When we talk about the taste of food, we most often use expressions that have something to do with the five basic tastes, sour, sweet, salty, bitter and umami. We also often express our perception of the food’s smell. However, if the food does not live up to our expectations and we do not like it, it turns out that in most cases it has little to do with taste or smell, but rather how the food feels in our mouth. Unwanted texture of food is the most frequent reason for us to reject it, and the quality of a food is often judged by the texture, we expect it to have.
The food’s flavour thus has a physical dimension: we sense and feel its structure. The perception of the physical dimensions of food is often completely unconscious, but we immediately become aware that it plays a role if the food is not as we expected; we react immediately on crisps that have gone soft, a gravy that has turned gritty, a piece of bread that is tough or a sip of tea that purses the lips because it has infused for too long. This important physical dimension of the total taste experience is called mouthfeel or, in the technical language, texture. Texture describes the part of the food’s physical structure that our senses can perceive.
Crucial to the experience of quality
Texture and mouthfeel are absolutely crucial and even more important than taste and smell when it comes to for example crisps and meat, and we set the quality (and price) very differently for tough and dark meat, no matter how it tastes. Mouthfeel is equally important as taste and smell, when it comes to food such as bread, vegetables, cheese and fruit, while it is of lesser importance for most beverages and fluid foods.
Cooking is very much about changing the physical and chemical properties of commodities such as meat, crops and vegetables and transforming the commodities into food and meals. Thus, cooking can be viewed as an exercise in creating a wanted texture of the food prepared by the given commodities.
The overall sensory impression of taste, smell and mouthfeel is integrated in the brain and give us flavour. Flavour is not in the food, though, it is in the brain!
Mouthfeel is probably the most overlooked component of flavour in the Western world, while in Asia, for example in Japan, it has a prominent position; here, much food, such as seaweed and jellyfish, is eaten specifically for the mouthfeel.
Mouthfeel: How does it work?
Mouthfeel is sensed by the so-called somatosensory system. This system is not only found in the mouth, but all over the body, for example in skeletal muscles, joints, internal organs and in the cardiovascular system. The system is stimulated when affected by physical impacts such as pressure, touch, traction and vibrations (tactile sensation), pain and temperature. In addition, it includes perception (kinesthesis) of position and posture of the body and body parts which plays a role to the mouthfeel, when the tongue’s movements investigate and identify the size, form and texture of a food subject, for example during chewing.
The nerve ends in the teeth report about the structure of the food as well: how hard it is, whether it crunches or is elastic, and the size of the particles in the food. In the same way as taste impressions, the nerve signals from the mouthfeel travel through the brain stem to the brain (to the thalamus and from there to the somatosensory center).
The mouthfeel is activated on nerve ends that have receptors in the epithelial layer of the mouth. These receptors are related to receptors elsewhere in the body’s somatosensory system, for example in the skin and muscles. But in the mouth the density of receptors is much larger than anywhere else. That indicates that the mouthfeel is a very perceptive sense, which is especially important for survival.
The physiological basis of mouthfeel: the somatosensory system
We have four different kinds of nerve ends and matching receptors, namely those sensitive to temperature, to pain, to touch and to pressure.
Temperature sensitive nerve ends
These nerve ends are slow and by means of certain receptors (‘transient receptor potential channels’, TRP-channels, that involve transport of both sodium and calcium ions) they can register a number of different sensory impressions, for example temperature, cold and heat as well as pain. It is this system that tells us that ice cream is cold and coffee is hot.
Pain sensitive nerve ends
These nerve ends also have a kind of TRP-channels (called nociceptors), and they are stimulated by mechanic and chemical actions or extreme temperature conditions, which can be a sign of danger or potential damage to the cells and tissue. We can distinguish different types of pain: tingling (for example from isothiocyanate in mustard and horse radish), burning (for example from the substance capsaicin in chili and piperine in black pepper) and general pain (for example from a fishbone or a sharp salt crystal). Temperatures below 15ºC (det må være -15 grader, ikke?) and above 52ºC are registered as pain.
The fact that the channels registering temperature and pain are related, is the reason why our senses occasionally play a trick on us and we sense something as cold (menthol, peppermint and camphor) or hot (piperine and capsaicin), even though the temperatures have not changed. The pain sensitive nerve ends are also affected by electric stimulation and they can also be pH-sensitive. The tingling and sour taste experiences of bubbly beverages containing carbonic acid are thought to be caused by a combination of both pH-sensitive receptors and somatosensory nerve ends.
Touch sensitive nerve ends
These nerve ends are faster than the two previously mentioned, and they are controlled by a special type of receptors (mechano-sensitive sodium channels, also called tactile receptors) in the membranes of the nerve cells. These receptors are sensitive to mechanical impacts on the cell membrane, and thus they signal minor deformation, stretching and compression of the mucous membrane as well as size, shape and roughness of solid subjects.
The touch sensitive nerves are especially activated during chewing of the food, and when the tongue examines the food by means of the proprioceptic system, which contains a certain kind of receptor, proprioceptors, that can detect the tongue’s position and movement pattern (kinesthesis).
Pressure sensitive nerve ends
These nerve ends are very fast, and they are especially sensitive towards vibrations caused by fast movements by tongue and jaws. For example, in our fingertips we have certain nerve ends that can feel texture differences down to a fifth of a millimeter and vibrations up to 250 Hz.
Astringent – not really mouthfeel, but still so
Astringency used to be considered a taste. Today, you would probably say that it is a taste impression caused by chemical reactions, leading to a mechanic sensation, and thus a sort of mouthfeel.
A well-known example is the dry, rubbing and astringent feeling, that emerges in the mouth when we drink tea or a young red wine, or when we eat unripe fruit, for example khaki or sloe, that all contain tannic acid substances (tannins).
The effect may be caused by chemical reactions between the food and the surface of the tongue and the saliva, when proline-rich proteins contained in the saliva make the tannins aggregate. This leads to a feeling of lumpy particles, and to a reduction of the saliva’s ability to make food slide smoothly across the tongue and up the sides of the oral cavity, because the saliva turns more viscous.
It has also been suggested that the effect may be due to a shrinking of the mucous membranes as a result of cross-binding of the proteins in the epithelial membranes, which in turn leads to a tension that activates touch sensitive mechano-sensitive channels.
Mentioned in the article
Dr Ole G. Mouritsen is head of centre in Taste for Life and head of the Gastrophysics focus area. He is a professor in Gastrophysics and Food Innovation at the Department of Food Science, University of Copenhagen.
He is an expert in bio- and gastrophysics with a special focus on mediation of the natural sciences to the general population through knowledge about food and taste.
Klavs Styrbæk is part of the focus area Gastrophysics. He is the head chef and manager of STYRBÆKS, that houses both a restaurant, cooking classes, meeting facilities, and product innovation.