A feast for the senses

Diggy has inherited his sense of smell (or, as science magazine readers like to say, olfaction) from wolf ancestors whose pups we stole and domesticated tens of thousands of years ago.

Suppose I told you that you can actually outdo Diggy at olfaction, would you believe me? Probably not. Would it further surprise you to be told that if you drink your tea in a red-coloured mug, it will taste sweeter than in a white mug? And would you go "no way" if I said that even if you hate airline food, using a saline nasal spray before eating and wearing good noise-cancelling headphones will improve the taste of the food?

Welcome to the Science of Flavour.

Let's start with an equation.
Flavour = (Taste + Smell + Sound + Sensation + Sight) x Nostalgia
(Nerd alert: please assume that some weights apply to each one of those variables.)

Let's start with taste: it's a word we tend to use interchangeably with flavour, but in reality, it represents only 10-15% of the perception of flavour. Taste is detected by thousands of taste buds on your tongue. Some people, typically men, have fewer taste buds. (Men also detect fewer colour shades, something my wife gets exasperated about as she tries to explain to me the difference between 'M.S. Blue' — a sari colour identified with M.S. Subbulakshmi — and cerulean.)

Some others have more taste buds: the ones that do are called 'Supertasters'. Some 35% of women are supertasters, but only 15% of men are. When women say that men don't have good taste, it's quite literally true.

The notion that different parts of the tongue have taste buds dedicated to detecting different tastes is a myth. Many high school textbooks still feature a map of the tongue with sweet-, salt-, bitter-, sour- and umami-detecting regions marked out. But that's not how it works.

A single taste bud is made up of 50 to 100 taste cells, each of which can respond to different tastes. When the cells detect and respond to sugars, they trigger the brain to give you a sweet sensation. When they detect protons (hydrogen ions), you get a sour taste. When they detect alkaloids, you get bitter. When they detect sodium, you get salty. And when they detect glutamates (for instance, from the internet's favourite bogeyman, MSG), you get the savoury umami sensation. Taste buds can detect all tastes, and depending on what you eat, your brain will make sense of these signals – and, in some cases, highlight or mute some tastes.

Good chefs have always known this intuitively. Adding a bit of sugar to your dal or chicken curry amplifies other tastes. Sweetness suggests a lot of calories, and your brain wants you to eat more of it. Likewise, adding a pinch of salt to your desserts amplifies sweetness. And umami amplifies all other tastes. At the same time, sweet can also suppress sour, and sour can suppress sweet. For instance, when you eat gooseberry (amla), it's very sour because it has a lot of citric acid. Acids donate protons, and protons trigger the sour taste buds, so when you see an ingredient with acid in its name – lactic acid in curd, tartaric acid in tamarind – it's going to be sour. Citric acid in amla triggers the sour taste cells, but the same molecule also triggers sweet, even if not as well as sour. When you eat amla and then drink water, the water dilutes the acid, so the sour receptors turn off, while the sweet detectors are still working, so your brain switches to telling you that it's sweet. This is why drinking water after eating things that contain citric acid (like gooseberries or lemons) will give you a sweet sensation.

On a side note, flavour experiences don't happen in the mouth. The chemistry happens there. The experience happens in the brain. And our brains are easily tricked.

That's about taste, which accounts for only 10-15% of the perception of flavour. The big boss of flavour is aroma. Dogs and most other mammals are fantastic at smelling things from the outside in. In fact, a dog's long snout is designed to smell the smallest trace of certain molecules. But here's the catch: dogs don't smell very well as they breathe out. You might wonder why that matters. Isn't the only goal of smelling to detect stuff on the outside?

Surprisingly, the answer is 'No'. And the reason for that is fire.

Early human beings who discovered fire also discovered that applying heat to food makes it easier to digest. Cooking breaks down large molecules into small, easily digestible ones. That allowed humans to evolve larger brains because we didn't have to use most of our energy on just digesting raw food, as other animals do.

So cooking is what makes us uniquely human. And a side effect of that is the evolution of sensors inside our nose purely for detecting aromas in food. Dogs don't care about detecting cinnamaldehyde from cinnamon in your biryani. But when you chew food, spice molecules get released and travel to the back of your mouth, and as you breathe out, they get detected by your nose. And remember: cooked food has smaller-sized molecules. Smaller molecules are volatile and can be detected by your nose.

This is called retronasal olfaction. And no animal does this better than humans because we cook our food. Our noses can detect more than 10,000 different molecules, unless those receptors were damaged by coronavirus: many people lost their sense of smell for long after they had COVID-19. Again, women on average smell better than men do, and I don't just mean body odour. Humans can tell the difference between saunf and cumin, between green cardamom and black cardamom, between saffron and turmeric – and not because we smell it from the outside in, but from the inside out.

This is why when you have a cold, your olfactory receptors are blocked and food tastes terrible. Smell is 80% of flavour. This is also why when you're sick, your mother or grandmother likely made you salty and sour food (like rasam or khichdi) or high on umami (like chicken or mutton soup), because only your tongue works when your nose is blocked.

And that brings us to the universally terrible experience of airline food. The pressure in an airline cabin is kept lower than atmospheric pressure to reduce the stress on the structure of the plane. And your nose and taste buds don't work well at lower pressures. Second, in-cabin humidity levels are kept low to prevent fungal and bacterial growth: planes carry thousands of people every day, so keeping the humidity low reduces microbial contamination. And in low humidity, your nose doesn't work very well.

Which is why it helps to squirt a bit of a saline spray into your nose before you eat: a wet nose is better at detecting food aromas.

So, what do noise-cancelling headphones have to do with food, you may wonder. The third component of flavour is sound. Ancient humans used the crunching sound of biting into an apple as an indicator of its freshness. This is why we like crisp, crunchy textures. Soft and mushy sometimes signal rotting, while crunchy suggests low moisture and thus freshness. In an airplane, the engine white noise blankets your soundscape, so you are unable to hear the sound of yourself munching on food. Wearing noise-cancelling headphones will help you enjoy your food better. Business class cabins give passengers noise-cancelling headphones, which is why people will say business class food is so much better, but it's usually the same food cooked in the same kitchen. So, the next time you salivate upon hearing a sizzling sound or sigh at the satisfying crunch of a potato chip, thank your ears for that flavour experience.

And then we have the trigeminal nervous system, which has millions of nerve receptors inside your mouth to detect heat, cold, sharpness, and so on. This is to prevent you from eating things that could damage you physically. Bitter taste buds detect alkaloids, which can be poisonous, but taste buds won't tell you if your food is boiling hot. Nerve receptors do that. However, the TRPV1 receptor, which detects high temperature, is tricked by the capsaicin molecule in chillies. This is why chillies give the illusion of heat and also why we characterise the sensation as "hot" even if it has nothing to do with temperature. Likewise, menthol in mint tricks the TRPM8 cold-detection receptor, so you get that chill sensation when you eat mint or a mouth freshener that contains menthol.

There are also receptors to detect food texture. We can detect soft, slimy, doughy, rough, nutty, crunchy or crisp. Texture is a critical element of flavour. Most people find a mix of textures more interesting than single-texture food. We tend to like contrasts. For example, when it's raining, and everything is cold, wet and soggy, hot, crisp bajjis and pakodas offer contrasting textures to the weather.

And then we have sight. Our brains have, over the years, learned to associate certain colours with certain tastes. Yellow signals sour (think citrus fruits). Green signals bitter (leafy vegetables) and red suggests sweet (ripe fruits, which tend to be red because they lose green chlorophyll and add red carotenoids as they ripen). So, when you drink tea from a red mug, it feels sweeter than it actually is. If you use a red mug, you can make do with smaller amounts of sugar, and that's good for you.

Experiments have established that we strongly associate flavours with colours. Between strawberry-flavoured candy and lemon-flavoured candy, our brains expect the former to be red and the latter to be yellow. If you switch the colours (to a yellow-coloured strawberry-flavoured candy and vice versa), your brain gets confused.

And finally, we have this mysterious 'nostalgia' factor, which seems to multiply all the other components of flavour. Because flavour is a multi-dimensional experience, we use the limbic system to store flavour memories. The nose is directly connected to the limbic system in our brains, which processes emotion and memory. This is why when you eat certain foods, they can virtually transport you to a specific time and place – say, your grandmother's kitchen. If what you eat evokes those memories, the experience of flavour is hugely amplified. This in essence is how "comfort food" works. The dish might be a simple dal-roti, or a fish curry and rice, but if it evokes nostalgia, you'll find it delicious.

This explains why it's hard to enjoy street food in a five-star, fine-dining setting. The chef may be talented and make the most multitextured and richly flavoured chaat, but if it does not evoke memories of you enjoying it in a streetside setting, it won't have the same flavour impact.

This is also why people find it hard to eat dishes they grew up eating at home elsewhere. North Indians find the chapati-dal in Chennai or Kochi to not taste "good": it tastes fine, but it does not evoke nostalgia. Likewise, South Indians react viscerally to a 'Schezwan dosa' in Mumbai: nostalgia for a simple, home-cooked dosa gets in the way of their being able to appreciate novelty. It is difficult to appreciate culinary novelty in dishes we grew up eating.

Understanding this should give you a deeper insight: it's nearly impossible to be objective about reviewing food. When people leave negative comments about a restaurant, they might say "the food was terrible", but more often than not, they are actually saying, "the food was unfamiliar; it did not evoke nostalgia".

So be mindful and appreciate flavours, even the ones you don't like. You have the ability to enjoy food far more than Diggy ever will.

See also:

Passing the sniff test
The NEW Science of Nutrition