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Science, Maths & Technology
  • Audio
  • 15 mins
  • Level 1: Introductory

Ever Wondered About... Sugar?

Updated Wednesday 17th February 2016

It might help the medicine go down - indeed, in the most delightful way - but that's only one of the amazing qualities of the sweet stuff.

A person holds a sugar cube in their mouth Creative commons image Icon Com Salud under Creative Commons BY 4.0 license Copyright The Open University


Paul Merrett: Hi, I'm Paul Merrett, chef and presenter of the BBC Open University TV Series Ever Wondered About Food?- a programme that explores the undeniable benefits of mixing a dollop of science in with all the other ingredients involved in your recipes. And, throughout the series, I've had the support and advice of David Shuker of the Open University, making sure that what I say is scientifically right; and I hope it is most of the time, David.

One of the things that I've been fascinated to learn is that everything that we eat, - or nearly everything that we eat - has a flavour, and the flavour of the things we eat is received by something called the "olfactory bulb" at the back of the nose. But there are also tastes, and there are only five tastes: there's sweet; there's sour; there's umami, which is a strange one; and there's bitter; and there's salty.

Now we're dealing with sweet - we've just done a programme about sugar - and what I'm wondering is, if we're advised to cut down on our sugar, why are we so pre-programmed to enjoy it? Why do we seek sugar in foods?

David Shuker: Well, of course, the thing about sugar is that it's a good form of energy and I think, when we were hunter gatherers, finding sources of sugar was very quickly recognised as being very efficient. So, for example, if you found a natural bees' nest, a natural honeycomb, then of course that was a source of quite concentrated sugar, and I think we realised that, if you could get sugar then that was an efficient form of energy and not only that, of course, but it tasted nice.

And so I think, once people had discovered sources of sugar - honey originally - but gradually people discovered sugar cane, and then sugar beet, then it began to be much more widely used as flavouring. And also in processed foods it's a cheap way of bulking up foods.

Paul Merrett: It's something that has a huge role to play to the cook, obviously in the sweet section, but not only the sweet section, as we demonstrated on the programme. I mean, sugar can turn up in savoury-style dishes as well. We've done a very quickly cooked piece of chicken which has been marinated in a ginger syrup which has sugar in it, but we've also made with that a mango relish which uses raw sugar in it, but it also has the sugar in the mango as well, we used a very ripe mango for it. It balances very well with savoury things doesn't it?

David Shuker: Well that's right, and I think you mentioned the taste receptors, and of course this is a fascinating area, and something which interestingly we're only still beginning to discover how that works. You mentioned, for example, umami. I'll come back to the relation in a moment, but umami is an interesting one because that was originally identified as the fifth flavour by a Japanese researcher, about almost hundred years ago now, and he proposed that there was this taste, which I think the Japanese word umami is kind of meatiness or savoury or something like that. For a long time it was not considered to be a true basic taste, a flavour. But actually, a few years ago, it was discovered that there is in fact a specific receptor in human taste organs that actually responds to glutamic acid - the famous monosodium glutamate which is often added - and of course mushrooms are very rich in it and I remember in the mushrooms programme we mentioned this: why you add mushrooms, because it enhances the flavour. And the reason is because glutamic acid is specifically detected by the taste receptors, as it gives a signal, and somehow that signal gets transmitted to the brain, and the brain says "Um, that tastes nice!"

Now we still don't know how that works, and it's all into the sort of psychology of why things taste good, but underpinning that are these very specific sort of molecular receptors and food molecules that actually seem to do the job.

Paul Merrett: Why do you think that sweetness goes so well with savoury flavours? I mean why the two work so well together - if you think of sweet and sour, or you think of pickles and chutneys with cheese, there seems to be some sort of connection between… the mouth seems to enjoy those two flavours working at once?

David Shuker: Yes, I think it's interesting, isn't it? Because, I mean, I don't know how true it is, and maybe you as a chef can help me with this, and that is, I've always felt that things which are sweet have a certain fullness of flavour in the mouth, as opposed to things which are savoury, which often can taste thin. I mean if you think, for example, vinegar - what vinegar tastes like. It's got a very strong taste but the texture of it is quite, sort of, thin and watery.

Something which is sweet like, for example, honey or even a dilute solution, tastes somehow rich. And I think it's the combination of those that come together, you get the sort of famous sweet and sour combinations.

I mean in your relish recipe it is fascinating how it starts off - and I have to say I did have a shot at this recipe, and it really is good, fascinating to see how the flavour changed during the process. And so when you've mixed everything together (and it was all fairly liquid) and I would taste it, as good cooks do, I think - taste every so often just to make sure it's going okay - there was this interesting transformation between obviously detecting the vinegar and detecting the sweetness.

But as the whole thing cooked - and my guess is that, as more flavour molecules got released into the whole mixture, and the whole texture changed - it suddenly sort of merged into this rather subtle thing. Now I guess that's what a lot of cooking is about but I was astonished how that worked and I don't understand - I don't think anybody understands - how that all works. But certainly at its base it seems to be a delicate combination of sweetness and sourness working together to give you something, and of course the trick is getting that right.

I'll tell you another thing which I've found over the years - I think I've picked this up from when I was living in France - if I'm making a sort of gravy, a sauce, a meat sauce, I'm always amazed how a little bit of Madeira, which is a fortified wine which is actually fairly sweet, suddenly changes not only the flavour but the sort of feel of the whole thing. So you pop a bit of that in, and it makes all the difference.

Paul Merrett: …and it enriches the whole thing.

David Shuker: And I think the sugar's in there, you know.

Paul Merrett: I think you're right, and I have a theory that when coming up with dishes, you have to think about where those flavours happen in the mouth.

David Shuker: Yes.

Paul Merrett: As you were saying about vinegar, if you had to describe a flavour, to describe a flavour as thin and watery, I know exactly what you mean by vinegar, and it's true, and I do think that that does play a part. When I'm thinking of dishes, it's thinking whether I could balance - for me chocolate and fruit isn't a great.

David Shuker: No.

Paul Merrett: Chocolate and fresh fruit isn't a great connection because I think that they happen in different parts of the mouth, and the flavours of fruit are very springy and short lived and they're on a quick assault, whereas chocolate sort of lies on the mouth, and it has a far more longer lasting sort of coating effect in the mouth, and so I think that's really true. But of course sugar in the relish recipe we made, whilst sugar, both the sugar we added and the natural sugar, is essential to the flavour of the dish, it's also acting as a preservative. It's also giving life to the dish, which I guess in itself develops the flavours, because it's allowed to develop because the sugar's allowing it that longer shelf life.

David Shuker: Well that's right, I mean we often think of sugar primarily in terms of its flavour, but of course in terms of the relish, and more generally in terms of jams and things like that, sugar is a preservative. And the reason it's a preservative is that bacteria don't like to grow and in fact they can't grow in a very strong sugar solution. It disrupts the cells, and so sugar is a very effective preservative. So actually it's a dual role: one which is as a strong flavouring but the other thing is very practical, it's almost the sort of sweet equivalent of salt - I mean salt is used for preserving. It does that preserving in a particular way; sugar does the same thing.

Paul Merrett: Would the vinegar that we added have changed the pH value to such an extent that bacteria wouldn't have grown anyway because of the vinegar, or is it really the sugar that's doing that job?

David Shuker: I think in the case of the relish it's really the sugar, although in fact of course vinegar is another preservative, and the pH of vinegar is the thing which is doing the job; the pH of vinegar is not compatible with bacteria, they don't like living in that acidic pH. But the interesting thing about vinegar in this particular recipe is, I think there is a chemical reaction going on between the acidity of the vinegar and the sugar.

Now we talk about "sugar" but we should be saying "sugars", because the sugar which you add, which in that recipe was demerara sugar, is sucrose (that's the chemical name for it). But I don't know what the sugars are in mango, because they're probably a slightly different form of sugar. And what the acidity does is to actually partially break down those sugars - because one of the sweetest sugars is glucose - but in the case of sugar, sucrose, that glucose is locked up with another sugar molecule, and what happens is, when you treat it with a bit of acid, it partially breaks it down so you release that sweetness.

So, again you see this is an example of the multiple roles: you've got the taste of the vinegar, but you've also got the acidity of it which is subtly affecting the way that the sweetness is tasting.
Paul Merrett: Yes, purely on a "what's good for you" level - and I'm just thinking about the different sugars you've just mentioned: sucrose and glucose, and lactose of course, and fructose - is there one that's better for you, or are they all really doing the same thing?

David Shuker: Well in a sense they're all doing the same thing.

Paul Merrett: I mean is an apple better than a Mars bar?

David Shuker: I think I wouldn't hesitate to say that an apple probably is better than a Mars bar.

Paul Merrett: Shame - what a shame!

David Shuker: But my favourite source of sugar as a kid was, in fact, Mars bars. I used to love them, so I mean I'm not against Mars bars. But it is interesting, because there is some evidence that certain forms of sugar are handled by the body in a way which puts less strain on the body. And the problem is, you see, with sugar - particularly glucose, glucose is the sugar that the body has to actually handle and insulin, which is released by the pancreas, is the way that the body controls the blood sugar level, and maintaining a blood sugar level is something the body needs to do.

Now it turns out when you're young, for example, your pancreas can produce enough insulin rapidly enough to maintain the blood sugar levels, so kids who eat lots of sugar in the form of chocolate bars or fizzy drinks, or whatever, can much more readily handle that than adults. And one of the problems that we have at the moment is an increase in rates of adult diabetes, and it's fairly clear, or beginning to be clear, that some of that risk of developing adult diabetes is probably related to the large amount of refined sugar that we eat.
Now it turns out that fructose, which is essentially fruit sugar, is handled in a way by the body which doesn't put so much strain on the pancreas, and so there is some evidence - and in fact I have to say that at home we have very little white sugar, or at least cane or beet sugar - we tend to use fructose. Now it's more expensive, and interestingly - spoonful for spoonful it's actually quite a lot sweeter, so you can use less of it.

Paul Merrett: Really? Is there any cooking role that it can't carry out?

David Shuker: I don't think so. It's got a slightly different kind of sweetness, but in all the recipes that we normally use sugar in, we use fructose, and as long as you remember not… because if you add too much, it's too sweet, so you just back off a bit on the fructose, and it actually tastes as good. So I mean we read some articles which suggested that fructose was better for you than eating sucrose, which is the normal sugar that we have and, well it seems to be.

Paul Merrett: Made the change - that's a good idea.

David Shuker: Yes.

Paul Merrett: One of the amazing things I think about sugar is the way it affects temperature. If you add sugar to water and boil it you raise the temperature of the boiling water.

David Shuker: Yes, that's right.

Paul Merrett: If you add it to, for instance, a sorbet, it has to have sugar in order to be malleable at that very low temperature, so if you add it to water and then freeze it you lower the temperature of the freezing.

David Shuker: That's right.

Paul Merrett: What is this little magic crystal actually doing in order to govern those temperatures?

David Shuker: Well it's a very interesting thing, actually and it's one of those nice bits of science that I think we understand fairly clearly now: pure water, which is made up of billions of molecules of H20, will either boil at a hundred degrees centigrade or freeze at zero centigrade. If you add sugar to pure water the effect is you either raise the boiling point or you lower the freezing point. Now what's happening is that, effectively, by adding sugar to the water, you're separating the water molecules. But the sugar molecules can actually absorb heat, so what happens is, if you have enough sugar in the solution, then those sugar molecules absorb heat, which means that the water isn't getting heated, and that's part of the reason why a concentrated sugar solution boils at a higher temperature. Now that's got certain advantages, of course, because that means you can cook things at a slightly higher temperature.

Paul Merrett: Yes, and that's exactly what I was going to say. I mean at the real top end of boiling sugar, I've worked with - they would call themselves chefs, but these guys aren't chefs, they're far beyond that, they're real craftsmen that treat sugar like glass. I mean they blow sugar into the most amazing shapes, and pull the sugar, and really it's an art form and it's incredible. But, sort of bringing that back to the sort of cook's level, working with caramels is really good fun. It adds a completely different flavour and I think almost de-sweetens the sugar slightly as soon as you've gone into a sort of caramel stage. It's not as sweet to the palette, certainly; it's not as sweet as it is in sugar form.

David Shuker: Well I think what's happening is that in a caramel you're starting off with a solution of the pure sugar which has got a certain sweetness. Now as soon as you start to heat that solution, you're going to start breaking down those sugar molecules, and some of those breakdown products will have a different flavour. And I think you're right, they're less sweet. And then it's a question of whether you want a caramel which has really got that sort of almost toffee taste, which is almost bitter. And, interestingly, it's a way of generating a sort of sweet and sour thing all in one go from sugar, and that is you break it down enough until you've got that consistency and taste which is just right.

Now, of course, that's where the judgement comes in and I don't think there's any means at the moment where a chemist, like myself, can come into your kitchen and measure something and say "That's the perfect caramel."

Paul Merrett: That's the point, yes.

David Shuker: No, I think it's really a question of taste, and of course, interestingly, this is another aspect to think about, is that our taste buds, our taste receptors, are very sensitive instruments - we think about the difference between science and cooking but of course our sense of smell, our sense of taste, are the sort of instruments that we use to test, and that's why you as a chef, and me as an amateur cook, taste things because we're continually testing; we're sampling.

Paul Merrett: Yes, assessing the flavours.

David Shuker: It's like assessing the flavour and saying, "Yes, I'll stop cooking now because the flavour's just how I want it." But that's because we've got built-in little scientific instruments which can detect these key things, which help us to judge.

Paul Merrett: It's an amazing thing, the human body. David, thank you very much. If people want to find out more about the science of food, or get any of the recipes that we've cooked on the series so far, or maybe another dose of David and I talking food, please visit our website at I'm Paul Merrett, and I've been talking to David Shuker of the Open University. The producer was Michael Brodbin, and this is a BBC Worldwide production for the Open University.


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