Along with the traditional sweet, salty, bitter and sour, in 2002 scientists added a fifth flavour to the list of primary flavours —umami— a Japanese word that means “pleasant flavour” and that describes the taste of Serrano ham or Parmesan cheese. More than a decade later, researchers continue to identify new flavours, with the hope that one will come to occupy the coveted sixth place.
But this is not a race with a clear winner. In recent years, at least three are posited as favourites in this taste competition. The difference will be found in the sensory mechanisms and in their unique identity.
Neither acidic nor bitter: calcium
Calcium is a mineral that could awaken new taste sensations in the discerning foodie. In an article published in the journal Neuron a few weeks ago, an international team of scientists suggested that this taste, slightly bitter and sour, was on the short list.
As Craig Montell, the main author of the work, explained to OpenMind, the five traditional (or primary) tastes are on the list because they meet two criteria: they are perceived by different taste receptors and they can be distinguished from the point of view of behaviour, that is, their signal provokes a response in those who taste them.
“Based on this definition, it would reasonable to think of calcium taste as distinct,” says Montell, a professor in the Department of Molecular, Cellular and Developmental Biology at the University of California at Santa Barbara (USA).
To reach these conclusions, the scientists used fruit flies. When they were given high doses of calcium mixed with sugar to try, the flies rejected it, showing a response to this new flavour. And the receptors that the insects used to identify it were a unique type of gustatory receptor neuron (GRNs).
The taste of fatty acids
But this is not the only flavour that fruit flies have identified. A few months ago, another investigation concluded that this species was also able to identify the fatty acid taste. A set of fruit fly neurons that are sensitive to sugar differentiated fatty acids as an independent taste.
Richard D. Mattes, director of the Ingestive Behavior Research Center at Purdue University (USA), described this flavour in a previous experiment and named it oleogustus: “oleo” is a Latin word meaning fatty and “gustus” refers to taste.
The participants had to differentiate salty, sweet, umami, bitter, acidic and fatty flavours. Although the samples of fatty acids were initially grouped with the bitter taste, in a second classification the volunteers separated them from the rest. According to Mattes, one must not confuse the taste of fatty acids, which is not pleasant, with the sensation of fat, much softer and creamier.
In his opinion there are more than six flavours. “It doesn’t really matter what the sixth taste is. The point is that there are more than 4 or 5. It may end up that we have 7 to 10,” he tells OpenMind.
According to the expert, there is no widely accepted definition of the requirements that a primary flavour must meet. In addition to specific receptors and the response it provokes in the taster, the specialist adds that the taste should provoke a signal emitted by the gustatory system, that the new taste has a purpose or that it is unique and perceptible.
Returning to the fatty acid taste, a study conducted in rodents revealed that a protein (CD36) had the ability to identify lipids in the diet of these animals. When the gene that produced the protein was inactivated, they were not enticed by foods rich in fatty acids. According to the authors, controlling this protein (preventing the body from producing it) could be useful in treating the pandemic of obesity, which is based on an excessive intake of fats. Differentiating fatty acids as a unique flavour, with its specific molecular and sensory routes, can facilitate the design of new strategies to combat the disease.
Along with calcium and fatty acids, starch also wants to join the taste triad. Foods rich in carbohydrates, such as pasta or bread, possess this flavour and according to several investigations, we are able to identify it in its own right.
Although until recently it was thought that we tasted starch as something sweet, a study published in the journal PLOS ONE that featured specialized tasters concluded that this new flavour has its own set of taste receptors. The problem is that they have not yet been identified, as Montell confirms.
And what is the use of differentiating different types of flavours? The main function of taste is simply to identify the substances that give us energy and, at the same time, avoid those that are toxic. In addition, flavours also prepare the body to better assimilate the nutrients that we will shortly ingest. For example, when we eat snacks, their bitter flavours make us salivate and generate acids in the stomach as a preliminary phase to the consumption of the food.
In the case of fruit flies that differentiated and rejected calcium in high quantities, their brains identified these high levels as harmful to their body, and hence their opposition to ingesting them.
Something similar happens with fatty acids. “When concentrations of fatty acids are high in a food, it is typically rejected, as would be the case when a food is rancid. In this instance, the fat taste sensation is a warning to not eat the substance,” says Mattes.
In his opinion, the new research will cause the traditional belief that taste is made up of a limited number of flavours to change over time. If we manage to identify the sensory mechanisms involved with each new flavour and what makes them unique, it will be an endless party for the senses.