There is much more to flavour perception than just the sum of all our senses. Our brains respond much more to changes in which molecules are in the nose and mouth than they do to what is actually there.
By Jeremy Ryland
For example, if you chew a piece of gum, the flavour will disappear after a few minutes, as your brain gets "bored" by the aroma in the nose – but there is virtually no reduction in the number of flavour molecules in the nose.
However, if you simply change the input from your tongue, by, for example, taking a sip of sweetened water, the full flavour will be instantly restored. The area of flavour perception is one of the most exciting areas for scientific research - it holds out the promise of helping us find ever better ways to produce truly wonderful food.
Did you know, that what we call flavour is actually a mix of taste and smell?
Flavour and taste are often confused. We smell and taste our food simultaneously and what we call flavour is really a combination of the two senses.
Our tongues have five different types of sensors called taste buds that detect sweet, sour, salt, bitter and umami – the taste of glutamate. (This last one has only recently been recognised as a separate taste sensation, the taste of monosodium glutamate, MSG - found in tomatoes, parmesan cheese, soy sauce, etc - and there is also some research that we have a sixth taste, fats and oils).
Our sense of taste is quite crude: we each have between 2000-5000 taste buds which can distinguish the five tastes.
We are born with a liking for sweet and salty. We need sugar as a source of energy, so we like sweet-tasting foods; if all we taste is sweetness we will eat the food.
We need salt to survive; salt has many essential roles – affecting the electrical conductivity through the body, governing how our hearts beat, how signals are transmitted along our nerves and in our brains, and controlling many other processes. We like salty food – and a little is good for us.
On the other hand, Bitter and Sour are warnings of potentially bad food. Sourness often accompanies foods as they are going off due to bacterial action, think of sour milk, so recognising sourness helps us decide not to eat some foods.
Most poisonous berries taste bitter, so we need to recognise and dislike bitter foods. If we eat a bitter food we will not only spit it out, but follow that up by vomiting to get rid of any trace that may accidentally have got into our digestive systems. Eating bitter foods, like coffee, has to be learned.
Umami is the fifth taste – that of glutamate, the meaty, Vegemite flavour of protein needed for repair and growth. Glutamic acid is one of the essential amino acids that form the building blocks of proteins – so recognising foods that provide it is important. It is; therefore, not surprising that our "Umami" taste receptors are particularly attuned to the sodium salt of glutamic acid (monosodium glutamate).
Taste is a survival instinct hard-wired into our brains. When we put food in our mouths, we need to decide whether to eat it or spit it out – this can be a life or death decision and needs to be made quickly. Give a baby something sweet – they will smile. Give them something sour or bitter, they will grimace and spit.
But taste is just the last line of defence – we use all our other senses first – and these affect how we react to different tastes. First, we look at the food: is it the "right" colour? Next, we touch it: is it firm or soft? At the same time, we listen to how it sounds when we break it: is it crisp or soggy? Then we sniff it: are there any unpleasant odours? All these impressions tell us what to expect when we put food in our mouths.
If we are eating berries, we will be looking for sweetness, combined with "fresh" and "tangy" aromas; if it is meat we will be looking for saltiness without any sour "off" aroma. The type of food and our memories of similar foods tell us the key aromas and tastes to look for in the "flavour". All this complex information is processed by our brains and interpreted as the "flavour" and is tasted in our mouths.
Whilst our taste buds can only detect five tastes, our noses can detect many thousands of smells. In fact 80 per cent of what we call “flavour” comes from smell.
The organ we use to detect aromas is the olfactory bulb, located at the back of our noses near the middle of our heads. Inside the olfactory bulb, we have at least 700 different types of sensors and can use them to distinguish many millions of different molecules.
As an aside – I have a beagle. Beagles love food. And beagles have one of the most sensitive noses on the planet. A beagles nose has over 225 million scent receptors, compared to five million or so in a human nose. A beagle's olfactory bulb is 40 times larger than a humans and their sense of smell is 10 000 times greater than a humans – capable of detecting a teaspoon of sugar diluted in two Olympic-sized swimming pools, which may not be very useful – but my beagle can smell Vegemite being spread on toast or a chicken leg from over half-a-kilometre away. This makes beagles very useful at airports and borders to detect fruit, vegetables and illegal contraband.
So whilst we can only taste five different flavours we can smell thousands of different aromas. It is not surprising that wine tasters sniff the wines first – their noses are attuned to look for a range of aromas that give clues to the grape variety and region, etc.
And whilst taste is “hardwired”, our sense of smell is largely learned and is affected by many things. As we eat, the volatile molecules released from food are carried in our breath to the flavour detectors. We need air to detect flavour, which is why it's hard to taste food if you hold your nose or have a heavy cold which stops the air flow. Remember when your mother told you to hold your nose if you didn’t like the taste of something!
Flavour is affected by air pressure (due to the blood capillaries in the nose expanding and “blocking” your nose) – food has less flavour at high altitude – which is why airline food often tastes bland!
Indeed, in space, astronauts lose their sense of taste and smell.
Flavour is affected by temperature: cold foods taste different when hot, and vice versa. Frozen foods have very little flavour whilst hot foods have stronger flavours. Ice cream gets sweeter as it gets warmer – and because it is eaten cold, often contains a lot of sweetener and extra flavours.
Flavour is affected by colour and vision. It is often difficult to correctly identify foods in the dark – although some flavours may be intensified by a lack of visual perception. Give someone lime juice coloured orange and they will tell you that they are drinking orange juice.
Flavour is affected by texture. Increase the thickness of a cheese sauce and it will taste less cheesy – even if it's exactly the same strength of flavour.
The tongue also enhances the eating experience, sending information to the brain along the trigeminal nerve about whether food is hot or cold, spicy or cooling.
Flavour is affected by our Hearing – crunchy crisps taste fresher… and too much noise makes food taste different. Flavour is affected by Touch – fresh, ripe, hard. Flavour is even affected by the people who you are with – food tastes better when you are relaxed and with people you like! You can read more about this phenomenon in my article on food & mood - How does food affect your mood?
Flavour is also affected by what the food looks like, in particular its colour. Next week, I will discuss the importance of colour in our food.