Skatole is a naturally occurring organic compound from the indole family, widely recognized for its contribution to the odor of feces. Despite this unpleasant association, skatole is also a component of some flowers and is used in products with pleasing fragrances, such as perfume.
Biological Origins of Skatole
Skatole is produced within the digestive tracts of mammals from microbial activity in the large intestine. Specific bacteria decompose the amino acid tryptophan, a common component of dietary protein. This metabolic breakdown results in 3-methylindole, the chemical compound known as skatole.
This compound is not exclusive to animal biology and is also found in the essential oils of plants and flowers. It is present in jasmine, orange blossoms, and Ziziphus mauritiana, where it contributes to the flower’s complex scent profile.
The Duality of Scent
The perception of skatole’s scent is dependent on its concentration. At high concentrations, the human olfactory system registers it as foul and fecal. This response is a function of how odor molecules interact with receptors in the nose, as a high number of skatole molecules leads to the perception of an unpleasant smell.
A shift in perception occurs when the concentration of skatole is extremely low. At near-trace levels, the smell is no longer perceived as foul, but as pleasant and floral. This phenomenon is due to the activation of a different combination of olfactory receptors.
This change can be likened to using a potent spice in cooking. A large amount can be overwhelming, but when used sparingly, it adds pleasant complexity. Similarly, a minuscule amount of skatole can introduce a desirable floral note to a fragrance composition.
Researchers have found that as an odor’s concentration increases, it recruits a wider variety of odorant receptors. High concentrations can also silence the most sensitive receptors. This interplay means the brain receives different signal patterns at low versus high concentrations, explaining the different scent experiences.
Commercial Uses of Skatole
Skatole’s concentration-dependent properties are used in perfumery, where it is added in minute quantities. It functions as a fixative, reducing the evaporation rate of other scent compounds and prolonging the life of the perfume. It also imparts a warm, floral, or animalic depth to the fragrance profile.
Its application extends to the food industry, where it is used as a flavor additive at very low concentrations. It helps create or enhance flavors such as vanilla, strawberry, and raspberry. The amount used is well below the threshold where its fecal odor would be detectable, providing a subtle enhancement.
Conversely, at high concentrations, skatole is utilized for its offensive quality as an animal repellent. The foul odor is used in products to deter animals like deer and rabbits from gardens. This application leverages the natural aversion most animals have to the scent of waste.