Whether it's a guy in a TV ad taking a huge bit out of a chocolate bar or Homer Simpson slobbering at the thought of sugary glazed doughnuts, it's clear that pretty much everyone likes sweets. It's believed that we eat desserts last because we love that sweetness lingering in our mouth. But why do we like sweets anyway? And what does "sweet" really mean?
Biologists think that humans love sweets because we evolved from fruit-eating tree dwellers. Since fruits sweeten as they ripen, we associated a sweet taste with food that would give us lots of energy. People unconsciously eat more of something if it's sweetened. Many diseases that have been blamed on added sugar are actually due to overeating in general.
Did you know? Cats and dogs have been shown to prefer canned foods that contain a sweetener. Pigs are fed sweetened foods to encourage weight gain.
Sweetened foods are bought more than any other type of processed food, literally billions of dollars worth. So if someone can come up with a sweetener that is completely nutritious, doesn't cause cavities, can be heated (for baking), and is cheap to make, that someone will become rich.
At first, food chemistry would seem to be about ingredients. Saccharides like sucrose are carbohydrates: they contain carbon (C), hydrogen (H), and oxygen (O). But aspartame (NutraSweet®) also contains nitrogen (N), and sucralose (Splenda®) instead of N has chlorine (Cl). The ancient Romans loved a syrup called sapa made by cooking aging wine in lead pots: lead acetate (C, H, O, and Pb) is also sweet. So if sweetness isn't the ingredients, what is it?
Did you know? Lead acetate is toxic. Roman Emperors like Nero and Caligula were famously crazy — historians believe this was caused by lead poisoning. An Empire falls because of too many sweets!
When we chew food, molecules contact our tongue. Basic flavours like sweet, salty, sour, and bitter are detected by taste buds. To activate taste nerves which signal the brain, molecules have to fit into receptors like a key opening a lock. Otherwise, everything would taste the same!
In the case of "sweet," the molecule MUST have a triangular shape. Two corners of the triangle are tipped with atoms that can slightly stick with waiting tongue atoms — a hydrogen bond. The third corner must absolutely not want to stick. Molecules that do not fit this exact "sweetness triangle" do not taste sweet.
For example, aspartame is made up of the two amino acids (aka protein building blocks) aspartic acid and phenylalanine. Each amino acid can be "D" or "L" mirror images — like right and left gloves. D-phenylalanine fits the triangle and is sweet. L-phenylalanine does not fit and is bitter!
Did you know? Starch is a chain of sugar molecules — something referred to as a polysaccharide. Chew bread for long enough and it'll become sweet. Your saliva breaks the chain into molecules that fit the sweetness triangle.
Food scientists engineer sweets by choosing molecules that fit the sweetness triangle. Thaumatin, a protein extract from the African plant "katemfe," fits into several sweetness triangles at once, so it's 2000X sweeter than sucrose. Artificially, chemists have added an extra hydrogen-bond-hating group to aspartame so it better fits the sweetness triangle. The resultant neotame is over 8000X sweeter than sucrose!
So the next time you're in math class, staring at triangles and chewing flavourless gum, think about how these things could stick together. Molecularly engineering something sweeter than neotame means a kilogram of it could flavour millions of sticks of gum. Have it survive baking and stop cavities, and everyone will be your customer. If wealth is sweet chemistry to you, then sweet chemistry could lead you to wealth.
Wikipedia on Sugar
Canada's comments on artificial sweeteners. Click Here
A short TV clip discussing some principles of sweeteners. Click Here
Does the colour of orange juice affect our perception of its sweetness? Click Here
A recipe for edible "sugar glass": Click Here
US Food & Drug Administration article on artificial sweeteners: www.fda.gov/fdac/features/2006/406_sweeteners.html
Eggers, S.C., Acree, T.E., and Shallenberger, R.S. (2000). Sweetness chemoreception theory and sweetness transduction. Food Chemistry, 68: 45-49.
Emsley, J. (1994). The Consumer's Good Chemical Guide. Corgi Books, London, UK.
Gonick, L. and Criddle, C. (2005). The Cartoon Guide to Chemistry. HarperCollins, NY.
Immel, S. (1995). Computer simulation of chemical and biological properties of saccharides: sucrose, fructose, cyclodextrins and starch. Ph.D. Thesis, Darmstadt University of Technology.
Lessler, M.A. (1988). Lead and lead poisoning from antiquity to modern times. Ohio Journal of Science, 88(3): 78-84.
Morris, D. (1967). The Naked Ape. Bantam Books, Toronto.
Darcy J. Gentleman has a Ph.D. in Analytical Chemistry from Arizona State University and lives in Toronto, where he did his undergrad in Chemistry and Planetary Science. He spends his time writing about molecularly-sized things like, well, molecules, but also atoms and clusters of molecules. Presently, he’s working on a book about nanotechnology, or as he likes to call it, kilopicotechnology. Darcy enjoys cycling and once tried mountain biking really fast near cacti – he now sticks to roads and city park paths because they have fewer spiky things.