Flavor Investigator: Taste

“Everyone eats and drinks, but few appreciate taste.” Confucius (551-479 BC)

Confucius may have been right about everyone’s eating and drinking habits. However, I believe it is fair to say that everyone has one fond memory of a meal that they can taste simply by thinking about.

Take a moment to think about one of your most favourite meals. Whether it is a savoury bag of Lay’s Salt and Vinegar Chips or a sweet triple chocolate cheesecake. Just the thought can start salivations in your mouth.

One of my most favourite treats which I could eat again and again is the simplistic cookie. The taste of bittersweet chocolate which is not too sweet but has a natural bitterness inherently from the cocoa bean. There is a slight acidity that comes from the baking powder and a creaminess which comes from the butter and completes the entire taste.

In contrast, even the least extravagant meals also have a distinguishable taste. While it may not be memorable all the time, they are still present in your weekly meals. Many instances you  eat similar things everyday and don’t even notice how they taste-like that breakfast muffin you ate this morning. Though how does the food evoke such a spectrum of taste and how exactly does that happen?  Today on the Flavour Investigator we are going to answer these questions. Try not to get to hungry!

How do we taste?

According to the Gale Encyclopaedia of Science:

“Taste is one of the five senses through which all animals interpret the world around them. One of the two chemical senses (the other being smell), taste is stimulated through the contact of certain chemicals in substances with clusters of taste bud cells found primarily on the tongue.  An individuals unique sense of taste is partially inherited.” [1]

Ultimately, taste is a chemosensation.  This is because the sensation of taste is produced when a substance in the mouth reacts chemically with taste receptors located on taste buds on your tongue. However, eating a meal is a complex experience because other factors come into play such as smell and stimulation of your trigeminal nerve to register texture, pain and temperature. That is what gives us that unique sensation when we eat spicy food. Ouch!

These nerves here are what are responsible for the sensation of spiciness! [2]

Currently scientists have identified five separate entities of taste: sweet, sour, bitter, umami and salty. These all work in unison to create a unique taste combination in our brains, creating the iconic meals that we all love and remember. So let’s dive right into it!

Sweetness

The first stop on our list is the basic taste of sweetness. I’ll admit it, I am a person with a sweet tooth. Cookies, cakes and ice cream are just a few I can think of that set my taste buds in a whirl spin.

Our perception of sweetness comes from the presence of simple carbohydrates and a few other substances. This includes sucrose, saccharin, sucralose, cyclamate, aspartame and many more.

Structure of beta-D-Lactose: The major sugar found in milk and milk products. This is what gives milk its unique sweetness [3]

Sweetness is recognized by our gustatory system (the sensory system responsible for the perception of taste). When stimulation occurs specialized taste cells for sweetness are activated, causing neurotransmitters to fire up and send information to the brain.  [4]

Our individual taste cells each express a certain type of taste receptor. The Type 1 receptors, T1R2 and T1R3 bind together to form a G-protein receptors on the G protein gustducin found on your taste buds. Calcium can now enter the cell, depolarize it and a perceived sweetness signal is sent to the brain.  As you can tell, there are quite a lot of receptors flaring up when you eat that last piece of chocolate cake! [4]

For your benefit- a chain reaction of what is going on when you take a bite of that chocolate cake [4]

An interesting scale that is used to measure sweetness is how sweet a substance is relatively to sucrose as sucrose registers as a 1. While 30 millimoles per liter would product a sweetness index of 0.3.

Saltiness

There is a consistent saying in the kitchen-that you always have to remember to season your food.  Although it may appear annoying to hear at first, there is truth to the reminder. By salting your food an over better food experience occurs, as it better helps to bring out other tastes.

The simplest taste receptors you have in your mouth are for sodium chloride (regular table salt). Sodium chloride (NaCl) creates a salt taste when it is dissociated into the ions sodium (Na+) and (Cl−) through your saliva. There are other ions that also elicit a perception of “saltiness.” These include ions from the alkali metal group such as potassium and lithium. However, they do not draw out as much of a response as sodium. [5]

It must be quickly stated that there is still research being conducted on the taste recognition of salt. Therefore, all that is researched is still a hypothesis as gaps in understanding still remain. The leading hypothesis is as follows:

When you take a bite from a salty chip sodium chloride enters you mouth. Epithelial sodium [Na] Channels: ENaCs found on the outside of the taste receptor cell allows for sodium cations to enter the cell. Depolarization of the cell occurs causing a voltage-dependent calcium channel, flooding the cell with positive calcium ions and leading to a neurotransmitter release. [5]

The size of lithium and potassium ions are most closely resemble those of sodium and the saltiness is most similar. However, the specialization of these channels do create some implications on the food industry. If these theories are correct then this means that there is a low chance developers will ever find an equal substitute for perceived saltiness in regards to sodium chloride.  [5]

Bitterness

Although saltiness may be the simplest of the taste receptors that we have. Bitterness is the most sensitive. This is why when we take a swig of coffee many people experience the unpleasant and sometimes sharp flavour.  Similar to that of saltiness there is still quite a lot that is not understood about the biology of bitterness. This is in part because there are many compounds that create a bitter taste in our mouths, unlike saltiness which have specific ions that react with taste receptors.

There is a wide variety of compounds which are bitter tasting; they include amino acids, phenylthiocarbamide (PTC), esters, phenols and a whole lot more. A theory has been proposed by McBurney that there are more than three different bitter taste receptors. Though most commonly thought is that there is a mechanism that works to perceive both sweet and bitter. Research has shown that TAS2Rs (taste receptors, type 2) coupled with G proteins are responsible for the human ability to taste bitter substances. [6]

Although it can be pleasant at certain situations, having slightly bitter food is tasty. However, this is not always the case as a problem facing the food industry is that consumers tend to reject this taste. Vegetables have a bitter flavour to them most likely leading to a low acceptance of consumers.  Plant-based phenols, flavonoids, isoflavones, terpenes, and glucosinolates are almost all bitter, acrid or astringent. [6]

Sourness

Aside from bitterness a lot of people find the taste of sourness not pleasurable. However, a splash of lemon juice to a piece of salmon can add another dimension of flavour. When you taste something that is sour you are actually detecting the acidity. Though how exactly do we taste this?

Like our previous tastes, the perception of sour is detected by cells that are found across your tongue. There is still a poor understanding of how exactly the mechanism for sourness works. However, it is known that when the protein, PKD2L1 is expressed you can tell that sour taste cells are being identified.  If you are familiar with ion channels, sour substances which have protons are what enter located ion channels in your mouth and cause an electrical response. [7]

Though where exactly do these protons come from? This depends on what type of food that you are consuming.  For the purpose of this example, we will talk about the juice of a lemon with contains about 5% to 6% citric acid.  Citric acid is a weak tricarboxylic acid which means that it contains three carboxyl functional groups. As a result, there are three regions which are capable of donating a hydrogen (proton/hydrogen ion H+). The more dissociated and free hydrogen ions, the higher intensity of sourness you taste. [7]

Umami

One of the newest flavour categories to join the table is umami.  This english loan word comes  from the Japanese word meaning good flavour or good taste umami (旨味). Umami or scrumptiousness was studied by the scientist Kikunae Ikeda who analyzed Kombu (an edible kelp) in attempting to isolate dashi taste. He isolated monodium glutamate (MSG) and sold it as a food additive as it produces strong umami taste. This discovery was based on Ikeda’s observation that umami had a distinct taste that was different from sweet, sour, salty and bitter.  This sensation of umami can also be tasted in western cooking when you eat cured meats. The so called savory flavour can be seen as umami. [8]

There are some umami taste buds that respond to glutamate in the same way that “sweet” ones do with sugar. The taste receptors which are responsible for umami are the modified forms ofmGlur4, mGluR1 and taste receptors type 1 (T1R1 + T1R3). Glutamate (Glutamic acid) binds to a variant of G protein coupled glutamate receptors. It is believed that the amino acid L-glutamate bonds to a type of G-protien-coupled receptors (GPCRs) known as metabotropic glutamate receptor (mGlur4). This causes the G-protein complex to create a series of events which activate a secondary receptor and eventually a neurotransmitter release. [8]

Though as you might be able to tell, the science behind “umami”are still not well known.

The Geography of the Tongue

If any of you are familiar with taste at some point in your lives you have heard of a tongue map or taste map. This map basically refer to the different sections of the basics tastes. This map was originally came from a Harvard psychologist Edwin G. Borin which wrote a paper entitled Zur Psychophysik des Geschmackssinnes which was written in 1901. The paper shoed small difference in threshold detection levels across the tongue. Certain regions did not taste the different tastes at the same levels. These the differences were taken out of context and they were placed a difference in sensitivity.  In 1974 Virginia Collings investigated the topic again and confirmed that all taste exist on all parts of the tongue.

This is not a correct representation of the human tongue!

References

[1] Petechuk, D. (2004). Taste. In K. L. Lerner & B. W. Lerner (Eds.), The Gale Encyclopedia of Science (3rd ed., Vol. 6, pp. 3957-3960). Detroit: Gale. Retrieved from http://go.galegroup.com/ps/i.do?p=GVRL&sw=w&u=rpu_main&v=2.1&it=r&id=GALE%7CCX3418502257&sid=summon&asid=34090a1a3b8bc9faebfcc8a93f5b2466

[2] By Grant, John Charles Boileu (An atlas of anatomy, / by regions 1962) [Public domain], via Wikimedia Commons

[3] By Yikrazuul (Own work) [Public domain], via Wikimedia Commons

[4] Fernstrom, J. D., Munger, S. D., Sclafani, A., de Araujo, I. E., Roberts, A., & Molinary, S. (2012). Mechanisms for Sweetness. The Journal of Nutrition, 142(6), 1134S–1141S. http://doi.org/10.3945/jn.111.149567

[4] Assadi-Porter, F. M., Tonelli, M., Maillet, E. L., Markley, J. L., & Max, M. (2010). Interactions between the human sweet-sensing T1R2-T1R3 receptor and sweeteners detected by saturation transfer difference NMR spectroscopy. Biochimica et Biophysica Acta, 1798(2), 82. http://doi.org/10.1016/j.bbamem.2009.07.021

[5] Institute of Medicine (US) Committee on Strategies to Reduce Sodium Intake; Henney JE, Taylor CL, Boon CS, editors. Strategies to Reduce Sodium Intake in the United States. Washington (DC): National Academies Press (US); 2010. 3, Taste and Flavor Roles of Sodium in Foods: A Unique Challenge to Reducing Sodium Intake. Available from: http://www.ncbi.nlm.nih.gov/books/NBK50958/

[6] Drewnowski, A., & Gomez-Carneros, C. (2000). Bitter taste, phytonutrients, and the consumer: a review. The American journal of clinical nutrition, 72(6), 1424-1435.

[7] Li, H., & Liu, F. (2015). The chemistry of sour taste and the strategy to reduce the sour taste of beer. Food Chemistry, 185, 200-204. doi:10.1016/j.foodchem.2015.03.135

[8] Lindemann, B. (2002). The Discovery of Umami. Chemical Senses, 27(9), 843-844. doi:10.1093/chemse/27.9.843