A toddler may pull a face of pure disgust upon tasting spinach for the first time, but eventually, that same child can grow to tolerate the vegetable and eventually — gasp! — even like it. And even after childhood, a person's flavor preferences can continue to evolve. The question is, how does that happen?

Our flavor preferences are shaped by many factors, including our , our mothers' and our nutritional needs in childhood, said Julie Mennella, a biopsychologist and member of the Monell Chemical Senses Center in Philadelphia. But our biology doesn't dictate which foods we come to adore or despise over time. Rather, our preferences are quite malleable, or "plastic," and change depending on which flavors we get exposed to, when, how often and in what contexts, she said. 

Studies hint that learning to accept new flavors may come easier in early childhood, before age 3, whereas by comparison, older children may need to taste a new food more times before they learn to like it, according to a 2014 review authored by Mennella and published in . But while toddlerhood may represent a unique window of opportunity for broadening a person's palate, "I don't think that the window shuts," Mennella told Live Science. 

So we all can learn to like new flavors, regardless of our ages, although bad memories of specific foods can be difficult to overcome, she noted. (For example, after an intense bout of food poisoning, you might feel queasy at the mere thought of the food that made you sick, psychologist and neuroscientist Hadley Bergstrom .)

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On top of this ongoing learning process, our flavor preferences in adulthood may somewhat shift as our senses of taste and smell become less sensitive with age, although flavor sensitivity is just one of several factors that shape elderly adults' food preferences, according to a 2017 report published in the journal . 

How we perceive flavors  

Our perception of flavor emerges from not only taste but also our sense of smell, according to , a public information initiative run by the Society for Neuroscience. That said, many other factors influence whether we actually like the flavor we're perceiving, Mennella said. These factors include innate, evolutionarily driven taste preferences; the physical properties of a food, such as its texture or temperature; and our previous experiences with a given flavor or similar flavors.

When we bite into a food, like a chunk of cheddar cheese, chemicals in the snack spill out into the oral cavity. Some of these molecules plug into sensory cells called taste receptors, located on the and along the roof and back of the mouth. These cells detect at least five basic tastes: sweet, salty, bitter, sour and umami (savory). 

Each taste receptor specializes in one of these broad taste categories, so there are sweet receptors and salt receptors, for instance. But that's not to say that all receptors within a category react to the same exact taste molecules. For example, humans carry 25 types of taste receptors for bitterness, ; some bitter receptors detect only a few compounds, while others are sensitive to many, Mennella noted. And depending on their genetics, different people carry slightly different versions of each receptor, and in various quantities, which, in turn, affects their sensitivity to various tastes.

And to a certain extent, the community of microbes living in our mouths — called the oral microbiome — may also affect what molecules get released from our food as we chew, and therefore, which receptors switch on in response to said food, . 

A single bite of cheese sends taste receptors into a frenzy of activity as they shoot off messages to the . At the same time, some small, airborne molecules released from the snack get swept out of the oral cavity, through the throat and into the nasal cavity, where they touch down on smell receptors. Some smelly compounds from the cheese also enter through the front door of the , the nostrils. Upon activation, smell receptors send a surge of messages to the brain, which integrates this information with that from the taste receptors to bring us the distinct flavor of an aged white cheddar. 

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While the sensitivity of an individual's taste and smell receptors shapes their flavor perception, "to measure how sensitive someone is … that doesn't tell you anything about how much you like something," Mennella said.

Why we like what we like 

To a certain extent, human underlies our love of particular tastes. From birth, infants show a heightened preference for sweet tastes, compared with adults, and this sweet tooth persists until mid-adolescence, around age 14 to 16, when a child's growth begins to slow. At that point, children's fervor for sweets typically drops off and their tastes become more adult-like, according to the 2014 The American Journal of Clinical Nutrition review.  

This early love for sweetness is common across primates, as sweetness serves as a general signal for high-calorie foods that would be key for growth, development and survival, Mennella said. In general, compared with adults, children also show a heightened liking for salt, an essential mineral for brain and function.   

While sweetness and saltiness signal helpful attributes of foods, "bitter, on the other hand, was most likely our signal for 'Beware, this might do harm,'" meaning the taste might denote something poisonous or spoiled, for instance, Mennella said. Babies show a heightened sensitivity to bitter tastes, compared with adults, and in this way, the taste system acts as a "gatekeeper" of sorts, ensuring that growing children ingest plenty of calories while avoiding toxins, she said. Of course, these inbuilt preferences also sway how babies react to nutritious-but-bitter foods, such as dark green vegetables; so while infants are drawn to the sweetness of breastmilk, they usually abhor the first taste of pureed they're offered after weaning. 

But evolution doesn't hold all the influence over our food preferences in childhood; from the moment their senses of taste and smell develop in the womb, fetuses begin learning to like different foods, Mennella said. Foods and beverages consumed during pregnancy "flavor" the amniotic fluid, thus exposing the fetus to new flavors and relaying information about which flavors are safe to consume, according to a 2019 review in . 

And after birth, flavor molecules can also pass through breast milk and color a child's impression of those flavors. For instance, in a study Mennella led in 2001, published in the journal , babies ate carrot-flavored foods more readily when their mothers drank carrot juice during pregnancy or during breastfeeding, and in general, they appeared to like the flavor more than babies who hadn't been previously exposed to it in utero or through nursing.

These early experiences lay the foundation of our flavor preferences, and through repeated exposure to new foods, our palates expand. that, for children 4 months to 2 years old, getting just a taste of a vegetable each day for eight to 10 days can increase their acceptance of that food going forward. These flavor-related memories we forge in childhood leave a lasting impression on our preferences. However, the process of learning to like new foods can continue into adulthood.

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"We can all learn to like new foods," Mennella said in 2010 at the . "But it's these foods that we experience in our childhood that bring us to our past, and that is because of these emotionally potent, flavor-evoked memories." Flavor-related memories carry a lot of emotional weight, in part, due to the direct line of communication between smell receptors and centers for emotion and memory in the brain, . 

Besides this continuous learning process, our tastes may shift as we get older due to changes in our ability to taste and smell. In youth, taste bud cells regenerate every week or so, but with age, this regeneration process drastically slows, . And around middle age, in our 40s and 50s, the total number of taste buds in our mouths begins to decline and the remaining taste receptors become less sensitive, . 

Our sense of smell also declines with age, both on its own and in conjunction with age-related diseases such as and , . Similar to taste, this is due to a reduction in smell receptors and slowed rate of regeneration. Medications, such as antibiotics and blood pressure pills, can mess with taste perception, and radiation treatments and can undermine both the senses of taste and smell. Cigarette smoke and chemical pollutants also damage the taste and smell systems.

In some cases, these declines in taste and smell can deter people from eating altogether, since everything tastes bland; in other cases, individuals seek out foods with extreme flavor profiles, . In particular, the consumption of super sweet and salty foods tends to increase in old age, some studies suggest, but this trend doesn't show up consistently, according to the 2017 Critical Reviews in Food Science and Nutrition report. Other attributes of foods — such as their visual appearance, texture and how convenient they are to prepare and eat — may weigh just as heavily on older adults' dietary preferences.  

Originally published on Live Science.