Aromas are chemical constituents that are smelled by the human olfactory system, i.e. aromas are what people smell; more particularly, aromas that are perceived directly through the nose are "smells" (direct olfaction) and those that are indirectly perceived through the mouth to the nose are "aromas" (retronasal olfaction). Scientifically, this sense of smell (olfaction) arises when volatile (gaseous) chemical are inhaled directly into the nose or indirectly through the mouth where olfactory receptors line the nasal passage. Aromas are a crucial party of the experience of food and drink, forming one of the constituents of flavours. But with over 10,000 distinct aromas, smells are almost impossible to name and there is effectively no shared language on how to describe aromas or flavours between different people:
- "Olfactory abstraction is impossible. We can easily abstract the common shared colour - i.e. white - of jasmine, lily-of-the-valley, camphor and milk, but no man can similarly abstract a common odor by attending to what we have in common and setting aside their differences."
The Science of Smell
The relationship between smell and the chemical properties of a food are complex: (i) smell is from volatile chemicals whereas taste relates to water-soluble substances; (ii) typical chemical stimuli are organic, volatile substances not inorganic chemicals; (iii) usually complex mixtures of chemical compounds emitted by plants, decaying matter and scent-producing glands of animals. The capacity to sense these natural odours has a survival value in enabling humans to identify foods and sense when foods might be toxic or decayed to a dangerous level.
Molecules enter the nasal cavity, where the air is warmed and humidified by baffles. There are about 10 million olfactory receptors on the "olfactory epithelium", which are replaced every 4 - 8 weeks. Only 10% of the aromas that are inhaled reached the receptors, because our noses have protection systems that filter out unpleasant smells and pollutants. Specific aromas are picked up by specialised binding proteins in the mucus and then transported to receptor sites on finger-like projections (called cilia) and the end of each olfactory receptor cell. The chemical binding of specific molecules to their receptor triggers a biochemical process that converts this sense into neural information that shifts along the olfactory nerve to the olfactory bulb in the brain and several other regions - the olfactory cortex (below the anterior portion of the frontal lobe), thalamus and lower brain centres of the limbic system.
In retronasal olfaction, i.e. smelling through the mouth, the aroma compounds reach the olfactory receptors internally when we exhale. Exhaling through the nose causes a depression in the pharynx which sucks the air from the mouth into the nasal cavity and towards the olfactory epithelium. Importantly, the smells of food and drink received via the nose and mouth vary, because of their different environments - in the mouth, chewing (mastication), temperature and the action of enzymes in your saliva start chemical reactions that change the nature of the food molecules, while many aromatic molecules are insoluble so do not enter the saliva and cannot be made volatile for transfer to the olfactory regions of the nasal cavity.
The olfactory system is very sensitive to aromas. Women are more sensitive to aromas than men. The elderly are less sensitive to aromas than young adults. There are over 70 different types of odour blindness.
Categorization of Aromas
In 1916, Hans Henning, a German physiologist, came up with only six fundamental aromas:
- Flowery (fragrant)
- Fruity (ethereal)
- Foul (putrid)
But six primary categories of aroma is far too small to capture the 10,000 distinct odours, so a more typical classification of aromas from the 1940s comprises a still-too-small 9 categories:
- Alliaceous (onion-like)
- Hircine (goat-like)
However, our sense of smell depends upon its associations with information stored in our memory, so our past experiences influence how we perceive smells in the moment. These senses of smell are filtered by subjective experiences and infused with emotional connotations - pleasure, pain, distaste etc - and cultural factors - revulsion for dog or horse meat in some cultures but not others. Adding a lack of shared descriptive language, this makes articulating our experience of aroma extremely complex.
- Ledgeway, T. (undated) Taste and smell, University of Nottingham, Cognitive Psychology 2 (C82COG) - Yea 2 Semester 1, accessed 10 July 2016 
- Henning (1916) Der Geruch, Leipzig, Barth.
- Delmas, F.-X., Minet, M., Barbaste, C. (2007) The tea drinker's handbook, p. 130-1, New York, Abbeville Press. ISBN 9780789209887.
- Davidson, A. (1999) The Oxford Companion to Food, Oxford, Oxford University Press. ISBN 0192115790.