Science of Seduction, Pheromones and the Food of Love

Chris - So Peter, take us through it. What actually is a smell?

Peter - A smell is a collection of molecules. Something like coffee has many different molecules that make up the smell. What the brain has to do is identify that specific combination of molecules that contribute to coffee odour, and distinguish it from the complex combination of molecules that gives you something like strawberry odour, and other odours.

Chris - So these molecules are drifting around in the air and go up your nose. What happens then?

Peter - They go up the nose and dissolve in the mucus up your nose. They then bond to receptor proteins. These are molecules which have some sort of complementary site that the odour molecules fit into.

Chris - Like a docking station.

Peter - Yes, that's right. What happens then is that they activate that cell in the nose which then depolarises, or changes its electrical potential, and that signal is then sent down a long process of the cell to the brain. Here it triggers changes of other cells in the brain.

Kat - So do you have a different receptor up your nose for every different kind of smell molecule? And are some people missing some types of smells?

Peter - That's a really good question. There are lots of these different receptors. Humans have about three hundred and fifty different types of functional receptor, but that doesn't mean that humans can only smell 350 different types of molecule. These receptors will all bind to a range of different molecules and so what the brain has to do is to look across the whole population of these receptors and identify the pattern of cellular activity.

Chris - It's a bit like light. The screen in front of me is making a range of different colours, but all the colour son the screen are produced by just three colours: red, green and blue. However, I'm not restricted to seeing just red, green or blue. Depending on how much of each of those is present, I see a different colour.

Peter - That's right, yes. If the brain was to look at a signal from any one receptor type, it wouldn't know which of five or six molecules was out there. By looking across a whole population of these receptors, then it can pick up this pattern.

Kat - How do humans compare in the grand scheme of smellers in the world? We all hear about how dogs have an amazing ability to smell. Where are we on the scale?

Peter - Quite poor really. There are animals that don't have a sense of smell like whales, for instance. But things like dogs or mice have much greater abilities to both detect both really faint smells and to distinguish lots of fine differences in the sense of smell that we would just think smelt the same.

Kat - Can you get humans that are super-smellers? Do you get people who have very special smelling ability?

Peter - That's a good question. People who have lost the use of other senses, such as by going blind, can sometimes have very acute hearing or sense of smell. As far as people with the whole range of sensory abilities are concerned, then certainly people who have trained as perfumers or wine tasters or tea tasters, have much greater ability to certainly talk about smells and to put labels on them.

Chris - Is it because they've spent their life doing that and they've got better at doing it, or is it that they've gone into those jobs because they have such a superior sense of smell?

Peter - That's a good questions and I don't really know the answer. It could work either way. Certainly learning and memory are very important for actually distinguishing smells and the way the way the whole of the smell sense works.

Chris - An intriguing thing is that smell seems to be very powerfully linked to memory. So one whiff of one particular odour, such as coffee, sets off a whole train of thoughts. Why is that?

Peter - Smell is quite an ancient sense. It's been around for many millions of years. The big brains we've got at the moment, most of the cortical area developed from areas of more primitive brains devoted to the sense of smell. And so the sense of smell is quite primitive in brain terms and feeds into lots of the emotional areas much more directly than some of our other senses.

Chris - But it's also one of the most underrated of our senses. If you do a survey of people and ask them which of their senses they'd give up if they had to, an enormous number of people choose smell.

Peter - But a lot of people do lose their sense of smell for periods of time when they have a cold or an allergy. If the epithelium swells up and blocks air from flowing through it, you lose your sense of smell. What you will notice is that their sense of smell will also be diminished.

Kat - Is that true of smokers as well?

Peter - It's certainly diminished in smokers, but normally they wouldn't lose it altogether.

Kat - It just becomes numbed a bit.

Peter - Yes.

Chris - It's nearly Valentine's Day and we did say that we'd talk a bit about the science of seduction. So let's move on to another family of molecules: pheromones. These are of dubious significance in humans, aren't they?

Peter - There's more and more evidence accumulating at a low level that humans are producing chemicals which can have an effect on other humans, either in terms of their reproductive state or their menstrual cycle, or in terms of their behaviour or mood. It's quite difficult to actually quantify the effect because it's normally quite a subtle effect. Whether it plays that much of a role in today's society is very much open to question.

Chris - Do you know why it is that women living together end up with their menstrual cycles synchronising? Do you have nay feel for why that happens? Do you know what the molecule is that's driving that?

Peter - There are some molecules which Martha McClintock, who's a researcher in the States, has found in the underarm secretions of women which can have an effect on the menstrual cycle length of other women. If she takes these odours from under the arm just before ovulation, then they can shorten the length of the menstrual cycles of other women, and if these odours are taken from under the arm after ovulation, then they tend to lengthen the cycle. This seems to act to co-ordinate the cycles.

Chris - One quick question then. That's all very nice, but what's the point of it?

Peter - You can make up all sorts of stories but no-one really knows and I don't have any great idea as to why this might be important now.

Kat - If we discover a human pheromone, is it going to smell like sweaty armpits?

Peter - One of the main roles of the underarm secretions is probably pheromonal communication. We have lots of glands that produce particularly smelly compounds under our arms. All these hairs under our arms are acting as wicks, which draw out the secretions and release them into the air. So it might well smell sweaty and horrible, but some people might find it attractive. Maybe if they don't have a sense of smell they might!

Kat - What about the role of smell in mother and baby bonding? I remember reading a paper about mice in which a part of their brain involved with smell gets bigger when a female mouse gets pregnant. Is that the same case in humans?

Peter - I don't know about the brain getting bigger, but certainly smell does play a role. If you take newborn babies, then just the day after they're born you can present them on one side of their head with breast pads from their mother, and on the other side of their head breast pads from another mother in the same hospital. They turn their head towards the breast pad of their mother.

Chris - So they've almost certainly learnt to recognise their mother from a very early age?

Peter - That's right. And they may well be doing this either from compounds in the mother's milk or in the uterus as they've been developing.

Chris - How does the nose get used to smells, because that's a characteristic of the nervous system and the way it works, isn't it?

Peter - Yes, and it's particularly important for the sense of smell because there are molecules given off all around us by the carpets, furniture, the walls, other people, and yet we have to get used to them so that we can respond to new odours that come in from the cup of coffee or the strawberries. It's actually some connections between the nerve cells in our brain between the first and second stages of processing which actually get reduced in their efficiency, and they therefore don't transmit the signal as efficiently after they've been stimulated for a certain amount of time.

Kat - So Pete, what's the best thing to serve on Valentine's Day?

Pete - Goodness me that's a difficult question! The best thing to serve on Valentine's Day is whatever your partner likes the most.

Kat - But what makes things taste good to people?

Pete - That's largely a matter of individual experiences. We all have these senses in our noses and in our mouths that tell us what we like, and as you've already noted, flavour and taste are very strongly related to memory. So things we like tend to be things associated with really good memories. So something that we really liked or enjoyed in our childhood tend to give us something that we like more. It's not quite as simple as that, but that's the basic idea.

Chris - Let's just get into the nuts and bolts of how taste works. First of all, when I put something in my mouth, what's actually happening at a neurological level for me to be able to taste it?

Pete - That's a question no-one knows the answer to. Where do you think we taste something?

Chris - Most people believe that taste is all down to the tongue, but that's not true is it?

Pete - Not at all. Our tongue can detect five different sensations: salt, sour, sweet, bitter and umami. Those are the five basic senses we've got in our mouth and they really are pretty crude. They can't distinguish much at all. Then they give signals to our brain. When you're eating and chewing, some of the molecules from your mouth go up the back of your nose and you breathe them out. It's mostly on the breathing out that you detect them. You nose has hundreds of different sensors in it, and it can recognise millions of different molecules. The nose is where most of the taste information going to your brain comes from.

Chris - So there is truth in the claim that if you hold your nose when you're eating something you don't like, such as brussel sprouts, you should be able to abolish the unpleasant taste.

Pete - You will change it. Of course, with brussel sprouts you've got that sour taste if they're over cooked. That acidic taste will still be in the mouth because it's coming from the tongue, but it's not just your tongue and your nose. You also taste with your eyes and hands and ears. If you think about it, when you're eating something you see it first. The very first thing you do is look at and say 'What am I expecting?' Expectation is set up. If you don't see it or if you see something unexpected, your brain won't work right. A good example of that is if you give professional wine tasters a red wine to taste and a white wine to taste, they will tell you that they're different. But if the red wine is just the same white wine with food colouring added, they still won't recognise them as the same wine because they expect something different.

Kat - A lot of people for Valentine's Day will be giving each other chocolates. Why does chocolate taste so good?

Pete - Of course chocolate has a combination of two of the things that we really like: that's sugar and fat. That's probably built into us from evolutionary times before supermarkets, when if you could get sugar you were able to run around and catch food, or if you ate fat you could lay it down and store it for the future. Those were really important things of which people wanted to get as much as they could. Put them together in something and that makes it really good. Sugar and fat alone is one reason for liking it.

Chris - Have people built systems so that people can work out what the taste profile of a substance is and then predict what it will go well with?

Pete - There are lots of people out there in various research institutions and food companies who are very concerned with knowing what the main flavour molecules are in any specific product. So far in coffee, over a thousand different molecules have been identified. Even if you know what they all are, knowing which the important ones are is quite difficult because you can feed different people different combinations of them and ask them what they taste, but different people will give you different answers. But yes, people have done research on it, and once you've identified the key molecules then you can look at profiles and starts to work out whether one food will go well with something else.

Kat - So what are particularly romantic tastes? Are there certain tastes that are more sensual?

Pete - I think the sensual tastes are those which coat the mouth and give a nice warm feeling. Chocolate is great for that as it cools the mouth when you put it in because it melts the chocolate. Other ones are things that are very crisp, clean and clean the palate, such as caviar and champagne. These are all things that people tend to like as very special foods. Rare and expensive foods also tend to be sensual because you fell as though you're being treated to something important.

Chris - Do you think there'll be a day when we might be able to go to the supermarket and buy foods depending on their profile? Someone will say that you can mix two items because the taste profile of one thing will map onto the taste profile of another and you can get a really bizarre combination that tastes great. This is the basis of molecular gastronomy isn't it?

Pete - It's something that some people call molecular gastronomy, yes. Some chefs are doing exactly that. I think to do it in a supermarket would be very boring!

Chris - I think it'd be really interesting!!

Why things taste disgusting together is largely because they have nothing in chemically common.

You have your nose and tongue largely set up with receptors to respond to food.

However, when certain combinations of receptors are set of at the same time, such as with milk and orange juice, the receptors that are set off act in a similar way to a jarring musical chord: it tastes completely wrong.

On the other hand, if you play something smooth that goes together, they go together pretty well. So foods that have molecules in common tend to taste very good together!

I think that's highly likely. Dogs have a fantastic sense of smell. They have an enormous amount of surface area in their nose that's taken up with cells that respond to smells. That's why dogs are used for tracking people's odours, finding explosives at airports and for hunting out victims of avalanches, which they can sniff out under the snow. So dogs will be much more sensitive to the changes in the odour of someone due to the hormonal changes during the menstrual cycle than humans will.

There's possibly some basis to this. There has been some evidence from quite special populations of Hutterites in the United States that the family trees aren't decided by random assortments of pairings. Instead it seems that they are actually biased by genetic identity, and this could actually be due to the sense of smell.