When adding fragrance to a malodor, fragrance strength is not the only thing that matters. In a 2023 study, peppermint and eucalyptus essential oils were judged to harmonize well with a prepared fecal odor, whereas floral extracts such as jasmine did not harmonize as well. Combining these with fragrance materials that suppress the response of a particular olfactory receptor also lowered ratings of unpleasantness for model odors.
The study focused on developing a fragrance that combined two approaches, with odors found in nursing care settings, where excreta and body odors mingle, in mind. One approach selected materials based on their action on receptors; the other selected them based on how well the malodor and fragrance harmonized when smelled together.
First, finding a receptor that responds to skatole and indole
The researchers focused on skatole, which contributes to the characteristic smell of feces. They expressed each of 398 human olfactory receptors in cultured cells and examined the response when skatole was added. They measured light emission using a system in which receptor activation increases luminescence through intracellular signaling.
The receptor found to respond was OR2L3. Its response increased as the concentration of skatole rose, and it also responded in a concentration-dependent manner to indole, another component of fecal odor. Comparison cells without the receptor did not show a similar response.
Next, they screened approximately 800 fragrance materials for ones that suppressed the OR2L3 response to skatole. From several dozen candidates, they selected three materials with woody-amber, sandalwood, and tropical-fruit notes, together with clary sage extract, for detailed testing. All four suppressed the cellular response to skatole and indole.
Materials like these, which interfere with a receptor’s response, are called antagonists. Rather than chemically breaking down odor molecules, this selection approach focuses on signaling on the receiving side. Cell-based tests can help narrow down candidates, which must then be tested by people smelling them.
People also rated malodor intensity lower
A panel of 20 experts smelled skatole or indole together with a fragrance and rated malodor intensity on a scale from 0 to 5. Each bottle contained two cotton balls: one soaked with the malodor sample and the other with the fragrance. The fragrances were diluted so that their perceived intensities were approximately equal.
With the intensity of the malodor alone set at 4, adding any of the four selected fragrances lowered the mean malodor intensity. With clary sage extract, both malodors fell to approximately 1. The result suggested that materials selected in cells might also be useful in human evaluations.
However, the figures showing these sensory evaluations do not report tests of between-group differences. Statistical tests are provided for the cell experiments, but they cannot simply be applied to differences in human ratings. Here, the results are read as the direction of the reported means.
Herbal and green notes harmonized better than floral scents
Real malodors do not consist of skatole and indole alone. The researchers created a model fecal odor by adding sulfur-containing components and lower fatty acids to these two compounds, and also searched for materials that could give the mixture as a whole a more pleasant impression.
For this evaluation, a panel of six experts smelled essential oils or plant extracts together with the model odor and rated the combinations on a three-point scale from harmonizing well to not harmonizing at all. Peppermint, eucalyptus, and galbanum oils received ratings on the harmonious side. In contrast, jasmine, tuberose, and orange-flower extracts received ratings on the non-harmonious side.
The authors suggested that because the floral extracts naturally contain indole, combining them with fecal odor may have emphasized that characteristic. However, they did not test this explanation by, for example, removing only the indole, so it remains a hypothesis. What was specifically demonstrated was that harmony ratings differed for the materials, concentrations, and model odor tested.
The essential oils used for harmonizing did not suppress the OR2L3 response under the conditions tested. This supports a selection approach distinct from the receptor suppression described earlier. It does not mean that their effects on every olfactory receptor were exhaustively examined.
Bringing two selection approaches into one formula
The researchers created a floral base fragrance without the materials intended for receptor suppression or harmonizing. The paper calls this the blank fragrance, but it was a scented floral comparison formula, not an odorless control.
In the model fecal-odor test, they compared versions of this base fragrance with receptor-suppressing materials alone, harmonizing essential oils alone, or both. Twenty people responded on a five-point scale from unpleasant to pleasant. The formulas incorporating either technology were rated on the more pleasant side relative to the base fragrance, and the combined formula had the highest mean rating.
They also added a model age-related body odor containing 2-nonenal to the model fecal odor to create what the study calls a nursing care odor model. Here, they compared the malodor alone, the addition of the base fragrance, and the addition of a formula combining both technologies. The combined formula was rated more pleasantly than the base fragrance. Because the nursing care odor model did not include separate conditions with each technology alone, its results must be distinguished from the fecal-odor model when considering comparisons with either technology on its own.
The authors explained that the two technologies complemented one another by suppressing receptor responses to prominent malodor components and harmonizing the remaining mixture. They also discussed effects on the complex model odor in light of earlier findings that some materials suppress other body-odor-related receptors. The present sensory tests alone did not establish the individual contributions or a statistical synergistic interaction.
Summary
This study identified OR2L3 as a receptor responsive to skatole and indole, along with fragrance materials that suppressed its response. In parallel, human evaluations identified essential oils that harmonized with model fecal odor, leading to a formula combining the two technologies.
In the fecal-odor model, the combined formula had a more pleasant mean rating than either single-technology formula, and in the nursing care odor model it also reduced unpleasantness relative to the base fragrance. Effects in actual care facilities, or on users’ health and quality of life, remain questions for future work. The study nevertheless outlines a path for fragrance design that moves between receptor information and evaluations of the entire smelled mixture.
All nine authors were affiliated with Takasago International Corporation at the time of the research. No separate funding or conflict-of-interest statement was found in the article.
Reference
Mihara H, Murai M, Terada I, Kuwahara Y, Kobayashi T, Sano N, Ishida K, Hoshino K, Hirano N. (2023). How Can We Reduce the Discomfort of Malodors in the Nursing Care Scene? Development of Novel Fragrance Using Olfactory Receptor Technology. Journal of the Society of Cosmetic Chemists of Japan, 57(1), 42–49. Original article
