Think about biting into a slice of lemon: its acidic sourness makes you wince. Now imagine a sip of double espresso: the bitter pang of caffeine is a wake-up call. Next imagine ripe mango, and the flood of sweetness when you sink your teeth into it. Now think about the salty hit from biting into the first crisp in a packet. Round it off with thoughts of a beefy, brothy stew.
"Taste is a powerful sense and can evoke vivid memories, just as the sense of smell can, but the primary function of our senses is far more basic: they help us to survive," says Dr Mary O'Connell, a senior lecturer at Dublin City University's school of biotechnology. "The taste system is the only one of our sensory systems that can completely regenerate if damaged. That gives us some indication of how important taste perception has been for our survival. "
While many factors affect our choice of food and how we perceive its taste, the chemical end of where food meets our sense of taste happens in raised bumps on our tongues and other parts of our mouths called papillae, where our taste buds lie waiting to be tickled. Suites of taste- sensing molecules called receptors lie in those papillae.
“Little taste receptors are clustered together in buds that include several different types of taste receptors,” says O’Connell. “Then nerves carry the signals about taste to the brain.”
Our tongues may harbour chemical mechanisms for tasting fat in foods too. In 2012 scientists in the US found a link in humans between versions of a gene called CD36 and the ability to detect fat in the mouth. “There seem to be genetic differences in how people perceive fat, with some being less sensitive to it than others and requiring more fat in their diet to reach satiety,” says O’Connell. “Studying the fundamentals of taste has important applications in obesity and diabetes research.”
Evolution of taste
O’Connell has studied the evolution of taste in hummingbirds and other vertebrates, or animals with a backbone: “All vertebrates have taste buds, with the exception of hagfish,” she says. “Taste perception has been around for more than 500 million years.”
Our ancestors had to seek out their food from the environment, and their perception of taste was important for survival and thriving. “For our ancestors, the ability to assess the caloric and nutritive value of food in the environment and to detect dangerous or poisonous foods (which would tend to be bitter or sour) were probably critical to survival, particularly as they moved from one ecological niche to another and encountered new potential sources of food,” says O’Connell.
"In our ancestral human population, those who had the ability to perceive dangerous compounds in their food could better avoid them, and those who could better assess the nutritive and caloric value of their food could get a good supply of energy. But, of course, if you had both of these abilities, then you would have had a distinct selective advantage and were most likely better-nourished and healthier, and as a result more likely to reproduce and pass on your set of genes for taste."
Before food can zing our taste receptors, its components have to go through a “taste pore” at the top of the taste bud, says Prof Dolores O’Riordan, director of University College Dublin’s Institute of Food and Health.
“In order for compounds from the food to pass into the taste pore, the compounds need to dissolve in the saliva,” she says. “Then, once a compound goes into the taste pore, you will get the various chemical interactions taking place that stimulate our perception of taste within the brain.”
O’Riordan and colleagues at UCD want to change the dynamics of entry into the taste pore and thereby to affect how foods taste. They work with foods that are being produced in a manner that reduces calories or salt or adds ingredients with potential health benefits from plants. The researchers want to understand how tweaking the formulation and “macrostructures” of these foods can help ensure the taste remains palatable.
“We are interested in changing the structure of the food so that it alters the dynamics of how compounds are released in the mouth, which in turn affects how the person eating the food perceives the taste,” says O’Riordan.
Side effects
One of the side effects of reducing the amount of fat in a food is that it loses its creaminess, and the “mouth-feel” is different, says O’Riordan. It can also change the dynamics of how the food breaks down and how compounds are released in the mouth.
“If you take fat out, then you have got more water-soluble, structure-forming elements in the food,” she says. “So the release of flavoured compounds from that structure will be very different, and it is normally faster. Often you get overwhelmed with a quick burst of taste, and that isn’t always desirable.”
Bitterness is generally undesirable too, and plant extracts can pack a punch on those receptors. Again, O’Riordan wants to keep them from swamping the taste buds. “Regardless of what the potential health benefits are, as a consumer you won’t want the food if the bitter taste is overriding,” she says.
Has the work affected her own experience of foods? “I would be more aware of textures and structures, and, particularly if I try a new product, I do take notice of how it breaks down,” she says. “That is the natural curiosity that comes with science.”
OTHER SENSES: WEIGHING UP FOODS
Have you ever noticed how food isn’t that appetising when your nose is bunged up from a cold? Or how the sight of a blemish or weird hue on food can put you right off it? Taste is just one sense we use to experience a food. What it looks and smells like are also important factors, notes UCD professor of food science Dolores O’Riordan.
“Colour affects our food choices; if something looks the wrong colour, like bread that is mouldy, we won’t be keen on choosing it. And if you artificially change the colour of a food, like turn an orange-flavoured jelly sweet green, it is confusing, because the anticipated and actual tastes don’t match.”
Smell is a major part of how we perceive the flavour of a food too.
“We smell compounds from food before we eat it, and when compounds get released from the food as we chew it. So foods taste different – and generally not as intense – when you have a cold, because your sense of smell isn’t picking up the flavour.”
