The tiny capsules used to retain fragrance in laundry products and other applications have shells surrounding their contents. If those shells remain in the environment after use, whether microorganisms can break them down is an important property when choosing materials.
Yet a high “biodegradation percentage” does not necessarily describe degradation of the shell itself. In 2024, Katerina Karagianni and colleagues at fragrance company Givaudan investigated how much results change depending on how capsules are washed before testing and what is removed.
Oxygen consumption does not identify which component degraded
The method used in this study measures the oxygen consumed as microorganisms break down a sample. The sample and microorganism-containing sludge collected from a wastewater treatment plant are placed in a closed vessel at a constant temperature. Oxygen consumption in a vessel without the sample is subtracted, and the result is compared with the theoretical oxygen requirement calculated from the sample’s elemental composition to determine the extent of degradation.
However, the instrument records total oxygen consumption. It does not separately identify whether the shell, encapsulated fragrance, or residual materials not bound to the shell have degraded. Readily degradable fragrance or residues could therefore produce a high value even if the shell is poorly degradable.
The researchers based their measurements on OECD test method 301F, extending the observation period to 60 days. Results from the usual 28-day test need to be distinguished from the 60-day results here. This is also an oxygen-consumption-based assessment, not a direct measurement of the percentage of capsule mass that disappeared.
Removing the fragrance does not leave only the shell
The study examined five types of fragrance capsule with different shell compositions. The researchers compared five preparation methods with different degrees of purification. Not every method was tested on every capsule: all methods were first compared using one type, with selected methods then examined in the others. Each biodegradation test was performed in duplicate.
The simplest method removed only water from the dispersion, leaving the fragrance and residues. Other methods extracted the fragrance with solvent and additionally washed away water-soluble residues. Even then, fine water-insoluble materials could remain with the shells.
The most thorough method therefore separated the layers formed by centrifugation. Only the upper capsule layer was retained, while both the aqueous layer and other material sedimented at the bottom were removed. The fragrance was then extracted with solvent to prepare the shell sample. The original paper calls this Method E.
For methods that removed fragrance, analysis confirmed that residual fragrance was less than 1% of sample weight. But a low fragrance content alone does not establish that other residues have been adequately removed. Addressing both water-soluble and water-insoluble materials was the distinctive feature of this approach.
From 91% to almost zero for the same capsule
For the capsule tested with all five preparation methods, the sample with only water removed gave a value of about 91% after 60 days. Removing the fragrance reduced it to about 54%, while one method that also removed water-soluble residues reduced it to about 35%.
With the most thoroughly purified sample, prepared by Method E, almost no shell degradation was observed. The table gives a value of −2%, but this includes variability in the calculation subtracting the sample-free measurement; it does not mean degradation occurred in reverse. The authors interpret it as effectively no degradation under these conditions.
For another capsule, however, the value was about 84% after removing only water and about 62% even after purification by Method E. Once extraneous components had been removed, this capsule still exceeded the researchers’ criterion of 60% over 60 days. The result was not that purification made every material nondegradable.
When Method E was applied to all five capsule types, only this one exceeded 60%. Two capsules supplied as biodegradable gave values of about 11% and 35% after the same purification and testing. However, the suppliers’ stated values and this study’s results were not paired replication experiments using identical preparation methods. The authors state that their method did not substantiate the biodegradability claims for those two capsules.
Checking the effect of washing through analysis
The researchers did not simply assume that more washing was better. Analysis of the wash liquid for one capsule type showed that more than 95% of the recovered residues had been removed after two water washes, with later washes adding very little.
Biodegradation tests on samples washed two, three, and four times also produced differences within test variability. The authors stress the importance of combining such analyses to prepare samples suitable for evaluating shell properties while removing fragrance and materials not bound to the shell.
Summary
Evaluating the biodegradability of fragrance capsules depends not only on the test procedure, but also on what is placed in the test vessel. In this study, simultaneous degradation of fragrance and residues could lead to overestimation of shell degradation. Some shells nevertheless exceeded the criterion even after thorough purification, showing that appropriate preparation is also necessary to identify materials that do biodegrade.
The authors recommend Method E or an equivalent preparation procedure, adapted to the capsule system and accompanied by analysis of both the wash liquid and remaining shell. These findings concern accurate laboratory evaluation of shell properties. They do not show that the shells degrade at the same rate in the sea or soil, or that the entire product has a low environmental impact.
References
Karagianni, K., Kuntzmann-Dembele, F., Bocokic, V., Charbonnier, A., Harrison, I., Kreutzer, G., Mendoza, A., & Jenner, K.(2024). Fragrance Encapsulates: Effect of Polymeric Shell Purification Method on the Accuracy of Biodegradability Testing. Environmental Toxicology and Chemistry, 43(6), 1242–1249. DOI: 10.1002/etc.5852 / Full original article and rights information.
The authors are Givaudan researchers based in France, Switzerland, and the UK, and the tests were also conducted at company facilities. They declared no external funding and no conflict of interest. Original article copyright: © 2024 Givaudan France SAS.
