Taste, Chemical Senses & Perception

A Review

We set in motion the sequence of perceptual process (Goldstein, 2014) since birth. As we walk through all the different realms of senses and how we perceive our world physically, psychologically and physiologically, we stand at the gates of the tastiest of treats – our sense of taste and smell.
Taste begins with sensation in the form of electrical impulses. We have learned that sensations results from responses to stimuli like pressure and light but our sense of taste and smell comes from chemical stimuli. Different stimuli activates different sensory receptors and in a similar fashion, chemical stimuli activates the chemoreceptors responsible for gustatory and olfactory perceptions. Both our sense of taste and smell are closely related. For example, as you drink something, you smell it because molecules in gaseous state are entering your nose, and you taste it because molecules in liquid state are stimulating your tongue (Goldstein, 2014). Amazingly, some species such as worms lack distinction between taste and smell and instead differentiate between volatile and nonvolatile chemicals. In humans, the chemoreceptors responsible for detecting tastes are called gustatory receptor cells and one taste bud can include about 50 receptor cells, plus basal and supporting cells. These taste buds are contained in goblet-shaped papillae. Papillae are small bumps that dot our tongues. Each gustatory receptor cell has a protrusion also known as gustatory hair. This hair reaches out through an opening called a taste pore and interacts with the environment sending signals back and forth to the brain. The complicated process of taste and smell begins when molecules from various substances such as food or a perfume, float around in our mouth or in our nose. In both cases, these molecules must dissolve in watery mucous in order to bind to and stimulate these hair cells. Once stimulated, the receptor cells synapse with neurons and transmits the electric impulse to the gustatory area of the cerebral cortex interpreting the sensation as taste.
A misconception about taste is that we have acquired four primary tastes (Sinauer Associates, 2001). We know them as sweet, salty, sour and bitter. These four tastes are also building blocks of flavors. Each taste triggers a particular gustatory receptor. If this misconception is true then all tastes could be represented as a combination of these four basic tastes but we do experience other tastes such as astringency, pungency, fat, starchy and metallic. A Japanese scientist in early 1900s isolated glutamic acid as a distinct fifth taste and named it umami.
The second misconception about taste perception is that sweet is perceived at the tip of the tongue, salt along its posterolateral edges, sour along the medio-lateral edges, and bitter on the back of the tongue (Sinauer Associates, 2001). All these tastes can be detected over the full surface of the tongue but different region of tongues do have different thresholds and function as a pleasure or protective measure. Tip of our tongue is more sensitive to sweet and helps activate feeding behaviors and back of our tongue is more sensitive to bitter activating protective reactions to prevent ingestion. Sour detecting region activates reaction such as grimaces, puckering and massive salivary secretion.
So, the four basic primary tastes plus other acquired tastes helped our ancestors learn about what foods are good, healthy and nutritious and avoid bad poisonous food. And, believe it or not, these preferences still remain. In fact, food scientists manipulate food taste sensations using chemicals in processed foods that can signify nutritional value that is actually not present. This can lead to serious issues such as obesity (Maliphol, 2013).
Our sense of taste can impact consumption since it functions to identify potential food items (Duffy, 2007). Thus, the taste system is a potentially important target in appetite regulation (Maliphol, 2013). Multiple studies have reported that the ability to detect taste stimuli is altered in obese humans and rodents. Research on sweet and umami preference in humans revealed that obese individuals may not taste these stimuli as well, but have increased preference for them compared to their lean counterparts (Donaldson, 2009). One possible explanation is that these individuals cannot detect appetitive compounds as well and require more to reach satiety (Maliphol, 2013).
Another study suggests that we tend to eat or gravitate fatty foods when our mood is negatively affected, for example depression. As mentioned before, recent studies suggest that fatty acid stimulates taste receptor cells and fat could easily be considered the sixth primary taste (Platte, 2013). Studies about psychological influences on taste perception indicate that the taste system is sensitive to emotional and stressful manipulations and it has been reported that there is a significant decrease in glucose and salt thresholds after exposing healthy subjects to a stress test (Ileri-Gurel et al., 2012). Patients with major clinical depression have elevated thresholds for sugars. There is no known reason why depression affects taste thresholds, but it sure can lead to overeating and weight management issues.
According to a study by researchers at the university of Malawi, people who had not eaten in 16 hours could perceive weaker sucrose and salt solutions than those who had eaten only an hour before. People who had eaten an hour before needed a sucrose concentration 50% higher and a salt concentration double that of those who had not eaten. We have all heard that everything tastes better when we are hungry. But our poison-detecting sense never rests. According to an article in New York Times, an empty or full stomach had no effect on the perception of bitterness.
Like taste, the sense of smell, or olfaction, provides information that can be important for survival (Goldstein, 2014). Humans can recognize only a handful of tastes but are capable of recognizing thousands of smells. It is the way the brain puts together the tastes with the smells, temperature and mouth-feel that creates our perception of “flavor”. It is actually surprising that our sense of smell is the most neglected of all of the senses in humans especially, when you consider that eighty five percent of what we perceive as taste actually comes from our sense of smell (Mathrani, 2008). Nose is commonly referred to as the organ of smell, but olfaction does not begin at the nose. Odor molecules do enter the two nasal passages but according to researchers, nose and nasal passages only serve as passages to channel the air containing molecules. The neuron detectors actually lie deep within the nasal cavity, in a patch of cells called the olfactory epithelium. Each of the two nasal passages has a 2.5 square centimeter patch containing about 50 million receptor cells and axons from these cells are extended to the olfactory bulb, which is a projection of the brain that lies over the nasal cavity, in the anterior region of the brain. There are about 10-20 cilia for every olfactory neuron, which are hair like projections. This is where the magic happens and specific odor molecules bind to their respective chemoreceptors causing slow electric signals. The signal reaches the olfactory bulb and then transmitted to the limbic system in the brain, where memory is used to recognize the odor.
As mentioned earlier, taste and smell are closely related. This is more apparent when we are suffering from cold or congestion. We have already mentioned that odor molecules from food that travels through the nasal passages for us to perceive good odor (perfume) or bad odor (garbage). If mucus in nasal passages gets too thick, air and odor molecules cannot reach the olfactory receptor cells, thus, eliminating perception of odor from the equation and everything tastes pretty much bland. Our sense of smell may be most primal but it is also very complex. For example: our brain analyzes over 300 odor molecules to identify the smell of rose. The average person can discriminate between 4,000 to 10,000 different odor molecules.
So, when we are walking in a busy market in the middle of the afternoon and smell the popcorn or rotisserie chicken, we should always remember that the food could only attract our attention when it not only tastes good but also smells good.

References:
Goldstein, B. (2014). Sensation and Perception Ninth Edition, Belmont, CA: Cengage Learning
Platte P., Herbert C., Pauli P., Breslin P. (2013). Oral Perceptions of Taste Stimuli Are Modulated by Affect and Mood Induction. Department of Biological Sciences, University at Buffalo, The State University of New York.
Maliphol A., Garth D., Medler K. (2013). Diet-Induced Obesity Reduces the Responsiveness of the Peripheral Taste Receptor Cells. Department of Biological Sciences, University at Buffalo, The State University of New York.
Duffy V. (2007). Variation in oral sensation: implications for diet and health. Curr Opin Gastroenterol 23: 171-177
Donaldson L., Bennett L., Baic S., Melichar J. (2009). Taste and Weight: is there a link? Am J Clin Nutr 90: 800S-803S
Ileri-Gurel E., Pehlivanoglu B., Dogan M. (2012). Effect of Acute Stress on Taste Perception: In relation with baseline anxiety level and body weight. Chem Senses
Biello D. (2005). Potential Taste Receptor for Fat Identified. Retrieved from: http://www.sciam.com.article.sfm?chanID=sa011&articleID=000AFE88=E770-1367-A6B083414B7F4945
Frasnelli J. (2012). The perception of flavor-retronasal olfaction. Retrieved from: http://blog.odotech.com/bid/83308/The-perception-of-flavor-retronasal-olfaction
Mathrani V. (2008). The Power of Smell. Retrieved from: http://serendip.brynmawr.edu/exchange/node/1887
Koerth-Barker M. (2008). The Surprising Impact of Taste and Smell. Retrieved from: http://www.livescience.com/2737-surprising-impact-taste-smell.html