The fragrance of jasmine and the stench of poop are actually the same smell component, can you believe it!

The sense of smell is one of the most important senses of human beings. It mainly transmits information about chemical substances in the air through the human body's olfactory nervous system and nasal trigeminal nervous system, and this information is converted into various different smells. The simplest and most intuitive language description of many smells is "fragrance" and "stink". But in fact, "fragrance" and "stink" are our subjective feelings, not absolute, and can even be converted into each other.

Image source: soogif

1. Indole, a "weird" odor

There is a little-known fact that subverts the common sense of many people. That is, the perfumes we use basically contain some chemical substance that can be extracted from feces. The name of this chemical substance is indole (yǐn duǒ).

Indole is an aromatic heterocyclic organic compound that is widely distributed in the flowers of various plants and is one of the main sources of the fragrance of various flowers. The first time humans synthesized it artificially, it was produced by mixing indigo and oleum. The English transliteration of the slightly difficult name indole, indole, is a combination of the English names of the above two.

Indole is a very interesting compound. In addition to being able to be artificially produced through the reaction of indigo and oleum, it is also "produced" all the time in nature. In nature, there are many types of bacteria that contain tryptophanase, which can decompose tryptophan, a component of protein, into indole, such as Escherichia coli.

Since E. coli is mentioned, it may trigger many friends' not-so-nice associations. That's right, the digestive tract of humans and animals is an important place for "producing" indole in nature! Therefore, feces and other excrement actually contain a lot of indole. In addition to indole, there is also its derivative - 3-methylindole (generated when the hydrogen atom in a carbon-hydrogen bond in indole is replaced by a CH3 methyl group), which is also called skatole. Both indole and skatole are the main sources of odor in feces, as can be seen from the alias skatole.

Comparison of the chemical structures of indole (left) and its derivative skatole (right) | Source: provided by the author

Indole is such a chemical substance with unique "fragrant and smelly" properties. In addition to being used to make perfume, it is also widely used in food, printing and dyeing, medicine and other industries. In the pharmaceutical industry in particular, due to its high biological activity, it has a place in drugs in many therapeutic fields such as anti-hypertension, anti-proliferation, anti-virus, anti-tumor, analgesia, anti-inflammatory, and antibacterial.

Although indole has a wide range of uses, some people may involuntarily associate indole with coming from excrement, and feel that the perfume on their body and the food in their mouth no longer smell good.

Image source: soogif

In fact, you can rest assured here, because humans do not use the above smelly raw materials to produce indole in large quantities in industry. It can be obtained by fractional distillation from coal tar, or it can be synthesized from different types of organic compounds.

In the more than 100 years of development of the chemical industry, dozens of chemical synthesis technologies for indole have been developed. The most popular one is the Fischer method, which mainly uses phenylhydrazine and organic compounds such as aldehydes and ketones under the catalysis of acid at high temperature.

2. Extremely “fragrant” leads to “stinky”, just because of different concentrations

Why is it that indole can be fragrant in flowers and the main source of stench in feces and other excrement? This is related to the concentration of indole. There is a famous saying in biochemical medicine that "discussing toxicity without considering the dosage is just hooliganism". In the case of indole, we can also say "discussing odor without considering the concentration is just hooliganism".

The indole content in the gas molecules emitted into the air by most plant flowers is often low, so the smell sensed by our olfactory organs has a faint fragrance; while feces and many other rotting protein substances, because they are decomposed by bacteria to produce a large amount of indole and 3-methylindole, the gas molecules released into the air will have a foul smell due to the excessive indole content.

Image source: pexels

In fact, even in the flowers of some plants, there are some that we smell bad because of the high concentration of indole released. For example, the famous Rafflesia in the plant world takes a different approach. Its flowers release a stench similar to that of a rotting corpse to attract flies for pollination, while other animals and insects avoid it. But from the perspective of the Rafflesia, it is also wronged. Like other plant flowers, it releases indole-rich gas molecules, but it "overdoes it" and releases a little more indole.

In addition, the flowers of plants such as chestnut and heather also release unpleasant odors when they open, which is also caused by excessively high indole concentrations.

Image source: pexels

On the contrary, there is also animal excrement that is used as a treasure of spices by humans, and that is ambergris. Ambergris is an intestinal stone in sperm whales, which is mainly formed by the long-term accumulation and fermentation of the indigestible parts of squids they swallow. It stands to reason that this kind of thing should be extremely smelly, but in fact, ambergris is a very precious spice. After being diluted and formulated, it will emit an indescribable unique fragrance. And these fragrances are a mixture of 3-methylindole in ambergris, as well as compounds such as ambroxan and ambroxan.

3. How to define fragrance and odor? Evolution determines the standard

As for why the increase in indole concentration causes human's sense of smell to change from "fragrant" to "stinky", the root cause can probably be explained from the perspective of biological evolution.

"Survival of the fittest" is the coldest law of evolution. In other words, most of the habits that humans have retained to this day have been preserved after millions of years of evolution and elimination. Our judgment of "fragrance" and "odor" also comes from this. The essence of smell is also a means for humans to perceive signals from the external environment. It can drive humans to seek benefits and avoid harm and survive in nature.

From this perspective, the substances that naturally emit low concentrations of indole gas in nature are mostly blooming flowers, which means the breath of life - fruits, honey, rich food sources, and ecological communities formed by large tracts of plants. When humans were still in the early stages of ignorance, our ancestors who could smell these smells keenly and feel pleasure from them would often trace the source of these smells and find an environment more suitable for survival than other places, thus having a higher probability of survival.

Image source: Pixabay

On the contrary, when a place emits a high concentration of indole, it represents the smell of death - spoiled food, decaying corpses, and plague caused by poor sanitation. This is a warning signal that makes humans stay away. Initially, those humans whose genes did not think these smells were bad had a higher mortality rate than others. This has been passed down from generation to generation. Over time, we now have the judgment standard of "low concentration of indole is fragrant, high concentration of indole is smelly" in our genes.

In fact, it is not just humans, many other animals have the same behavior pattern. For example, scientists once conducted an experiment with fruit flies. One of the fruit fly's favorite smells is ethyl acetate, a volatile aromatic substance widely found in fruits. When scientists released low concentrations of ethyl acetate in the experimental device, the fruit flies were attracted to fly toward the source of the smell, but when scientists increased the dose of ethyl acetate and made its concentration higher and higher, the fruit flies began to avoid in the opposite direction. This is because high concentrations of ethyl acetate can have an anesthetic effect on fruit flies, which puts the fruit flies' lives in danger. The survival instinct that has evolved over hundreds of millions of years makes fruit flies judge high concentrations of ethyl acetate as "stinky" and stay away from it.

4. Time dilutes everything, and the sense of smell will also become "aesthetically fatigued".

There is an old Chinese saying: "If you enter a room full of orchids for a long time, you will no longer smell their fragrance; if you enter a store selling abalone for a long time, you will no longer smell their stench." This is also caused by human physiological mechanisms.
As we mentioned earlier, the essence of smell is to sense external gas signals for humans. On the surface of sensory neurons in the nasal cavity, there are a large number of olfactory receptors that can recognize volatile molecules in the gas. When the receptors recognize the stimulation of volatile molecules, they will be activated to produce corresponding neurotransmitters, which are transmitted to the olfactory bulb of the brain, allowing us to "smell" a certain smell.

Image source: Pixabay

However, the human nervous system has another set of adaptive mechanisms, which is to reduce attention to irrelevant and repetitive information. The purpose of this mechanism is to save energy and resources so that the brain can focus on more important things. To use a figurative metaphor, after receiving a certain signal from the olfactory system, the brain will respond with a "got it". After that, although the olfactory receptors will continue to be stimulated by the same volatile molecules, the release of neurotransmitters will gradually decrease, and the neurons in the olfactory bulb will also reduce excitability, resulting in signal attenuation. At this point, we can think that the brain is "aesthetically fatigued" by this smell.

In fact, it is not just the sense of smell, other human senses also follow this mechanism. For example, when taking a shower, the skin will gradually adapt to the slightly hot water at first; after entering a noisy environment, the noise will not be so unbearable after staying there for a while; when going from a dark place to a bright place, the visual perception of light changes from glaring to not glaring, all of which are caused by this mechanism of the brain. It is precisely because of this physiological mechanism that we humans can quickly adapt to different environments while maintaining alertness to external environmental signals and have stronger survival capabilities.

Source: Chongqing Science and Technology Museum

Author: Yang Zhen, popular science writer.

Statement: Except for original content and special notes, some pictures are from the Internet. They are not for commercial purposes and are only used as popular science materials. The copyright belongs to the original authors. If there is any infringement, please contact us to delete them.