Oxidative Stability of Cosmetics and Pharmaceutical Raw Materials - Rapid and Comprehensive Analysis with the Rancimat Test Method

The cosmetics sector is among the fastest-growing industries in recent years. Research indicates that in addition to the growing importance of personal care, the development of e-commerce has played a significant role in this growth. For this sector, which is showing considerable expansion globally, demand for product quality is increasing daily.

Oils in raw materials used in the cosmetics and pharmaceutical industries tend to undergo oxidation. For this reason, in addition to the typical main parameters followed when evaluating product quality, determining oxidation stability is also highly important. In this context, the Rancimat test method has become one of the common analytical techniques applied in laboratories in recent years for rapidly and reliably determining the oxidation stability of cosmetics and pharmaceutical raw materials.

In the Rancimat test method, samples can be analyzed without any pre-treatment, and shelf life estimates can be made by directly relating the obtained results to the oxidation stability of the sample.

Principle of the Rancimat Test Method

Rancidity is a condition in which substances formed as a result of chemical changes in liquid and solid oils affect the odor and taste of the oil. During oxidative rancidity, in the first stage, unstable primary reaction products such as peroxides and hydroperoxides are released. These products are converted into secondary reaction products such as aldehydes, alcohols and carboxylic acids in the subsequent stages of the oxidation process. [caption id="attachment_118287" align="aligncenter"] Figure 1: Rancidity reaction mechanism[/caption]   The Rancimat test method is also referred to as an "accelerated oxidation test." This technique, which reduces degradation processes lasting weeks and months to a matter of hours, allows the aging behavior of the sample to be tested. During this accelerated test, the sample comes into contact with an air stream at elevated constant temperature and humidity. [caption id="attachment_118288" align="aligncenter"] Figure 2: Rancimat test measurement principle[/caption]  

Fatty acids, under a closed-loop dry air stream directed into the heated reaction vessel, are oxidized with oxygen to form low molecular weight organic acids, and these organic acids are transported to the measurement cell with the aid of the same air stream. The products transported into the low-conductivity (preferably less than 5 µS) pure water in the measurement cell gradually change the conductivity value of the water, and through continuous real-time monitoring, the "induction time" is automatically calculated by software from the time-conductivity curve.

[caption id="attachment_118289" align="aligncenter"] Figure 3: Typical induction time graph[/caption]   The Rancimat test method takes its name from the Rancimat device developed by Metrohm in close cooperation with the food oil industry specifically for this test, and today the most current version of this system is referred to as the 892 Professional Rancimat. [caption id="attachment_118290" align="aligncenter"] Figure 4: Metrohm 892 Professional Rancimat[/caption]   The 892 Professional Rancimat device stands out as a specially designed oxidation stability platform capable of parallel operation of up to 8 samples simultaneously at 2 different temperatures in a single run. System management, data collection and evaluation processes use StabNet software compliant with FDA CFR21 Part 11. Thanks to the clear user interface provided by StabNet, the 892 Professional Rancimat system is operated in a truly intuitive manner. [caption id="attachment_118291" align="aligncenter"] Figure 5: StabNet software system management interface[/caption]   Induction time is automatically calculated from the measurement data and analysis results containing all method and device parameters are recorded in the database. [caption id="attachment_118292" align="aligncenter"] Figure 6: StabNet software results database[/caption]   When repeated sample measurements are performed, StabNet not only provides statistical values such as relative/absolute standard deviation and average value, but also, if results are re-evaluated, archives all changes along with the original version and allows you to revert to the original data whenever desired. Thanks to the advanced search, filtering and sorting functions provided by the StabNet database, you can easily find your previous analyses and generate control charts matching various trend criteria. The temperatures selected for measurements in the Rancimat test method are the most important parameter during testing, and theoretically, every 10°C difference in sample temperature causes a two-fold change in induction time. [caption id="attachment_118293" align="aligncenter"] Figure 7: StabNet software results overlay[/caption]   Another notable feature of StabNet software is the ability to perform shelf life estimates with the help of the results extrapolation function. Using Arrhenius or Q10 approaches, mathematical relationships are established between results obtained at different temperatures for the same sample, and thus the theoretical induction time (in years and hours) at the target storage temperature can be calculated. [caption id="attachment_118294" align="aligncenter"] Figure 8: StabNet software extrapolation tracking[/caption]  

Experimental Content and Workflow

In this study, the oxidation stability of different raw materials used in the production of cosmetic oils was studied repeatedly on the 892 Professional Rancimat system.

Prior to analysis, the system was validated by checking all temperature, air flow and sample position conductivity measurement values of the Metrohm 892 Professional Rancimat system within the relevant calibration and verification protocols. Before sample measurements, ΔT measurements determining the sample heating offset value were performed separately for both temperature blocks, and the system was made ready for use. At the beginning of each actual sample set measurement, the system was brought to the target temperature and 60 mL of pure water was added to the conductivity monitoring cells for the sample positions, and caps with integrated Pt measurement probes were attached. For sample measurements, 3.00 ± 0.10 g of raw material was weighed into each reaction vessel and samples were placed in the temperature blocks, after which measurements were initiated. The parameters used during the experiments are specified in Table 2.

Analysis Results

Within the scope of this study, a wide range of more than 50 various pharmaceutical and cosmetic raw material oil samples were analyzed. The raw materials whose analyses were carried out within the study differ in quality or extraction methods: • Unrefined. • Refined. • Deodorized. • Cold-pressed. • CO2 extraction. • Organic. • Demeter. [caption id="attachment_118297" align="aligncenter"] Figure 9: Oxidation stability of refined mango oil, induction time.[/caption]   The results obtained from sample studies show that raw materials with different characteristics lead to different induction times. It was determined that oxidation stability also varies depending on the extraction method of the raw materials. As a result of the studies, it can be seen that the Rancimat test method can provide acceptable values at the ss(rel) ≤10 level for the analyzed samples (See Table 3). Table 3: Oxidation stability results of various raw materials with 892 Professional Rancimat (four replicates were performed for each oil type).

Conclusion

Classical oil parameters such as acid number or peroxide value only describe the current state of liquid and solid oils, but these values can only be used to make limited predictions about product stability, which is an important factor in determining the quality of natural solid and liquid oils processed in the cosmetics industry. This information gap can be filled with induction time, an additional oil index parameter. The results obtained in this study demonstrate that the Rancimat test method can be used as a quality control parameter for determining the oxidation stability of cosmetics and pharmaceutical raw materials and as a consistent tool for shelf life estimates. As Metrohm, we operate as a global sector leader in primary chemistry methodologies (potentiometric titration, Karl Fischer titration, ion chromatography, direct measurements, electrochemistry, etc.) that form the basis of analytical techniques in many industrial sectors, particularly in the pharmaceutical and cosmetics sectors. With our comprehensive portfolio of systems, software and applications compatible with international standards, we provide fully integrated, fast, precise and reliable solutions directly to your preference under one roof.

References: 1. Metrohm Stability Application Note AN-R-029, Oxidation stability of cosmetic and pharmaceutical raw materials 2. Metrohm Application Study AW ST CH7-0174-052020 Determination of the oxidation stability of pharmaceutical and cosmetic raw materials