You've never tasted the real taste of food

If saliva deceives you, don't be sad, don't be sad, because it deceives more than just you...

In the tropical regions of West Africa, there is a plant called miracle fruit (Synsepalum dulcificum).

Image credit: Hamale Lyman via Wikimedia Commons

The fruit itself tastes sour, but when humans eat it and then put a slice of lemon in their mouth, they will also feel that the lemon is sweet. This magic of making sour food "sweetened" can last for about an hour.

The reason why miracle fruit can deceive human taste is because of a special glycoprotein, miraculin. This substance has no taste in a neutral environment, but under acidic conditions, it will bind to the sweet receptors in the taste buds. In this way, when the miracle fruit protein stays on the tongue, people will think sour things are sweet.

So, without using such taste modifiers, can humans taste the real taste of food? It's not that simple. You think your taste buds can communicate directly with food, but in fact the taste you taste is not the flavor of the food itself, but the taste of the food after being adjusted by saliva.

When miracle fruit suddenly tells us that lemons are sweet, we at least know that it is a lie. But if saliva quietly changes our sense of taste, will we still be smart enough to discover it?

If you eat more, it won't be hard to eat.

Saliva is the lubricant in the mouth. With its protection, we can talk and eat smoothly without getting hurt during these daily activities. But saliva processes food more than just reducing the particles in it and turning it into a bolus that is easy to swallow.

For example, when we eat a piece of chocolate, the taste substances in it, such as sweet compounds, bitter compounds, etc., usually dissolve in saliva first, and then bind to the receptors on the taste buds, so that we have the opportunity to feel its taste. Rather than tasting the taste of chocolate, it is better to say that we are tasting the mixture of saliva and chocolate - this is also helpless, after all, without saliva, it is difficult for people to taste the taste.

One of the tasks of saliva is to ensure that our sense of taste can function normally. In addition, saliva may also transform our sense of taste unconsciously. In your memory, have you ever encountered a certain food that you thought was unpalatable at first and wanted to spit it out after taking a bite, but after eating it a few times, you found that the taste was not so bad and even gradually began to like it?

Image source: Pixabay

In 2017, a group of scientists trained the taste buds of rats with special "delicacies". The meals they designed for rats contained 0.375% quinine, a bitter stimulant that is not a friendly ingredient for ordinary rats who are not used to bitterness. The rats in the experimental group first ate normal food without quinine for 14 days, and then ate food with quinine for 14 days; while the rats in the control group always enjoyed meals without quinine.

The rats that first ate the quinine meal were not used to it and their food intake dropped sharply. But after two weeks, their acceptance of bitter food increased a lot and their food intake returned to the baseline level. In contrast, the rats that had never eaten the quinine meal did not have a significant change in their food intake during the whole process. So, did the rats trained with the quinine meal become able to endure bitterness because they got used to the bitterness? Or did the taste of the quinine meal change in their mouths?

Scientists found that during the period of feeding quinine, the secretion of various salivary proteins in the mouths of rats was higher than before. Moreover, when the rats that had eaten bitter food for two weeks returned to normal meals, these salivary proteins did not return to baseline levels. The researchers confirmed through experiments that the changes in the secretion of these proteins were not due to changes in the rats' food intake, but precisely because of the addition of quinine to their diet.

Later, the same research team conducted another experiment, which was to put the saliva of rats that had developed the ability to endure bitterness into the mouths of rats that had no experience of enduring bitterness. As a result, those rats that had not been trained to taste bitterness were also able to tolerate bitterness. At the same time, rats that had no experience of enduring bitterness and did not receive "bitter saliva" from their companions were still very resistant to the sudden arrival of bitter food.

Although scientists are not quite sure which proteins are the ones that help rats tolerate bitterness, different saliva does seem to give rats different tastes. So, do humans who have been trained to endure bitterness have different tastes than those who have not been trained?

In 2018, a team of scientists recruited 64 volunteers and asked them to drink bitter chocolate almond milk three times a day and rate the taste of the almond milk each time. The taste test lasted for a week, and over time, the volunteers reported a decreasing bitterness score.

In addition to the subjective perception that almond milk was becoming less bitter, the composition of the volunteers' saliva also changed: the content of several proline-rich proteins (PRP) gradually increased. Some of these proteins can bind to bitter molecules (such as tannins), and the reduction in people's perceived bitterness is likely related to this.

Sometimes, our evaluation of a food changes from unpalatable to not unpalatable, or even to delicious, perhaps as a result of changes in saliva composition. Researchers speculate that this is probably an adaptation that can help humans reduce the negative experience of bitter substances (or other unacceptable taste substances).

In addition to changing the chemical composition of saliva, we can also change our taste perception using some physical methods.

Why is Coke sweeter when it’s flat?

An important prerequisite for us to feel sweetness or bitterness is that the sweet compounds or bitter compounds successfully reach the taste buds and bind to the corresponding taste receptors. But even if there are these flavor molecules in the mouth, not all of them reach the taste buds. If there is a way to block the path of some flavor molecules to the taste buds, it can change the taste perceived by humans.

For example, in a study published in 2021, scientists believed that this may be partly why carbonated cola is not as sweet as still cola.

In fact, this phenomenon has long had a more well-known explanation, that is, when carbon dioxide is converted into carbonic acid, it will stimulate the nociceptors on the tongue, causing pain, distracting the brain's attention, and making people unable to feel too much sweetness. However, the research team insisted on exploring this issue from another angle.

Image source: Pixabay

They focused on the lubrication mechanism in the mouth. When humans eat, there is sliding between the tongue and the palate, and saliva is crucial as a lubricant, which can reduce the damage caused by friction between the uneven tongue and the palate.

For example, when the salivary film is too thin to completely prevent direct contact between the tongue and the palate, this lubrication mode is called boundary lubrication; when the salivary film is sufficient to completely separate the two surfaces and prevent any contact, the lubrication mode becomes full-film lubrication. There is also an intermediate state between different lubrication modes, called mixed lubrication.

In the laboratory, scientists used an artificial oral model to simulate the situation when the tongue and palate are lubricated by liquid. They found that when the relative movement speed of the tongue and palate in the artificial oral cavity is very slow, the liquid provides boundary lubrication. As the sliding speed increases, more liquid will be dragged into the contact surface between the tongue and the palate, and the fluid dynamic pressure pushes the two surfaces apart, and the lubrication mode changes to mixed lubrication and finally to full film lubrication.

The friction in the mouth will also be different between different lubrication modes. The researchers found that under mixed lubrication (speed 3-15 mm/s), when the thickness of the film formed by the liquid (~25 nanometers) is similar to the roughness of the tongue surface in the artificial mouth (~20 nanometers), the friction caused by soda is more than three times that of non-soda (under the same pressure).

The researchers explained that this is because the carbon dioxide in the soda will accumulate at the entrance of the contact surface between the tongue and the palate, restricting the flow of liquid, reducing the fluid dynamic pressure and increasing friction. Scientists are so concerned about friction in the mouth because it often affects the taste of food and also affects people's perception of taste.

Image source: Vlădescu et al, 2021

In addition, the researchers found another factor that increases friction in soda. They found that soda can reduce the thickness of the salivary membrane in the artificial mouth by 80%. Destroying the salivary membrane that has a lubricating effect can also cause greater friction in the mouth.

Scientists say that one of the reasons why carbonated cola is not as sweet as non-carbonated cola may be that when we drink carbonated cola, the friction-related mechanism affects the flow of flavor molecules to the taste buds. Of course, this does not mean that they want to overturn the previous explanation, but just provide another idea.

So what's the use of this line of thinking? By learning more about the laws of taste perception, food scientists can better deceive our taste perception. But it doesn't matter, we never taste the real taste of food anyway.