Protection against oxidation
The trend towards natural ingredients in cosmetics and also towards the use of vegetable oils continues to strengthen. Due to their composition, these harbour problems in terms of stability during the product’s shelf life. Arielle Springer and Dr Sandra Kiese present a process to increase the stability of the products.
To achieve the best effectiveness, various extraction methods and measurement techniques have been tested. Extraction with ethanol and measurement with folin assay were the most suitable.
Benefits and risks of natural oils
For the cosmetics industry, vegetable oils are very important due to their high content of physiologically valuable fatty acids. For the shelf life of creams and lotions, for example, the structure of the fatty acids in the oils is decisive1. Depending on their composition, oils consist of saturated fatty acids without double bonds, monounsaturated fatty acids with one double bond or polyunsaturated fatty acids with more than one double bond1.
Many vegetable oils contain a large percentage of polyunsaturated fatty acids2. Among other things, these factors can have a positive effect on the skin by helping to maintain the skin’s natural protective function, inhibiting inflammation, and moisturising the skin3.
Hemp oil and sunflower oil are vegetable oils that contain many polyunsaturated fatty acids and can operate as anti-inflammatory and regenerating1,4. Coconut oil contains only a small amount of polyunsaturated fatty acids but has a good spreading ability as it melts quickly on contact with the skin and spreads quickly on the skin surface. In creams, coconut oil creates a smooth, light, and cooling feeling on the skin5. Avocado oil and rapeseed oil offer a high amount of monounsaturated fatty acids and avocado oil is considered particularly skin-friendly and easily penetrate the horn layer of the skin1.
The biggest disadvantage of oils with polyunsaturated fatty acids, is that they tend to be oxidatively unstable and can reduce shelf life1. In combination with oxygen, fat oxidation of vegetable oils is triggered and the oil forms volatile oxidation products. Polyunsaturated fatty acids increase the oxidation rate during fat oxidation and increase the number of primary oxidation products formed6. These volatile oxidation products cause the typical rancid aroma of an oxidised oil1.
In cosmetic products, oxidation can influence the quality of the products and lead to unwanted aromas and odours7. In table 1, coconut oil, avocado oil, hemp oil, rapeseed oil and sunflower oil are described with the contained main fatty acid, the volatile compound mainly formed during lipid peroxidation and the aroma impression to be expected.
Therefore, it is important to know the benefits of using natural oils, but also determine the risks for the products due to oxidation.
Why stabilising against oxidation?
To determine how cosmetic emulsions and oils behave during oxidation, a study was conducted. Coconut oil, avocado oil, hemp oil, sunflower oil and canola oil were studied both as emulsion and as pure ingredients.
The different emulsions were prepared with 7% glyceryl stearate and 18% of the respective oil, added 3% glycerol as moisturiser, thick-ened with 0.2% xanthan gum and preserved with 0.2% potassium sorbate. In this process, oil, glycerol, and xanthan gum were suspended and then homogenised at medium speed (17,500 rpm) for one minute at 80°C with deionised water. After cooling, potassium sorbate and citric acid were added and homogenised again for one minute at medium level. The pH was adjusted to 4.3 – 4.5.
A mass of 20g of each sample was filled as a triplicate determination into measuring cells and stored in the dark at 30°C for 27 days. The oil was also stored in the same way. The starting conditions correspond to the atmosphere. Afterwards, the concentration of O2 and CO2 in the headspace was measured by a gas analyser. The oxygen uptake was calculated from the decrease in the amount of oxygen in the headspace (mg) relative to the mass of the product (100g) at the end of storage. The results are shown in figure 1.
At the end of storage, it was observed that both the emulsions and the oils absorbed oxygen. Since no CO2 could be measured in the headspace, microbial influences can be excluded.
It can be seen that both oils and emulsions with a high proportion of unsaturated fatty acids such as hemp oil or sunflower oil oxidise more quickly than those with a low proportion, such as coconut oil. This correlation has already been described in another article10.
It can also be seen that the emulsions show similar oxidation in all emulsion variants except those with sunflower oil. However, emulsions also contain only 18% oil and have about five times lower the amount of oxidisable substances than the pure oil, but do not oxidise five times slower. Thus, oxidation appears to be accelerated in emulsions compared to oils. It is therefore all the more important to prevent oxidation here. Antioxidants are very well suited for this purpose.
Stabilising with antioxidants
The oxidation of plant oils can be reduced or prevented by natural antioxidants such as the secondary plant substances (SPS), which are naturally characterised by their functional properties. There are more than ten thousand different SPSs, which perform various ecological functions, such as protection against pests, pathogens, herbivores, and UV radiation11,12,13. Due to their health-promoting and disease-preventing properties, SPS are widely used in the pharmaceutical and dietary supplement industries. However, in addition to the aforementioned properties, SPS are best known for their potent antioxidant activity, which enables them to scavenge reactive oxygen species and other free radicals. Therefore, they can be used in numerous applications such as food and cosmetics14 – 19.
To obtain the secondary plant compounds, there exist efficient extraction processes from various plant raw materials and residues, considering residue availability and prices. In order to avoid competition with food production, the focus is on the use of by-products from the food and agricultural industry, which up to now have not been utilised.
During the extraction process, parameters such as solid-liquid ratio, extraction time and temperature as well as the type of solvent are varied to optimally adapt the extracts to their target application. By adding different enzymes and using suitable stirring and reactor systems, the yield of functional substances in the extract can be further increased.
The antioxidant activity of functional additives using different methods was investigated, based on in vitro measurements and tests of the final products. The in vitro methods allow a quick statement about the efficacy of the extracts and require only very small sample quantities.
Based on these results, it is possible to perform the process adjustment of the extraction method quickly without having to produce large amounts of extract20 – 22. The antioxidant activity in the final product can be determined by the Rancimat method23, which accelerates the ageing process of a vegetable oil at elevated temperature and continuous air intake.
Since each of these tests have advantages and disadvantages, and different results are obtained depending on the extract ingredients, a correlation should first be determined before use as a rapid test for a specific product. Figure 2 shows the comparative results of two different plant extracts obtained with different concentrations of ethanol in the Folin-Ciocalteu spectrophotometric assay versus the results of the respective emulsions with canola oil from the Rancimat assay24.
Extract 1 showed significantly higher antioxidant activity than extract 2 in both the in vitro assay and the Ran-cimat test. Different trends were seen with respect to the effect of the solvent. For extract number one, the highest activity was obtained with pure ethanol, whereas extract number 2 was slightly more effective using a 70/30 mixture with water. For these two extracts, the correlation of assay and Rancimat test was high, so the Folin assay could be used here as a rapid test.
Conclusion
Natural oils can be used in cosmetics due to their beneficial properties directly on the skin and for the product. However, they are also sensitive to oxygen and can cause off-flavours. In this experiment, emulsions and pure ingredients with different oils were studied. It was shown that emulsions can oxidise to a similar extent as pure oils, even if the proportion of oils is lower there. It is therefore all the more important to protect not only the oils themselves but also the emulsions from oxidation. Secondary plant substances can be used for this purpose.Although natural oils and emulsions containing these can be susceptible to oxidation, the optimised extraction of plant materials to target secondary plant substances can effectively prevent this oxidation. This can improve the quality of the product while sav-ing raw materials and costs, making the overall product concept more sustainable and long-lasting25.
References
1 Krist, S., 2013: Lexikon der pflanzlichen Fette und Öle, Springer, Wien, 2. Aufl.
2 Sticher, O., Hänsel R., 2010: Pharmakognosie – Phytopharmazie, Springer-Verlag Berlin Heidelberg, Berlin, Heidelberg, 9. Aufl.
3 Böhning, K. Rossbach-Sotek, C., 2017: Naturheilkunde-Adenosinmangel, Die Naturheilkunde, 34 – 35
4 Website Olionatura, Hanföl, https://www.olionatura.de/oele-und-buttern/hanfoel (3rd May 2022)
5 Website Olionatura, Kokosöl, https://www.olionatura.de/oele-und-buttern/kokosoel (3rd May 2022)
6 Belitz, H.-D., Grosch, W., Schieberle, P., 2008: Lehrbuch der Lebensmittelchemie, Springer Berlin Heidelberg, Berlin, Heidelberg, 6. Aufl.
7 W. Rähse, Cosmetic creams: Development, manufacture and marketing of effective skin care products, Wiley-VCH, Weinheim 2020
8 Website Olionatura, Avocadoöl, https://www.olionatura.de/oele-und-buttern/avo-cadooel (4th May 2022)
9 Website Olionatura, Sonnenblumenöl, https://olionatura.de/oele-und-buttern/sonnenblumenoel (4th May 2022)
10 Springer, A.; Destler, E.; Pazurik, B; Platzer, M.; Kiese, S. Nachhaltig & Langlebig: Naturkosmetik aus rein pflanzlichen Inhaltstoffen. SOFW Journal 148 (2022), Nr. 3, S. 32 – 35
11 F. Shahidi und M. Naczk, Food phenolic: sources chemistry effects applications, Lancaster PA, USA: Technomic Publishing Company Co., 1995.
12 R. Bennett und R. Wallsgrove, „Sesondary metabolites in plant defence mechanism,“ New Phytol., Bd. 127, Nr. 4, pp. 617-633, 1994.
13 A. Crozier, M. Clifford und H. Ashihara, Plant Metabolies: Occurence, Structure and Role in the Human Diet, NJ, USA: John Wiley & Sons, 2008.
14 F. Shahidi und M. Naczk, Food phenolic: sources chemistry effects applications, Lancaster PA, USA: Technomic Publishing Company Co., 1995.
15 F. Shahidi, h. Zhong und P. Ambigaipalan, „Antioxidants: regulatory status,“ Bailey’s Ind. Oil Fat Prod, Bd. 1, Nr. 1, pp. 1-21, 2005.
16 W. Zheng und S. Wang, „Antioxidant activity and phenolic compounds in selected herbs,“ J. Agric. Food Chem., Bd. 49, Nr. 11, pp. 5165-5170, 2001.
17 S. Dragland, H. Senoo, K. Wake und R. Blomhoff, „Several culinary and medicinal herbs are important sources of dietary antioxidants,“ J. Nutr., Bd. 133, Nr. 5, pp. 1286-1290, 2003.
18 S. Wang, „Antioxidant Capacity of Berry Crops and Herbs,“ in Oriental Foods and Herbs, Wachington DC, USA, American Society, ACS Symposium Series Vol 859, 2003, pp. 190-201.
19 X. Wu, G. Beecher, J. Holfe, D. Haytowitz, S. Gebhardt und R. Prior, „Lipophilic and hydrophilic antioxidant capacities of common foods in the United States,“ J. Agric. Food Chem., Bd. 52, Nr. 12, pp. 4026-4037, 2004.
20 M. Platzer, S. Kiese, T. Herfellner, U. Schweiggert-Weisz, O. Miesbauer und E. P., „Common Trends and Differnces in Antioxidant Activity Analysis of Phenolic Substances Using Singel Electron Transfer Based Assays,“ Molecules, Bd. 26, Nr. 5, p. 1244, 2021.
21 M. Platzer, S. Kiese, T. Herfellner, U. Schweiggert-Weisz und P. Eisner, „How Does the Phenol Structure Influence the Results of the Folin-Ciocalteu Assay?,“ Antioxidants, Bd. 10, Nr. 5, p. 811, 2021.
22 Platzer, M., Kiese, S., Tybussek, T., Herfellner, T., Schneider, F., Schweiggert-Weisz, U., & Eisner, P. (2022). Radical Scavenging Mechanisms of Phenolic Compounds: A Quantitative Structure-Property Relationship (QSPR) Study. Frontiers in Nutrition, 663.
23 F. Rückle, „892 Professional Rancimat,“ Deutsche Metrohm GmbH & Co. KG, https://www.metrohm.com/de-de/products-overview/stability-measurement/rancimat/28920010. (14th January 2022.)
24 Platzer M., Kiese S., Herfellner T., Schweiggert-Weisz U.,Eisner P., presented during the 14th World Congress – Polyphenols Applications 2021 International Society of Antioxidants, (2021)
25 More about the use of secondary plant compounds: https://www.ivv.fraunhofer.de/de/recycling-umwelt/biobasierte-additive.html
Arielle Springer,
Business Development Manager,
Fraunhofer Institute for Process Engineering and Packaging IVV,
Freising, Germany,
www.ivv.fraunhofer.de
Dr Sandra Kiese,
Group Leader Process Development for Plant Raw Materials,
Fraunhofer Institute for Process Engineering and Packaging IVV,
Freising, Germany,
www.ivv.fraunhofer.de
Co-authors:
Melanie Platzer, Scientist Process Development for Plant Raw Materials
Bettina Pazurik, Master’s Thesis Student
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