At first glance, saliva seems like boring reading, a simple and convenient way to moisten our mouths, but scientists are beginning to realize that the reality is quite different.
This liquid interacts with things that enter the mouth, and despite being 99 percent water, it has a key influence on the flavors - and our enjoyment - as we drink and eat.
"It's a liquid, but not just a liquid," says oral biologist Guy Carpenter of King's College London.
Some of the functions of saliva have been understood by scientists for a long time: it protects the teeth, facilitates speech, and establishes a desirable environment for food that enters the mouth, but researchers now understand that saliva is both a mediator and a carrier, influencing how food moves through the mouth. and how it stimulates the senses.
The latest evidence suggests that the interaction between saliva and food can help us understand what foods we really like to eat.
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Saliva is essentially not very salty, which allows people to taste the saltiness of potato chips.
It's not very acidic either, which is why even a drop of lemon can be so stimulating.
Water and saliva proteins lubricate each bite, and its enzymes such as amylase and lipase start the digestion process.
This moistening also dissolves tastants, the chemical components of taste, in the saliva so that they can move around and interact with the taste receptors.
Jianshe Chen, a food scientist at Zhejiang Gongshang University in Hangzhou, China, says that through saliva, "we detect the chemical information of food: taste, aroma."
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Chen coined the term "oral food processing" to describe a multidisciplinary field that relies on the science of food, the physical properties of the materials from which food is composed, and the physiological and psychological responses to food, in a paper published in 2002 in the Yearbook of Science about food and technology.
When people eat, he explains, they're not actually enjoying the food itself, but the mixture of food and saliva.
So, for example, a person who eats can perceive sweet-salty molecules only if they reach the taste receptors, and for this to happen, they have to pass through the deposits of saliva lining the tongue.
That didn't happen just like that, says Carpenter and points out the example of an evaporated carbonated drink that tastes sweeter than a freshly opened bottle of the same drink.
Scientists hypothesize that this happens because the popping bubbles of carbon dioxide impart a sour taste that distracts the brain from the sweetness.
But when Carpenter and his colleagues decided to simulate this process in the laboratory, in a kind of artificial mouth, they discovered that it was saliva that prevented the bubbles from flowing between the tongue and the upper palate.
Carpenter thinks that these suppressed bubbles physically blocked the sugars from reaching the taste receptors on the tongue.
This was not the case with the evaporated drink, as there were no bubbles to block the sweet taste.
Saliva can also affect the aromas - which are largely responsible for our perception of taste - released by food in our mouths.
While we chew, some taste molecules from food dissolve in saliva, but those that do not dissolve can float into the nasal cavity where they will be met by countless receptors.
As a result, people with different saliva flow or composition can have very different taste experiences from the same food or drink.
So, for example, Spanish researchers measured the flow of saliva in ten volunteers who evaluated wines in relation to the fruit esters that were added to them.
The volunteers who produced more saliva rated the flavors as more intense, probably because they swallowed more often and thus let more aromas into their nasal passages, the scientists said.
So those wine enthusiasts who take particular pride in their ability to detect nuances of aromas should be at least partially grateful to saliva.
Saliva also plays a major role in our perception of texture.
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Take for example astringency, that dry feeling that happens in the mouth when we drink red wine or eat unripe fruit.
It's not the wine that dries out your mouth.
Instead, wine molecules called tannins can cause proteins in the saliva to precipitate, so that it no longer lubricates the mouth effectively.
Saliva also helps in distinguishing foods with a higher or lower amount of fat.
Even if two types of yogurt look the same, the one with less fat will create a drier mouthfeel, says Anvesha Sarkar, a scientist from the UK's University of Leeds.
"What we're trying to understand is not the property of the food, but how the food reacts in contact with the surface of the mouth," says Sarkar.
Milk fats can combine with saliva to create a layer of droplets that can mask the astringency and contribute to the richer taste of yogurt, she says.
Her research uses a mechanical tongue submerged in artificial saliva as a way to stimulate the processes that occur as food moves through the mouth and how this affects the sensory experience of eating.
Smudi with less fat, Sarkar says, may look creamy at first glance, but it lacks the textural richness that fat mixed with saliva produces.
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A full understanding of these interactions between saliva, food and the inside of the mouth—and how information travels to the brain—could lead to the creation of healthier foods, Sarkar says.
She foresees the development of "graded food" that could have enough sugar on its surface to dissolve in saliva and give a sweet taste, while at the same time having a lower concentration of sugar and calories in it.
She says that a similar conceptual approach could help reduce the amount of fat in food.
However, a good understanding of these processes and the creation of such food will not be a simple process, because saliva and perception vary in relation to the time of day and in relation to the individual characteristics of each of us.
In general, saliva flows slower in the morning and faster in the afternoon.
And the composition of saliva in each individual - say the amount of certain proteins - varies in relation to the time of day and depending on the presence of stimuli such as tempting aromas.
Oral biochemist Elsa Lami from the University of Evora in Portugal studied this on volunteers who smelled a piece of bread for four minutes with their eyes closed.
During that time, she observed changes in their saliva.
Two types of protein, amylase, which helps digest starch, and cystatin, which is associated with sensitivity and taste perception, increased their presence during exposure to bread, she says.
Her team did similar research with vanilla and lemon and in all cases changes in salivary protein levels were observed, although the specific changes depended on the food that was served.
Her team is now working to discover the functions that this discovery could help with.
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The composition of saliva varies from person to person—and depends in part on the person's past diet, says Ann-Marie Torregrosa, a behavioral neuroscientist at the University at Buffalo.
When Torregrosa fed rats food with additives that were bitter, she noticed a significant increase in the amount of salivary proteins.
As these changes took place, the rats slowly began to accept the bitterness in their food.
"We should look at it this way: If you keep eating broccoli, it won't taste so bad anymore," says Torregrosa.
In another experiment, Torregrosa used a catheter to transfer saliva from the mouths of rats that were accustomed to bitter food to the mouths of rats that were not.
Despite not being exposed to such foods, they also became tolerant to bitterness.
But control animals that were not pumped with bitter-tolerant saliva still refused to eat the bitter food.
Torregrosa says that she and her team had to figure out exactly which proteins are responsible for creating this tolerance.
In addition to a few likely candidates, among them proline-rich proteins or protease inhibitors, it is possible that there are others.
It is necessary to know which proteins these are before assessing how responses to bitter tastes change in the mouth and brain.
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Of course, rats aren't humans, but scientists have come across indications that saliva does similar things to taste receptors in humans, although things are much more complicated.
"There are a lot of other things in human diet and experience that affect the everyday experience of food and taste that rodents don't have to deal with," says Lisa Davis, a sensory and food scientist at Purdue University who studies taste and behavior.
If we could decode and understand these patterns, the potential would be enormous, says Lami.
If we could somehow provide children with an additive that will cause changes in their saliva, and thus make the experience of eating vegetables tastier, it could lead to a healthier diet.
If their first encounter with a new food is not associated with strong bitterness, then they "will probably experience eating vegetables as a good experience," she says.
Finally, by understanding how saliva affects taste—and how the type of diet, in turn, affects the composition of saliva—we can discover new ways to nudge our preferences for certain types of food toward healthier meals that we normally avoid.
“How do we turn haters into people who like this kind of food?” she asks.
"It became my obsession."
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