May 25, 2023
In today's blog we are going to know a little better this study that takes most of the time and budget of our developments: the stability study.
But let's start at the beginning, what is stability? Stability is the ability of a product to maintain its properties within the established limits throughout its useful life. In other words, that it maintains its properties despite the passage of time. And what are these properties? Well, the properties that define it: its appearance, color, odor, its applicability, its texture, its pH, its capacity to exert its action, ... We can divide these properties into 3 large groups: physical, chemical and microbiological properties.
So the objective of the stability study is to ensure that the cosmetic maintains its intended physical, chemical and microbiological quality, as well as its functionality and appearance during its shelf life.
And this is where the interesting part begins because, although the legislation in most countries requires that cosmetic products marketed must be stable, there is no standard that defines the design of a stability study in cosmetics. We have an ISO (ISO TR 18811) that gives us guidelines for creating study designs adapted to each particular product, but each one must design the study in a way that meets the needs of their product. To summarize, we will have to consider the following:
To design our study we will have to define, on the one hand, the temperatures at which we will store the product and, on the other hand, the parameters to be measured.
The definition of the temperature will determine the duration of the study and will be determined by the expected useful life and the countries to which we are going to sell. 25ºC is considered room temperature, so a study performed at room temperature will be in real time, not accelerated. If we consider tropical countries, the ambient temperature rises a few degrees and we consider 30ºC real time. Real-time stability is the only definitive one and the one that guarantees results equivalent to real storage conditions. The problem is that, if we did it this way, we would have to wait 30 months before launching a product on the market (clearly unfeasible).
The most common stability studies are those carried out under accelerated conditions, i.e. by increasing the temperature. The most common are performed between 40ºC and 50ºC, which according to the Arrhenius equation accelerates the reaction rate by about 10 times. Thus, 3 months at this temperature would be equivalent to 30 months in real time. These studies are called accelerated stability studies.
To define the parameters to be evaluated, we will take into account the product specifications. For example, for an aqueous solution it will be interesting to measure the pH, for an oil, perhaps the density, the refractive index or peroxides. It is always interesting to measure the most sensitive parameters, those that can change more easily, in order to detect changes quickly.
What we will always measure are the organoleptic parameters (appearance, color and odor) since, in addition to being important in determining the quality of the product, they can be indicative of hidden chemical or microbiological changes.
There is much debate about the need to chemically analyze some of the substances contained in the formula. For example, sunscreens or preservatives. As the design of each study is at the discretion of the person in charge, it will be the decision of the person in charge as to whether analyzing any substance adds value to the study.
Normally microbiological parameters are always included in stability studies to ensure that the product maintains its microbiological quality throughout its shelf life. But it is usually interesting to evaluate the robustness of the preservative system to contamination during use. For this purpose, a Preservative Challenge Test is performed, as mentioned in this Blog entry.
The impact of the primary container on the formulation must be taken into account, as well as any effect that the product may have on the container. For this purpose, it is recommended to perform the stability studies in the final container, or at least in one that shares the materials with the final container (e.g., if the final container is a special mold).
The interaction between packaging and product can be:
These tests are called compatibility tests and may even include an ink anchorage test in the case of screen-printed containers or label adhesion tests.
Other studies that can be performed are stability studies in the presence of light, especially for colored products that will be exposed to light in stores. Also freeze-thaw cycles or vibration resistance, to see the robustness of the product to transport.
In other words, we must think about the entire life cycle of the product and design as many stability studies (or as complete a study) as conditions the product will undergo.
Once such tests have been performed, a stability conclusion report shall be prepared. The report should include:
It is very important to keep this report in the product file, accessible at the request of any authority, as it is the proof that the product maintains its properties throughout the shelf life we have assigned to it.
Find out more about Celia Campos, Technical Director and co-founder at MuttuLab, author of this article.
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