Review of Approaches to Studying the Stability of Essential Oils and Development of a New Research Algorithm

ABSTRACT Determination of stability is one of the most important stages in the development of various products. In this case, stability refers to the compliance of products with fundamental quality parameters throughout the entire declared storage period. Manufacturers' activities to determine product stability are based on the regulatory requirements of the relevant regulatory documents in a specific field of activity. The study of the stability of pharmaceutical products should be based on current pharmacopoeial articles and current stability study requirements. The process of studying the stability of essential oils, which are multifunctional products, has the same goal: to determine the storage conditions and shelf life under which essential oils can be considered stable. The key differences between essential oils and food products and medicines are the absence of regulatory documentation not only governing the conduct of stability studies, but also establishing quality requirements for the vast majority of essential oils. In addition, essential oils are versatile products used in various industries: cosmetics, perfumery, agriculture, food, pharmaceuticals, etc. Based on this, it is clear that it is difficult to establish universal requirements for conducting stability studies. The complex of basic characteristics of essential oils (physical, aromatic, pharmacological properties) is determined by a mixture of individual organic substances—oil components, but studying changes in the content of a number of components does not allow us to make an unambiguous conclusion about the stability of the essential oil itself. Thus, it is clear that the main difficulty in determining the stability of essential oils lies in the need to conduct a comprehensive study and draw conclusions about the stability of essential oil as a complex multicomponent object. Also, as part of the stability study, it is necessary to determine the storage conditions under which possible changes in the component composition will not affect the unambiguous identification of the essential oil, and the content of a number of characteristic components will be within the acceptable range of criteria. It should be noted that changes in the composition of essential oils can lead not only to a loss of identity, but also directly to the formation of undesirable compounds that may have allergic and toxic properties. Thus, the issues of stability and safety are directly interrelated. Currently, the requirements for the quality of essential oils are most detailed in the texts of ISO standards dedicated to essential oils and in the texts of monographs of the European Pharmacopoeia. These standards include a list of indicators that allow for an unambiguous assessment of the quality of the essential oil in question. In addition to the quality standards described above, there are a number of other regulatory documents governing the composition of essential oils when used in cosmetics. In 1999, based on a review of the results of an assessment of the allergic effects of a number of fragrant substances, the Scientific Committee on Cosmetic Products and Non‐Food Products (SCCNFP) published two lists of allergens requiring special control if present in the composition. Currently, the IFRA has established permissible limits for the content of a number of aromatic components in perfumes and cosmetics (up to a complete ban) based on their toxicological properties. A different approach is used in the food industry. The FDA classifies essential oils as “generally recognised as safe” (GRAS), which allows them to be used without undergoing toxicological studies. When essential oils are used for pharmaceutical purposes, the key parameter of identity is their native chromatographic profile, i.e., the content of a number of characteristic components, which is not regulated by data on possible toxicological components. Due to the lack of uniform regulatory requirements for conducting stability studies of essential oils, the diversity of approaches used, and their wide application, it seems necessary to develop a comprehensive approach to studying the stability of this category of products. Purpose of this article is to develop a unified algorithm for conducting studies on the stability of essential oils based on an analysis of approaches used in modern practice. Research method: search and analysis of scientific information presented in specialised (Science Direct, Springer, etc.) and general (Google, Google Scholar, etc.) scientometric search engines. Search depth: literary sources for the period 1978–2025. The development of a new research algorithm is based on a comprehensive analysis of current scientific literature. The analytical review covers the main chemical processes occurring during the storage of essential oils, the methods of analysis used in studies of the stability of essential oils, and an overview of existing experimental designs for studying the stability of essential oils.