampling frequency.
In the cosmetics industry, European cosmetics regulation 1223/2009 stipulates that product stability must be proven before being placed on the market. This includes validating preservatives and other components to guarantee the product's safety throughout its shelf life. These regulations mandate extensive testing to demonstrate that products will remain stable and safe under normal storage conditions.
Why are stability tests mandatory?
Stability testing is mandatory to ensure that products sold to the public pose no health risks. Without these tests, it would be impossible to determine a product's shelf life or guarantee its long-term safety. The tests also verify that the product retains its essential properties (efficacy, safety, quality) throughout its lifespan.
In the pharmaceutical sector, for example, a lack of stability testing could lead to the marketing of drugs whose effectiveness declines over time, or worse, whose components could degrade into toxic substances. Similarly, in cosmetics, insufficient testing could lead to irritation or allergic reactions in consumers due to the degradation of certain ingredients.
Stability testing is also crucial to ensure product traceability and consistency over time. To meet regulatory requirements, each production batch must undergo rigorous testing. These results are then used to justify the expiration date or shelf life displayed on the label.
Post-market surveillance
Even after a product is launched on the market, monitoring its stability doesn't stop. Regulatory authorities require follow-up studies to ensure that products continue to meet safety standards throughout their lifespan. Follow-up or "monitoring" tests are often required for three representative production batches.
These tests validate product specifications and, if necessary, extend its "re-testing period" or shelf life. For example, in the pharmaceutical industry, these follow-up tests are often conducted over a period of up to 60 months, ensuring that medications remain safe and effective throughout their time on the market.
In conclusion, regulatory requirements surrounding stability testing are a fundamental pillar for ensuring consumer safety. They guarantee that products remain compliant with the strictest quality standards, from production to final use.
The role of stability testing in improving product quality
Stability testing not only ensures product safety, but also plays a key role in the continuous improvement of formulation quality. These tests allow manufacturers to identify weaknesses in their products, adjust formulations, and maximize the durability and effectiveness of the final product.
Contribution to research and development (R&D)
In the research and development process, stability testing is a crucial tool for optimizing formulations before market launch. Laboratories conduct tests on multiple versions of a formulation to determine which is the most stable and durable under varying conditions. This not only ensures product safety but also allows for improvements to its characteristics, such as texture, odor, and efficacy.
For example, a laboratory developing a new cosmetic product might test several formulations with different preservatives. Stability tests will indicate which formulation retains its original properties longer, without significant alteration, while ensuring optimal safety for the user. This also helps reduce costs by optimizing the composition from the outset, thus limiting the need for reformulation after market launch.
Optimization of formulations and selection of additives
Additives play a crucial role in product durability and performance. Stability tests assess the impact of various additives, such as preservatives, antioxidants, and texturizing agents, on product shelf life. By adjusting the quantity and type of additive used, product stability can be improved without compromising its effectiveness.
For example, in food products, antioxidants can be added to prevent the oxidation of fats, while in cosmetics, preservatives inhibit the growth of microorganisms. Through stability testing, manufacturers can refine these choices to find the perfect combination that guarantees product quality while meeting safety standards.
Improved shelf life
One of the goals of stability testing is to extend product shelf life. Extended shelf life is a major advantage for manufacturers, as it reduces losses due to product deterioration during storage and transport. Furthermore, it improves consumer satisfaction by offering products that remain effective for longer.
Stability tests simulate different storage conditions, such as variations in temperature or humidity, to predict how long the product will remain stable. These results help determine the most accurate expiration date possible, while ensuring the product retains its optimal properties until that date.
Impact on product effectiveness
Stability testing is also essential to verify that the product maintains its effectiveness throughout its shelf life. In the pharmaceutical sector, this means that the drug must retain its potency and therapeutic action for its entire validity period. For cosmetics, this can mean that the product retains its moisturizing, anti-aging, or cleansing properties without degrading.
For example, an anti-aging serum must maintain the same concentration of active ingredients throughout its use. Stability tests verify that these ingredients do not degrade and that the product remains effective until the end of its shelf life.
Therefore, stability testing is not merely a quality control tool; it is an integral part of the continuous product improvement process, ensuring not only safety but also optimal performance. It allows manufacturers to adapt their formulations to offer consumers high-quality products, while reducing the risks of non-conformity and product returns.
The challenges of laboratory stability testing
Laboratory stability testing presents complex challenges, requiring scientific precision and rigorous control conditions to ensure reliable results. Overcoming these challenges is essential to guaranteeing product quality throughout its lifecycle.
Common problems encountered during testing
1. Microbial contamination:
Sensitive products, particularly cosmetics and food, are susceptible to contamination by microorganisms. This contamination can occur during production, handling, or analysis, and compromise the validity of stability test results. Laboratories must therefore operate in sterile environments and implement strict control procedures to prevent any risk of contamination.
2. Chemical instability:
Chemical instability occurs when unexpected chemical reactions take place within a product over time. These reactions can include oxidation, hydrolysis, or interactions between different components, which can lead to a loss of efficacy or the production of undesirable substances. These phenomena are particularly problematic in pharmaceuticals and cosmetics, where the stability of active ingredients is crucial to ensuring their effectiveness.
3. Packaging defects:
Packaging plays a crucial role in protecting the product from external factors, such as humidity, oxygen, and light, which can degrade its stability. Packaging failures, such as leaks or poor resistance to storage conditions, can compromise product quality. Stability testing must therefore include studies on the interaction between the container and its contents to ensure that the packaging does not alter the product.
Solutions and innovations to improve testing
1Use of sophisticated climate chambers:
Modern laboratories use specialized climate chambers to simulate extreme conditions of temperature, humidity, and light. These chambers allow for testing the stability of products in controlled environments, often more rigorous than those to which the products will actually be exposed. This enables more reliable predictions of product shelf life.
Advanced climate chambers can also simulate conditions specific to certain geographical areas, allowing manufacturers to adapt their products to different markets, taking into account the climatic fluctuations specific to each region.
2. New equipment for accelerating testing (accelerated aging tests):
Accelerated aging tests are used to reduce the time required to validate product stability. Recent equipment allows storage conditions to be pushed to extreme levels of temperature and humidity, thus accelerating degradation processes to predict the effects of time on products in a matter of months, rather than years.
These methods are particularly valuable for companies that need to quickly validate their products before launching them on the market. They also allow for the earlier detection of potential stability issues, thus reducing the risk of commercial failure.
3. Real-time monitoring solutions:
Real-time monitoring of stored products allows for tracking changes in critical parameters, such as temperature and humidity, throughout the testing process. Smart sensors are installed to continuously collect data, enabling scientists to react quickly to any deviations or anomalies. This technology offers high accuracy and ensures that products are stored under ideal conditions throughout their stability testing.
Technological innovations, such as connected sensors and advanced analysis tools, have improved the accuracy and efficiency of stability testing, making these processes faster and more reliable.
