Sense of taste: tastes fat to us?

Probably everyone has a favorite food. But only a few people are able to explain why they enjoy the apple strudel from grandma’s house or the curry sausage from the snack bar around the corner so much. And the study of taste is also a surprisingly young discipline. Scientists have only been interested in this topic for about 15 years now. Since then, the five tastes sweet, salty, sour, bitter and umami (meaty) have been gradually deciphered, which antenna molecules on the thousands of taste receptor cells of the tongue pick up the signal, how this information is then processed in the cell and sent to the brain.

Little is understood, however, about how taste qualities and intensities are encoded, for example, how the brain receives the signal "sweet liquid" conveyed when drinking lemonade. It is clear, however, that the sense of taste should distinguish above all poisonous and less nutritious (bitter, sour) from non-toxic and caloric (sweet, salty, umami). The hedonistic pleasure is, so to speak, an addition of nature, so that we like to eat and do not starve to death.

This article is included in Spektrum – Die Woche, 49/2015

It is also becoming increasingly clear that we do not only taste on the tongue, but that the sense of smell also plays a decisive role, identifying various aromas from the food we eat. And also Trigeminal, a nerve that runs through the mouth and nose conveys certain tastes, such as a furry, astringent feeling, triggered by certain tannic acids, for example from rhubarb or tea, or by the taste of alcohol. In addition, the mouthfeel, i.e. the sense of touch, and also the color of the food play an important role so that there is talk of delicacies. Especially when we consume fatty foods such as chocolate, the creamy, soft mouthfeel is probably mainly responsible for the taste experience.

Fats are flavor carriers

Here, too, the sense of smell is important, since volatile aroma compounds from foods are often fat-soluble: Fats are flavor carriers. Until now, "fat" has been not as an independent sense of taste. But now U.S. researchers led by Richard Mattes of Purdue University recently claimed to have demonstrated in a study that "fatty" taste is not a sensory attribute taste is its own, and they have also given it a name: Oleogustus. What lies behind it? As early as 2011, Wolfgang Meyerhof of the German Institute of Human Nutrition (Dife) had found receptors (GPR40 and GPR120) on the human tongue to which only fatty acids in free form can dock quite specifically. Fatty acids, along with glycerols, are the building blocks of triglycerides found in oil, butter and lard. The fact that there are corresponding sensory cells for these fatty building blocks on the tongue, which also only recognize a specific taste signal, is the minimum requirement, so to speak, for being able to speak of an independent basic type of taste at all. Sweet taste, for example, is triggered by sugar molecules, while umami is triggered by two specific amino acids found primarily in animal foods. The Meyerhof study formed the cornerstone for further research in this direction.

Now the U.S. scientist Mattes has conducted tests with 28 gourmets, tasting all flavors, equally prepared so that the 15 samples did not differ in color, smell or mouthfeel. For example, oleic acid, citric acid, glutamate, caffeine and glucose were found in the test jars in quantities similar to those found in foodstuffs. The result: more than half of the test eaters were able to recognize bold as a distinct flavor, which was definitely different from the five already known tastes. In a second test with more than 100 subjects, however, fat was only tasted if the comparison samples were bitter, sour or umami. "Until now, pure fat taste was often described as bitter or sour in such tests – simply because it is perceived as unpleasant", reports Mattes. "Short-chain fatty acids in particular taste sour, while long-chain fatty acids evoke a different sensation." This fulfills another criterion on the way to the independence of the fat flavor. "Fat would have to be considered as a sixth sense of taste", demands Mattes. In view of the study, numerous media even headlined that fat is now "official" has been recognized as the sixth sense of taste.

What characteristics must a sense of taste fulfill?

However, this is not the case. Wolfgang Meyerhof, for example, says that it is still too early to give fat flavor the rank of a quality in its own right. Other factors were also missing. If, for example, in addition to the stimuli transmitted via the taste cells, there were also a signal transmitted via the trigeminal nerve, this would be a criterion for exclusion. Studies in the past have already shown that fatty acids stimulate the trigeminal nerve. In addition, it would still have to be proven that the signal triggered by the fat receptor is transmitted to the brain via specialized taste cells and downstream nerve tracts. Hubert Preibl, a medical scientist at the University of Tubingen, who recently published a review article on the current state of research on the subject of "fat perception says: "I think that the question of fat taste has certainly not yet been conclusively clarified."

However, there is no unanimous opinion in the scientific community on exactly what characteristics a sense of taste must fulfill in order to be named as such. Mattes points this out when criticizing his conclusions. However, he admits that there need to be more studies with more test subjects to substantiate his findings and definitively settle the question. Currently, his research group is analyzing data from thousands of subjects. An important indication that fat has its own flavors was provided by a study conducted last year by Meyerhof himself together with researchers at the Technical University of Munich. They proved in it that there are fat-splitting enzymes in salivary glands of the human tongue. These lipases probably work so fast that the triglyceride compound is broken down on the spot and the fatty acids could be recognized as free molecules by the receptors. "The study demonstrates the presence of these enzymes in the immediate vicinity of the taste buds and thus provides further evidence that humans can perceive fats tastefully", wrote the study authors at the time.

People could be trained to taste fat better

But why is it even important to understand in detail how we taste fat? Fatty foods have the reputation of making you fat. "After all, they have the highest calorie count at nine calories per gram of fat", says Preibl. Carbohydrates and proteins provide only four kilocalories per gram of nutrient. In addition, fat is found in many foods. By understanding exactly why French fries and the like are so popular, researchers hope to counter rising obesity rates. Thus, experiments showed that a person’s taste receptors react very differently to stimuli depending on his or her genetic makeup. People who barely notice fat in food consume larger amounts of calories than fat-sensitive people, who are also quick to notice the rancid, unpleasant tones in lard sandwiches, roast crust and chips. The latter are also actually slimmer. "While there is no proof that there is a causal chain here, these are nevertheless indications of a link between genetic variability of receptors and various diseases", writes Mattes. Initial experiments showed that less sensitive subjects can be trained to taste fats more strongly.

If fat were recognized as a sixth sense of taste, it would be possible to develop foods that taste good even with less fat. Because up to now, low-fat foods have been designed primarily to imitate a creamy mouthfeel. But if fat itself tastes like something, that could explain why light products often have little appeal among consumers.

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