Food Impacts On Brain Activity

Multisensory Taste Perception Modules

5 sensesFood provides a multimodal stimulus; it excites more than one sensory system. During the process of eating, all of the five senses are used. With our far senses vision and olfaction we see and smell foods from a distance. With our near senses somatosensation and gustation we feel and taste the food during handling and oral processing. In many cases foods elicit our auditory system as they emit sounds during chewing and other oral processing. We use the term flavor to describe our perception of a food, generally thinking of the senses of only taste and aroma. However, in a scientific context, flavor may be defined as “the complex combination of the olfactory, gustatory and trigeminal sensations perceived during tasting. The flavors may be influenced by tactile, thermal, painful and/or kinaesthetic effects and expectations from visual presentation of the product”.

Taste: Specialized chemoreceptors on the tongue, palate, soft palate, and areas in the upper throat (pharynx and laryngopharynx) detect sensations such as bitter, for example, from alkaloids, salty from many ionic compounds, sour from most acids, sweet from sugars, and umami, or savory, from some amino acids and nucleotides

The receptors reside in taste buds mostly located in fungiform, foliate, and circumvallate but not filiform papillae on the tongue. Taste buds, as the name indicates, are bud-shaped groups of cells. Tastants, the molecules being tasted, enter a small pore at the top of the taste bud and are absorbed on microvilli at taste receptor cells. (for more information click here)

Smell: While the taste receptors in the mouth detect small molecules dissolved in liquids, the receptors of the olfactory system detect molecules in the air. The range of receptors provides a wide sensitivity to volatile molecules. Some of the most potent thiols can be detected in concentrations as low as 6 × 107 molecules/mL air (2-propene-1-thiol), whereas ethanol requires around 2 × 1015 molecules/mL air. Thus, there are at least 8 orders of magnitude between our sensitivity to the most and least “smelly” molecules. The sensitivity of the sense of smell varies quite significantly between individuals. Not only do different people have different sensitivity to particular aromas, some people suffer anosmia, odor blindness to specific odorants. People can be trained to become sensitive to some odorants, such as for the unpleasant smelling androstenone.

To complicate the picture further, the sense of smell develops during the human lifetime; we tend to lose sensitivity at an older age, especially after the seventh decade. An odor is detected by sensors in the nose, the odorant receptors. The way these sensors recognize aroma molecules is by “combinatorial receptor codes”, i.e., one odorant receptor recognizes a range of odorants and one odorant is recognized by a number of odorant receptors. The distinct odor identity is created by the pattern of odorant receptors activated by the odorant’s shape. Thus, slight changes in an odorant or even in its concentration can change the identity of an odorant.

The olfactory system consists of other areas in the temporal and frontal parts of the brain. The orbitofrontal cortex is of particular importance for food behavior since nerve cells in this area play a large role in the computation of hedonic properties of smell stimuli and have also been implicated in the representation of flavors of foods.

Sensory scientists usually refer to smelling through the nostrils as “orthonasal perception”, whereas the aroma compounds that gain access to the olfactory epithelium through the nasopharynx (i.e., molecules released in the mouth) are referred to as being perceived retronasally. The latter is often mistakenly referred to as taste by laymen. It should perhaps more correctly be referred to as flavor, although we prefer to think of flavor as the combination of the perception of taste in the mouth and retronasal aroma in the nose (see section 2.6 below). It is one of the challenges for Molecular Gastronomy to develop an appropriate language that can be used by chefs, the general public, as well as the scientific community to describe the various ways we interpret the signals from our chemical senses. When eating a food the initial olfactory stimulation takes place as we smell the aroma of the food before the food is in the mouth. Thus, orthonasal perception is often said to be of the external world. In contrast, the aromas perceived retronasally are said to be of the oral cavity (the interior world). (For more information click here)

Chemesthisis: As we have already noted, the overall “flavor” of a food is determined by the combination of many stimuli both in the mouth and nose. Most authors argue the important senses are those of taste, (retronasal) smell, as well as the less wellknown, mouthfeel and chemesthesis. Chemesthesis mediates information about irritants through nerve endings in the skin as well as other borders between us and the environments, including the epithelia in the nose, the eyes, and in the gut. Chemesthesis uses the same systems that inform us about touch, temperature, and pain. In humans, sensory nerve endings from branches of the trigeminal nerve are found in the epithelia of the nose and oral cavity. Signals transmitted by these nerves are responsible for the pungency of foods, as exemplified in carbonated drinks, chili, ginger, mustard, and horseradish; accordingly, chemesthesis is also sometimes referred to as the “trigeminal sense”. Hot spices are typical stimulants of trigeminal sensory nerve endings, but most chemicals will stimulate these nerve endings at sufficiently high physical concentration

Without pungency many foods would be bland; imagine horseradish without the heat or garlic with no bite. Clearly, the sense of chemesthesis must play a crucial role in our the evaluation of the palatability of any food. The sensation of oral pungency differs in many ways from the sense of taste. For example, pungency typically has a slow onset but can persist for prolonged periods, minutes to tens of minutes.

This is contrary to the sense of taste, which is most intense for the few seconds the food is in the mouth. This difference in the temporal nature of pungency and taste is of great interest when considering of the palatability of foods and the overall satiety they provide. In many cases, the long term effects of pungency will make foods both more palatable and more satiating.

Texture (sense of touch): Szczesniak39 succinctly defines of texture as “…the sensory and functional manifestation of the structural, mechanical and surface properties of foods detected through the senses of vision, hearing, touch and kinesthetics”. This definition clearly conveys the important point that texture is a sensory property and thus requires a perceiver. The distinction between texture and structure is sometimes ignored in the terminological practice, such that sensory and instrumental measurements can be confused. It is not touch alone that provides the sensation of the texture of food: vision is active in texture perception when we see the food; additionally, audition, somesthesis, and kinesthesis are active during handling of the food. Texture plays a major role in our recognition of foods. For example, when presented with blended food products 56 blindfolded young and elderly subjects were, on average, only able to correctly identify 40% of these foods.

Further, there is a marked difference between the food that enters the mouth and the wetted bolus that is later swallowed, and it is the summation of sensory impressions during the whole process from seeing the food, picking it up and putting it our mouths, chewing it, and eventually swallowing it that we perceive as the texture of the food. This has been termed the “philosophy of the breakdown path”. In this view, individual foods follow specific paths during oral handling along the axes “degree of structure”, “degree of lubrication” over time, or number of chews. Foods interact with the eater during consumption, the saliva lubricates the food, and enzymes in the saliva affect the viscosity of semisolids and liquids.

Temperature: From cold ice cream on a hot summer day to hot cocoa after a trip on the skating rink in winter time, temperature is part of our perception of foods. We have expectations for the serving temperature for most foods and beverages; an inappropriate serving temperatures leads to reduced liking or even rejection of such foods and beverages. We sense the temperature of food in our mouth through nerve endings.

Temperatures above 43 °C and below 15 °C are accompanied by a feeling of pain. However, we routinely consume hot beverages well above both pain and tissue damaging temperatures. A study of ingestive behavior of hot coffee coupled with measures of temperatures during sipping and in mouth showed that minimal cooling occurred during sipping and ingestion. The authors hypothesize that during drinking, the hot coffee is not held in the mouth for a sufficiently long time to heat the epithelial surfaces sufficiently to cause pain or tissue damage. The perception of temperature changes in the mouth is very precise; under experimental conditions sensitive subjects feel changes in temperature of as little as around 1 °C. The ability to sense changes is asymmetric: increases in temperature are sensed much more rapidly than decreases.

The sensation of temperature can be affected by various chemestetic agents, with menthol as a well-known example of cooling and capsaicin for heating. The temperature of a food or beverage affects the release of airborne molecules, with an increase in temperature leading to increased release.

Adaption & Suppression: In addition to the actual signals from the sensors, there are further, perhaps surprising ways in which we perceive the environment around us which can significantly affect the flavor of the food we are eating. Two of the most important are adaptation, when we ignore a constant stimulus, and suppression, when we find the effect of a stimulus in a mixture less than on its own.

Adaptation: When subject to a constant stimulus, the senses become less responsive. When holding a solution of a tastant (e.g. sucrose) motionless in the mouth, the solution will become completely tasteless after a while. This phenomenon is well known to us all, although we tend to ignore it. Whenever we leave our homes for a prolonged period, to go on holiday or to a week-long conference, we find on our return that as soon as we walk in the front door our home has a slightly “musty” smell. Thus, we open the windows and “air” the house. The odor quickly goes away. Of course, in actual fact, our homes always have that smell, it is what our friends and neighbors perceive as the smell of our home. However, because it is always present in our environment we rapidly become adapted to it and simply do not notice it at all. When eating, we will quickly become “bored” with a dish which appears to lose its flavor if we are subject to the same taste or aroma continuously for a prolonged period. Variety becomes the spice of life. In the gastronomic kitchen, some chefs, having recognized the adaptation phenomenon, have tried to create dishes that continually provide a diverse range of stimuli so as to retain (and hopefully enhance) the diner’s interest.

Mixture Suppression: The phenomenon that individual taste and smell characteristics are perceived as less intense in mixtures than alone.63 Thus, when preparing a complex recipe and mixing several foods with different flavors or tastes, the perceived intensity of the flavors of the separate ingredients is decreased relative to that of the same tastes or aromas of the ingredients on their own. There is a very interesting and useful exception to this phenomenon, Release from suppression; when adapting to one component in a mixture, other components are less suppressed and will then be perceived as more intense.

All the information in this article has been extracted from “Molecular Gastronomy: A New Emerging Scientific Discipline” by Peter Bruham et al. 2009. For the full document download click here

Leave a Reply

Your email address will not be published. Required fields are marked *