Biting into a tomato unleashes nearly five hundred aromatic chemicals. Working together these chemicals create the smell and the taste of tomato. No single chemical carries the tomato odour. Being an edible fruit, the most prominent odours are fruit, and vegetal. Of the nearly five hundred chemicals only sixteen reach the threshold of human perception. Of those sixteen, the four most striking chemicals convey very different things. They range from the almond and cocoa of 3-methyl-butanol to banana and gasoline (3-methyl-1-butanol). Others convey apple and cut grass (3- hexanal), and citrus and mushrooms (6-methyl-5-hepten-2-one). As we eat a tomato, we release a flood of chemicals into our mouth. Some even reach our smell receptors through the backs of our mouths. Some stimulate our taste buds. They create a wonderful “tomato” combination of sugar, acid, and umami.

Sensing the “chemical world” around us

Smell and taste have their own set of detectors and are our window into the chemical world. They operate together to make "flavour". Taste is the minor partner in that process. Our sense of smell contributes eighty percent of our flavour “sense”.

Our sense of smell lies in a small patch of tissue, no more than nine square centimetres. It is high in the nose about seven centimetres behind the nostrils. There, millions of receptors lie. With them is a sources of mucus into which the chemicals can dissolve and hence be sensed. It is that mucus that we have all been dutifully collecting for our COVID tests. Doctors believe that there are only four hundred and fifty different types of sensor. There are literally millions of aromatic chemicals so how do we cope?

Each type of receptor is stimulated by many different odour molecules. Each odour molecule can activate many different receptors. The forces that bind the molecule to the receptor vary in strength. It is the complexity of those interactions that allows our olfactory cortex to detect a wide variety of smells. Our sense of smell has as much to do with our brain as it does to our senses. Even the act of detecting a smell requires our minds. We live in our own world of smell. There is up to a thirty percent variation in the mix of receptors across individuals. That is why we can argue about the smell of anything!

One Trillion Smells

How good our detection system depends on our brains not our noses. Our smell receptors can tell the difference between trillions of smells. Researchers started with one hundred and twenty-eight different odours and constructed random mixtures. Some with ten, some twenty and some containing thirty different molecules. They then presented each respondent with three phials. Two were identical and the third one different. The task was to find the “odd one out”. Mixtures that contained similar molecules were obviously more difficult to tell apart. From the data they could extrapolate. They could predict how well people would do if given every possible combination. The result was that our receptors could detect over one trillion smells.

Unfortunately detection does not mean recognition. The situation is made worse because chemicals can appear as key components in the smells of different foods. n1-octen-3-one appears is a key component of the smell of an oyster. Particularly a Brittany oyster. Its mushroom, citrus “note” appears in sardines. It is strongest in those that have been on ice in a trawler hold for a couple of days! Another oyster associated molecule also appears in lychee. Organic chemicals can cross the boundary to become malodourous. Methyl mercaptan appears in tomato paste but also the smell of rancid chicken.

The perceptual process of recognizing a smell lets us down. When faced with this complexity our brain lets us down. We have a very poor ability to identify smells in complex mixtures. This is according to Australian researcher David Laing. In a whole series of studies he mixed recognizable smells, in combinations of two, three, four and five at a time. He used things like spearmint, almond and clove. He then asked respondent to identify as many smells as possible from the mixture.

The results are surprising, given the sensitivity of our smell detectors. He started respondents with two smells and people did reasonably well. With three smells together only fifteen percent of his respondents could recognize even one. His conclusion was that we have a very poor “mental language” for describing odours. This constrains our effectiveness. People who work with smells develop such a language and hence can do somewhat better. These are people like perfumers or wine tasters. The “professionals” could identify two or three smells out of the three. Even they could not break the “four smell” barriers and at their best could only identify three out of the four.

Contrary to popular myths, the blind are not more sensitive to smell. They cannot perceive smells at concentration levels lower than the sighted. Their “detection thresholds” for their sense is no lower than a sighted person. What they appear to do better is to put a name to that smell or flavour. The have the same skill as other sensory experts. This in turn allows them to recall and process the remaining sensations better.

Women are able to detect odours than men, They are also able to identify more smells by name. This is not due to the physical receptors. Rather to the differences in the architecture of the brain itself .

Ageing Smell 

Thresholds for smell are higher for many older people. Some need higher concentrations of organic chemicals to senses them. A US study of people above fifty years of age showed that a quarter had a problem with their sense of smell. This was the result of a simple smell recognition test. They were given eight commonly found smells in the home e.g. chocolate. “Problems” were defined as the ability to identify only six or less smells out of the eight.

Even this was very much age dependent. There was not a straight-line relationship. Only about five percent had a problem at the bottom of the age range. About a fifth of Third Agers had any problems. This rose to sixty percent in the over eighties. Men demonstrated more decline with age. This is consistent with women having a better sense of smell throughout their lives.

Like all the other senses it is difficult to separate out the impact of healthy ageing from disease. How much decline is due to exposure to a lifetime of smoking or “toxic” environments. There are some effects that appear to be causally related to ageing. Our smell receptors have a remarkable capacity to regenerate themselves. As we age the regeneration process can slow and eventually stop. This leads to a decline in the number of receptors.