Extending the shelf-life of plant-based cosmetic oil-in-water emulsions

27 Feb 2023 --- As plant-based cosmetic emulsions are difficult to preserve in terms of quality due to oxidative spoilage, a study evaluated the shelf-life of such cosmetics under different storage conditions.

Emulsion samples stored in darkness showed insignificant changes, whereas, under light exposure, preferential oxidation occurred due to higher oxygen uptake and an increased formation of characteristic fatty acid oxidation products.

“Depending on the incorporated plant extract, significant differences in oxygen uptake behavior were observed under the influence of light that indicates possible antioxidative effects,” the study highlights.

Plant extracts of Riesling (Vitis vinifera) pomace, apple (Malus domestica) pomace, coffee (Coffea arabica) grounds and powder extract and cocoa (Theobroma cacao) husk were incorporated in stable O/W emulsion formulations. In contrast, an emulsion without extract functioned as blank.

The study aimed to determine the suitability of the method combination by measuring the antioxidant activity, oxygen concentration and volatile oxidation products via gas chromatography (hexanal) to characterize the influence of some plant extracts with antioxidant activity on the oxidative stability of natural cosmetic oil-in-water (O/W) emulsions.

The coffee grounds and powder extracts measured highest antioxidant activity in the plant extracts.“The oxidation mechanism in O/W emulsions is based on the lipid hydroperoxide reactions with pro-oxidants from the aqueous phase, which leads to radical formation at the surface of the micelles,” the study details.

“Consequently, this initiates the oxidation of fatty acids inside the micelle and finally leads to the production of undesirable rancid compounds. It is necessary that these unsaturated fatty acids are protected so that they can maintain their valuable effects on the skin and their oxidation products do not negatively affect the bulk.”

Cosmetic application of plant extracts
The researchers freeze-dried the raw materials and ground it to 0.5 mm particle size. In vitro antioxidant activities of the plant extracts were measured using the oxygen radical absorbance capacity (ORAC) assay.

The researchers also calculated the oxygen uptake of O/W emulsions with plant extracts. They found that it was generally higher for the samples stored under light exposure than in darkness over the entire storage time of 90 days.

“It is clear that the emulsion with apple pomace had significantly higher values for daily oxygen consumption rate than the other samples when stored in light. The other samples did not differ significantly from each other,” the study elaborates. 

Both coffee samples measured the highest antioxidant activity in the plant extracts. “When using plant extracts in preserved emulsions, it must be noted that a change in pH is possible depending on the composition of the extract. It is therefore important for good comparability to choose a preservative with a broad pH range,” the study states.

According to the study, natural oils are more susceptible to oxidative processes than synthetic emollients due to the content of unsaturated oils and the possible impurities from plant ingredients. Therefore, it spotlights the importance of choosing the ingredients in the right ratios suitable for the emulsion type.

“The results also showed a mathematical relationship between ORAC assay results, daily oxygen consumption rate and hexanal concentration of O/W emulsions during storage. These results show promising perspectives for the research areas of shelf-life modeling or modeling-assisted product development,” the study concludes.

Edited by Radhika Sikaria