By: Kelsey Tenney

(source: www.theconversation.com)

Sweet, salty, sour, bitter, umami. The five basic tastes. The subject of taste, however, has plagued scientists, philosophers, and epicureans for centuries. In fact, up until 2002 umami was not an accepted taste. There is currently a debate over a sixth primary taste. Should fat join the ranks of sweet, sour, bitter, salty, and umami?

Historically, taste was defined as a philosophical question. The Chinese generally counted sweet, salty, sour and bitter among their repertoire but added pungent into the mix. They related the tastes that they defined to the elements of Chinese philosophy: earth, metal, water, fire, and wood. The Western philosophers took a crack at taste as well, but they regarded taste as the gluttonous sense. While sight and hearing allowed us to develop and understand language and art, taste promoted self-indulgent behavior. Nevertheless, ancient Greek philosophers tried their hand at it. Plato perceived six tastes—bitter, sweet, salty, sour, astringent, and pungent. Aristotle made the matter much more complex by mapping taste between two poles. He asserted that sweet and bitter were on opposite ends of the spectrum with salty, pungent, harsh, astringent, and acidic somewhere in between. Finally, Theophrastus, who studied under both of the philosophers added oily to the mix making eight basic tastes.

This list was generally accepted as fact until the eighteenth century. The enlightenment was in full force in Europe, and scientists were challenging many common beliefs. Interestingly enough, taste became incredibly difficult to define. Carl Linnaeus named eleven basic tastes (bitter, fatty, sour, astringent, sweet, salty, sharp, viscous, insipid, aqueous, and nauseous) while Père Polycarpe Poncelet equated taste to harmonizing music notes naming seven notes (bitter, acid, sweet, peppery, astringent, sweet and sour, and weak or tasteless). Brillat-Savarin, on the other hand, was a renowned French epicure and believed that taste was infinite. He alleged that “the number of tastes is infinite, since every soluble body has a special flavor, which does not wholly resemble any other.”

The nineteenth century cemented our four tastes. Simplicity was ideal, and Adolf Fick, a German doctor, narrowed the tastes down to sweet, salty, bitter, and sour. In addition, taste buds were discovered when tongue cells were examined under a microscope. Taste buds looked like keyholes that bits of food might fit into. It was generally believed that there were four different keyhole shapes agreeing with Dr. Fick’s assertion.

Umami came onto the scene in the twentieth century challenging the status quo after French superstar chef Escoffier created veal stock and Dr. Ikeda decided that a savory flavor in his dashi could not be explained by the four tastes. Umami is now in the textbooks as the fifth taste. It only took 100 years.

What exactly constitutes a taste? There are five criteria: 1) There must be a class of stimuli responsible for the perception, 2) There must be mechanisms present that change the chemical code of the stimuli to an electric signal, 3) That electrical signal must be neurotransmitted to the brain, 4) The stimuli must be completely independent from other tastes, and 5) The stimulus must generate effects in the body downstream from the mouth.

Sweet is triggered by sugar telling us we are eating carbohydrate, sour is activated through hydrogen ions reflecting excessive acid, amino acids (predominantly glutamate) set off umami, salty is elicited by sodium and potassium ions indicating the presence of minerals, and many molecules meant to trigger toxin signals are perceived as bitter.

(source: www.npr.org)

But what about fat? Recent arguments have been made by Dr. Russell Keast toward such.

Criteria 1: The breakdown products from fats and free fatty acids are considered the stimuli for fat perception. Lingual lipase break down triacylglycerols, the predominant form of fat in food, so that the fatty acids and other breakdown products can be perceived.

Criteria 2: Recent research points toward two potential receptors responsible for transduction of the signal—CD36, a fatty acid transporter, and G protein-coupled receptor 120, a fatty acid receptor that activates a peripheral signaling cascade.

Criteria 3: The neurotransmitter carrying the fat signal to the processing region of the brain requires more research, but there is some indication that noradrenaline and serotonin are secreted in response to fatty acids triggering the orosensory perception.

Criteria 4: Perceptual independence of fat is proven through detection thresholds of fatty acids completely separate from other basic tastes. Fat taste quality, however, is different than other tastes. When you taste sugar, you automatically identify sweet. Fat is detected at a low level, but the recognition of that taste as fat is not apparent until supra-threshold is reached. At that point, other chemosensory systems are likely involved.

Criteria 5: A 2.8-fold increase in plasma triacylglycerol concentration has been recorded in humans following oral fat consumption. Additionally, fatty acids have been shown to activate signaling chains in the body that prepare it for fat digestion and absorption.

Fat meets the criteria of what a taste is, but it doesn’t act like any of the others. According to fat researcher Dr. Richard Mattes from a recent NPR interview, if fat were to become a primary taste, it would change “our basic understanding of what taste is.” A change in the taste repertoire could be extremely helpful for future product development. As we commonly associate fat being important to the texture and flavor profile of foods, fat replacements in low-fat foods are meant to mimic those characteristics. If we can actually perceive the taste of fat, that would explain why low-fat products just aren’t making the cut.

Dr. Russell Keast has also drawn a correlation between individuals with high BMIs and lower sensitivity to the taste of fat. In other words, more fat is required to get the same sensation. Individuals that are less sensitive to the taste of fat are likely to overconsume fatty foods. Increased understanding of fat as a taste could shed light on this pattern

What do you think? Should fat become the sixth taste? Only time and more research will tell.

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About Kelsey: I grew up in Minnesota constantly in awe of innovative companies like General Mills and Cargill. After touring Cargill kitchens in high school, I decided food science was my niche. I received my Bachelor’s from Purdue University and continued on to The Pennsylvania State University where I am working on a Master’s concentrating in food chemistry. In my free time I love to bake and cook in order to experience food science in all its glory. I recently started the blog www.appeasingafoodgeek.comdetailing my culinary adventures. When I’m not in the lab you can usually find me multitasking between testing a cookie recipe, watching an old episode of X Files, and trying to find my wine glass. 

References:

Keast RSJ, Costanzo A: Is fat the sixth taste primary? Evidence and implications. Flavour 2015, 4:5.

Keast RSJ: Tasty treat: how we showed fat to be the sixth taste. The Conversation 2015.

Singh M: Salty, sweet, sour. Is it time to make fat the sixth taste? NPR 2015.

Krulwich, R: Sweet, sour, salty, bitter…and umami. NPR 2007.

Aristotle: De Anima. 350 BC.

Chale-Rush A, Burgess JR, Mattes RD: Evidence or human orosensory (taste?) sensitivity to free fatty acids. Chem Senses 2007, 32(5): 423-431.

Galindo MM, Voigt N, Stein J, van Lengerich J, Raguse JD, Hofmann T, Meyerhof W, Behrens M: G protein-coupled receptors in human fat taste perception. Chem Senses 2012, 37(2): 123-139.

Abumrad N: CD36 may determine our desire for dietary fats. J Clin Invest 2005, 115(11): 2955-2967.

Keast R, Roper J: A complex relationship among chemical concentration, detection threshold, and suprathreshold intensity of bitter compounds. Chem Senses 2007, 32(3): 245-253.

Mattes RD: Fat taste and lipid metabolism in humans. Physiol Behav 2005, 86(5): 691-697.

Cover image: www.newston.net