Photo: iStock
Photo: iStock

What we taste: The science behind yum and yucky

Matters of taste don't end with the tongue. The nose, too, plays a crucial role. That is, flavour is more than just taste: it's a fusion of multiple senses

Remember being taught about the tongue map in school? Sweetness is tasted on the tip of the tongue, salty and sour on the sides and bitter in the back. As it turns out, scientists now say all that is wrong. It’s actually all over the place—you can taste everything everywhere on your tongue in varying degrees. But then, can taste buds perceive only sweet, sour, salty and bitter?

Many would like to add to the list—pungent, astringent, bland or even plain yucky.

But there is actually just one more taste to add to the basic four. Umami.

That’s what a Japanese scientist Kikunae Ikeda proposed way back in 1908. It was formally recognised as late as 1985 by scientists.

Umami is a pleasant, brothy and meaty taste that is mouth-watering and long-lasting. It qualifies as a basic taste as scientists have proved that human and animal tongues have specialised receptors to perceive it.

While tomatoes and soy sauce are rich in umami components, the taste is also artificially added to food in the form of flavour enhancers such as monosodium glutamate, or MSG. Remember the controversy with Nestle’s Maggi noodles which allegedly had MSG and excess lead?

But matters of taste don’t end with the tongue. The nose, too, plays a crucial role. That means if you can’t smell a certain food item, you cannot fully fathom its flavour. That is, flavour is more than just taste, it’s a fusion of multiple senses.

It’s the memory of food, the sound of chewing, the smell wafting into the nose from inside the mouth as one chews and chomps (retronasal olfaction), and the aromas that the nostrils pick up from outside (orthonasal olfaction). All that goes to the brain, to construct a flavour.

Smell plays a crucial role in perceiving flavour, but surprisingly the presence of odour receptors in the brain was not known until 1991. That discovery of the organisation of the olfactory system won Columbia University scientists Richard Axel and Linda Buck a Nobel Prize for medicine in 2004.

The importance of smell also means that for those without retronasal olfaction ability, food can lose its charm. Barb Stuckey, innovation officer from Mattson and Co., a US-based food firm, told National Geographic magazine (December 2015) the story of a woman who lost her sense of smell after a car accident. Her sense of taste was fine but nothing tasted delicious anymore—the connection with the odour receptors in her brain had been severed.

She came to Stuckey for help to prove in court—in the case she had filed against the person who had hit her—that she was disabled.

Stuckey gave her pieces of a plain rice cake with reference compounds for all the basic tastes (sweet, sour, etc.) on top of each piece. All these compounds on the rice cake were non-volatile, and therefore could not be smelt. The woman took them to the arbitrators to show that’s how food tastes when you cannot smell—the food loses its flavour. She went on to win.

But science could come to the aid of someone like her one day. For instance, a new field of research known as neurogastronomy is focusing on altering flavour experiences by manipulating neurological signals rather than changing how food is cooked or what goes into it.

The possibilities are immense: bitter gourd could taste like sweet pumpkin, and, more importantly, food could be delicious again for cancer patients whose taste and smell receptors have been altered after chemotherapy.

Gordon M. Shepherd, professor in the neuroscience department at Yale University School of Medicine, who pioneered research on neurogastronomy, writes in his book, Neurogastronomy: How the brain creates flavor and why it matters, that “by combining brain studies with food studies, and drawing on the wisdom about flavour exchanged within families every time they eat together, neurogastronomy holds the promise of healthy eating on a new scientific basis".

“How can we hold informed opinions on food and nutrition, gourmet eating, fast food, and obesity," writes Shepherd, “if the role of smell is not recognised and understood for its dominant function in the perception of flavour, if flavour itself is not recognised for the dominant role it plays in our daily lives, and if the brain regions involved in cravings for food are not recognised to be the brain regions involved in cravings for drugs of abuse?"

That brain and neural connections play an important role in eating habits is also shown by what is known as ‘the buffet effect’. This means one stops eating when full when served a particular food, but keeps eating when someone brings another plate with a different food on it.

“Fast-food companies know this, and they sell a series of things in addition to the Big Mac," Shepherd told in an interview (November 2011).

“Thanksgiving is a good example of how with a dozen different dishes we just keep eating and eating. It’s a very real thing and people who are obese lack control over it."

So matters of taste are as much matters of the head, as they are of flavour.