Our sense of taste is a chemosensory system that evolved to help us be efficient at finding nutritionally useful foods and recognising harmful foods.
Throughout the evolution of different species, dietary feeding behaviours and therefore taste perceptions have changed. As humans our evolution as omnivores means we have genes that encode our taste receptors to respond to a wide variety of tastes; sweet, salty, sour, bitter, and savoury (otherwise named umami) to support us selecting the nutrients we need.
The tastes we can decern are due to different genes. For example, in the case of bitter flavours, humans produce 25 separate proteins encoded by 25 separate genes. By contrast, cats can no longer taste sugars as one of the two genes required to detect sweet has been switched off. Pandas and koalas, whose diets are restricted to certain plants, cannot recognise savoury umami since this relates to proteins. Without receptors for a particular flavour, we will not be motivated to eat it.
Human Taste Senses and what they do for us
Sour, or more correctly acid taste, suggests that foods are off and to be avoided. Acid balance is a fundamental aspect of physiological homeostasis in all vertebrates because blood and tissue acidity can have severe or lethal effects on function and life. As it is so fundamental to life it is thought that Sour/acid is the first taste to have evolved. Only as species evolved in complexity and moved onto land and accessed more varied diet a greater range of tastes evolved.
Humans are unusual as we like sour flavours and have for thousands of years employed microbes to make foods that are not sour become acid (eg through fermentation). Most mammals produce their own vitamin C. Humans do not and so it may be that liking acid flavours ensures we take on sufficient vitamin C (ascorbic acid) Rot, due to lactic acid bacteria and yeasts, often increases food calories, amino acids, and vitamin content.
Sweet – indicates sugars and carbohydrates needed for energy. Our ancient ancestors looked for sweet foods as the calories would help them survive harsh conditions. Sugar is the easiest taste for us to detect. Sugar is found in varying amounts in most foods including meat. In nature sugars don’t come alone; they are found within foods that contain other nutrients. The sugar content likely to encouraged us to go out and make the effort to collect food containing sugars which would also provide other nutrients.
We have evolved to process sugars in a way that we can store them as fats for later use in harsher times. Sugars in this way are useful to us. The challenge we have today is that we have refined sugars to be pure and hold no additional nutritional benefits to us.
Consuming sugars creates a dopamine release which encourages us to consume more. In our long-ago past this was useful; now however, there is little effort required to consume sugar and the dopamine release within the nucleus acummbens when we constantly consume it, supports cravings and addiction – something not lost on the biscuit manufactures of the world.
Salt – indicates sodium, an essential element for water balance, blood circulation and nutrients.
Bitter – indicates the presence of toxins which suggests poison. But bitter can also indicate substances that are powerfully nutritive for the body and so for humans, bitter flavours can promote foods that are positive for health. In fact, so important is this aspect, there are, as noted earlier, 25 ways to taste bitter. Note that babies cannot taste bitter until about 4 months of age. It appears that preference for bitter increases with age – eg coffee, beer, chocolate. Ayurveda practice and Chinese medicine place emphasis on the profound impact of bitters on our health. Sources of bitters include horseradish, parsley, water cress.
Savoury (Umami) indicates the presence of proteins. This taste is elicited by L-amino acids and therefore has a special role of detecting nutritious, protein-rich food since amino acids are the compounds that create proteins.
Why do we not taste fats? Why does the tongue not have a receptor for fat given they are an important part of our diet? This is being studied and it seems that the brain does know when fats enter the body, these being recognised as quickly as the other tastes. Some feel that it is the texture of fats – ie a tactile element – that we perceive rather than the chemicals that are sensed. However recent research may have fat become our next recognised taste.
We also recognise ‘hot’ and ‘cool’ tastes. For sensation of heat, for example pepper, the chemicals directly lock into our temperature receptors but cleverly lower the activation temperature of the cells by several degrees, leading our brain to perceive ‘hot’ in the absence of heat.
The same for cool flavours such as mint. The temperature cells for cold are activated by the chemicals released to change their thresholds and send messages to the brain that something is cold.
Human milk is, not surprisingly, comprised of nutrients that are of most benefit to the body and brain growth of the baby. There is broad consensus that breast milk decreases mortality rates, respiratory and gastrointestinal infections, and neurodevelopment for babies born prematurely or term.
Human milk contains 87% water, 1 % protein, 4% lipids (fats), 7% carbohydrates including 1-2.4 % oligosaccharides (act as a prebiotic – food for gut bacteria) and many minerals and vitamins.
We have very low proteins in comparison to other animals which links to our very low growth rate in comparison to other animals. On the other hand we have the highest proportion of long-chain polyunsaturated fatty acids important in brain development. In comparison with cows milk, human milk has greater levels of cholesterol which is needed for hormones involved in brain development.
When babies are born prematurely the breast milk has a slightly different composition for first several weeks. It is higher in protein and different fats that are more easily absorbed.
Brain growth in the last 4 to 8 weeks of gestation is extremely rapid and is crucial for later development. Supporting feeding behaviours in premature infants is critical. Smell and taste are strong stimulators of digestion and metabolism. It is therefore suggested that giving an infant a taste of milk during tube feeding will support feeding behaviours.
Neuroanatomy of Taste
Stimulation of taste receptors initiates a cascade of neurological and behavioural responses.
Taste sensory cells are housed in the taste buds of specialised papillae of the soft palate of the tongue, pharynx, and upper oesophagus. Taste cells have a life span of approximately two weeks. The entire tongue can actually respond to all of the tastes, but some regions of the tongue have lower thresholds to some tastes over others.
The sensory nerve fibres of three cranial nerves take taste information to the brain, these
• facial nerve (CNVII),
• glossopharyngeal nerve (CN IX)
• vagus nerve (CN X)
These messages travel to the gustatory area of the nucleus of the solitary tract which is positioned in the medullar area of the brain stem. This nucleus has far-reaching impacts on the homeostatic systems of the body.
When taste stimulus reaches the solitary nucleus, it passes the information on to the motor neurones of four cranial nerves (CN’s V, VII, VIX, X) which will generate sucking and chewing, swallowing and peristaltic activity in the upper gastrointestinal tract during swallowing.
The nucleus also receives signals of satiety and can change our sense of taste.
From here information heads to the thalamus and on to the gustatory cortex, the area responsible for perceiving and distinguishing tastes and which is situated within the insula. This area sits really close to the periform cortex, the cortical area for smell. From the gustatory cortex, messages travel to the orbitofrontal cortex where integration of taste and smell takes place.
Information also makes its way to the hypothalamus and amygdala. The hypothalamus has an important role in glucose sensing and metabolic regulation, controlling feeding and hunger, also linking to the insular and reward system. The amygdala enables further processing, including taste intensity and taste aversion memory.
Currently we think we have 5 basic tastes, but these turn into an array of complex taste perceptions. That is because what we call “taste” is actually a multi-sensory construct. Smell plays an important role in the perception of flavour as does vision, tactile, proprioception and interoception and possibly hearing too! What is certain is that we are not done with identifying all that we taste. Interestingly, like other senses, we have some that are more or less sensitive to certain taste sensations. Interestingly, generally the more hypersensitive you are to a taste, the less you wish to ingest.
A final thought is that the multi-dimensional complexity of taste is reflected in the use of the term in common usage for things that may be hard to define – that jacket exhibits fine taste, that action leaves a bad taste in the mouth, that experience was bittersweet for me, that joke was in poor taste, an acid look, she is sweet as sugar, and of course a taste of summer. So, when you enjoy that ice cream this summer have a think about the tastes, the temperature receptors, the tactile receptors, the visual stimulus that encouraged you to consume and of course enjoy!