Dr Monika Fekete dissects sensory pathways and how taste molecules bind to receptors, one at a time, to give us the ultimate sensory experience in the cup.
Coffee is not only a feast for the senses, but one of the most complex beverages we consume, from a chemical point of view. Dissecting this multi-sensory experience in order to identify distinct flavours in your brew can be a difficult task, even if you have worked with coffee for many years. Understanding how the human body receives and processes chemical signals through smell and taste can be helpful towards getting a grip on those elusive flavours.
First, what is “flavour”?
Flavour is perceived through a combination of three main sensory pathways (see Figure 1 above):
- Basic tastes: Taste (gustatory) perceptions in the mouth caused by non-volatile chemicals in coffee or other foods.
- Aromas: Smells are a mixture of volatile chemicals that we perceive through our olfactory system. We can detect aromas through the nose (orthonasal perceptions) or through the mouth (retronasal perceptions).
- Chemical “feelings”, such as heat or texture, are the result of chemicals stimulating trigeminal nerve endings in the mouth, which communicate with our somatosensory system.
The distinction between taste and aroma has fascinated researchers for a long time. At the end of the 19th century, American psychologist E B Titchener stated in his best-selling textbook An Outline of Psychology that “taste has four qualities and no more. Smell seems to have a very large number of qualities”. Titchener was referring to bitter, sweet, sour, and salty tastes, to which a fifth basic taste, umami (savoury), has been added since. Almost everything else we think of as a taste, such as “this cake tastes like hazelnuts”, is in fact aroma perceived through the retronasal pathway.
Taste sensations come to us mainly through our taste buds. Most taste buds are found on the tongue, but there are also some dotted around the roof of the mouth and the back of the throat. Taste buds are located within three main types of structures:
- Fungiform (mushroom-like) papillae at the front of the tongue
- Foliate (sheet or fold-like) papillae at the sides of the tongue, and
- Circumvallate (tower-like) papillae at the back of the tongue.
Taste papillae make the surface of the tongue much larger, making room for a great number of sensory cells that can come into contact with the non-volatile chemicals in coffee or foods. Their structure is designed such that they direct liquids and food particles towards the taste buds. For example, the circumvallate papillae have a moat-like valley around the “tower” in the centre that allows liquids to collect and flow towards a set of taste buds below.
Zooming in to an individual taste bud (shown in Figure 2) we find that it’s made up of at least five kinds of sensory cells, arranged like orange sections around a fluid-filled funnel (the taste pore). Each taste bud connects up with a taste nerve and a trigeminal nerve, which is responsible for transmitting the “chemical feelings” of temperature or texture mentioned above. The messages sent through these nerves tell our body to get ready for digestion or alert it to dangers, such as poisons or extreme heat.
There are five known types of sensory cells. Type 1, 2, and 3 cells are active taste receptors. They all have hairy tips (microvilli) where they can interact with the chemicals in coffee or food.
Type 1 cells are mainly responsible for picking up salty taste. Their membrane is punctured by an ion channel that can sense sodium.
Type 2 cells can have receptors for sweet, bitter, and umami tastes (possibly a separate fat taste receptor as well). Each Type 2 cell only responds to one of these tastes. Type 3 cells respond to sour and possibly also salty tastes.
Taste receptor cells need to be regenerated every week. This is why Type 4 precursor cells sit at the base of the taste bud, ready to transform into type 1, 2, or 3 taste receptor cells. Type 5 cells are supporter cells that serve as scaffolding for the other sensory cells.
Imagine you are sipping on a cup of coffee, with the liquid swirling around your mouth. Can you isolate just one particular flavour molecule?
That might sound like an impossible task, yet this is exactly what taste receptors do. Each receptor is tuned to a group of chemicals they can grab out of the mixture that they come into contact with. One receptor interacts with one taste molecule at a time. Just like how a key fits into a lock, the taste molecule binds to the receptor. By doing so, it alters the receptor membrane’s structure – the key opens the lock. This subtle change in structure starts a chain of reactions that ultimately send a message to the nerve endings at the base of the taste bud, which in turn, send signals of the taste sensation to the brain.
Identifying flavours such as sweet, sour, and salty and their levels of intensity in odourless, transparent liquids, is one of the hardest exercises of the Q Grader training program. This task becomes especially difficult when the flavours are mixed together. We have all experienced how flavours can interact and mask each other. Soft drinks don’t taste excessively sour, even though they are often very acidic, just like lemon juice. The sourness is counterbalanced by lots of sugar or sweetener. This happens because of the way taste receptors communicate with each other.
Type 2 cells (bitter, sweet, or umami taste receptors) send their signals to the nerve endings through type 3 (sour taste receptor) cells, which then tell the brain if the food has bitter, sweet, or umami tastes, as well as sour. In addition, type 1 cells (salty taste receptors) can turn off type 2 bitter and sweet taste receptors. Bitter and sweet receptors can also turn off the other, depending on the concentration of flavour molecules present. This explains why adding some sugar to your coffee can decrease its bitterness. However, a pinch of salt would be more effective.
Next time you sip on your cup of coffee, think about the complex ways the individual components of this intricate mixture communicate with your senses. It’s an experience worth savouring.
This article appears in the April 2020 edition of BeanScene. Subscribe HERE.
For more information, visit www.coffeesciencelab.com.au
