Though chemistry teachers might have to regularly field questions about the chemistry of ‘Breaking Bad’ these days, baking bread is probably more likely to figure on a list of their recreational activities. Bread-making is a process that seems simple, essentially involving the mixing of just four ingredients. However, there’s a lot more chemistry to it than meets the eye; here we delve into the science to work out what’s going on in your loaf.
The process of making bread can be broken down at a very simple level into four steps. First, the ingredients are mixed; the four basic ingredients used to make bread are flour, water, yeast, and salt. Combining these creates a dough, which is then kneaded before being left to rise, before being baked. Sounds pretty straightforward, right? Perhaps, but at a molecular level, there’s a lot more happening.
We start our examination of bread science with the flour. Amongst the most important components of the flour are proteins, which often make up 10-15%. These include the classes of proteins called glutenins and gliadins, which are huge molecules built up of a large number of amino acids. These are collectively referred to as gluten, a name we’re probably all familiar with.
Without these proteins, making bread would be much more difficult; in the flour itself, they are inert, but as soon as water is added to the mixture the fun begins. The proteins are then able to line up with each other and interact. They can form hydrogen bonds and disulfide cross-links between their chains, eventually forming a giant gluten network throughout the dough. Kneading the dough helps these proteins uncoil and interact with each other more strongly, strengthening the network.
Another ingredient that can affect the dough’s gluten network is salt. It can help strengthen the gluten network, making the dough more elastic, and of course, adds flavour to the final bread. Ascorbic acid, a compound more commonly known as vitamin C, also helps to strengthen the gluten network.
This network is vital for the bread to be able to rise, but of course, the rising bread wouldn’t occur at all without one of the other ingredients. This is, of course, the yeast. Yeast contains enzymes that are able to break down the starch in the flour into sugars; first using amylase to break down the starch to maltose, and then using maltase to break down maltose into glucose. This glucose acts as food for the yeast, and it metabolises it to produce carbon dioxide and ethanol.
The sugar produced by this process isn’t all metabolised by the yeast, however. It can also get involved in some other chemical reactions during the baking process. Specifically, it participates in the Maillard reaction, a series of reactions between sugars and amino acids that occur rapidly above 140˚C. These reactions produce a whole range of products, which can add flavour to the bread, and also help to form the brown crust of the bread.
Back to the yeast though, and let’s look in more detail at the products of its sugar metabolism. Ethanol is of course simply the alcohol found in alcoholic drinks, but you don’t have to worry about getting a little tipsy as a result of eating a loaf of bread because it’s expelled from the dough during the baking process. So is the carbon dioxide gas, but before the dough is baked, it simply diffuses through the dough and enlarges pre-existing tiny air bubbles. This is another reason that kneading the dough is important, as it ensures that a large number of these pre-existing bubbles are present.
Exactly how the carbon dioxide is held in the bread has been a matter of some debate amongst scientists. The common explanation for a long time was that the gluten network helps to trap the carbon dioxide, and prevents it from escaping the dough. However, it’s become clear that the picture is a little more complicated than that. Though the gluten network is certainly involved, it turns out that proteins and lipids in the dough are also involved, and can help stabilise the gas bubbles.
Of course, bread doesn’t always have to be made with baker’s yeast. Sourdough breads are another way of producing a loaf. Sour doughs begin with a starter, which is begun by mixing flour and water. The natural microbes in the flour start to grow, and if this mixture is regularly ‘fed’ with more flour and water, you end up with a mixture containing a mix of bacteria and yeasts. These yeasts are wild yeasts, of a different variety to baker’s yeast. For starters, they have to be more acid tolerant, due to the acidic compounds produced by the bacteria, and they also differ in how they metabolise sugars.
Whereas baker’s yeast will quite happily munch on maltose, converting it to glucose before converting that in turn to carbon dioxide and ethanol, the wild yeasts found in sourdough are unable to process maltose. Luckily for them, the bacteria in the sourdough mixture can, and since maltose is simply two glucose molecules joined together, it produces food for both the bacteria and the yeast. This help is eventually paid back by the yeast, as the bacteria are able to feed on any dead yeast cells. The ultimate result is still the same, but the taste can sometimes be altered by the bacterial metabolites; compounds such as lactic acid can sometimes add a sour flavour.
Sometimes, we might want to cheat a bit and resort to quicker ways to get carbon dioxide into our bread. That’s where baking soda and baking powder can potentially help out. These both contain sodium bicarbonate, a basic compound that breaks down in the presence of acidity to produce carbon dioxide as one of the products. However, there is a slight difference between the two. Baking soda contains only sodium bicarbonate, which can leave a bitter taste in the bread if there isn’t enough acidity to completely break it down. Baking powder, on the other hand, also contains an acidic compound (commonly cream of tartar, potassium bitartrate) which helps break down the bicarbonate once mixed into the dough.
You might well be wondering how this whole process works for gluten-free bread. After all, without the gluten network forming, your bread would be pretty flat-looking. For gluten-free breads, gluten-free flours such as rice flour must be used, to which xanthan gum is commonly added. This is a polysaccharide produced by a particular bacterium which can help provide a similar elasticity to gluten.
After your bread’s cooked, if you don’t manage to eat it in time, it will of course start to go stale. This isn’t due to a loss of moisture, but due to the starch crystallising and hardening over time. Though this can be temporarily reversed by reheating the bread, this doesn’t last for long – it’s good if you’re going to be eating it immediately after, but not if you’re trying to save an entire loaf for future use. In the habit of storing your bread in the fridge? You’re actually accelerating the staling process: experiments have shown that bread stored in a refrigerator at 7˚C stales as much in one day as bread left outside in 30˚C heat does in six days.
There’s load more to bread science that it’s been possible to explore in this short post and graphic, but I’ve compiled some additional links below for those who are interested in delving further into this fascinating area of food science. In the meantime, you might look at the humble loaf of bread a little differently the next time you’re in the supermarket!
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References & Further Reading
- On Food & Cooking – H McGee
- Salt in bread dough – M Glezer, Bread Bakers Guild of America
- Yeast & Sourdough – Science in the Kitchen
- Enzymes: the little molecules that bake bread – E Buehler, Scientific American
- On the rise – B Reuben & T Coultate, Chemistry World
- Bread Science (£) – E Buehler
- The science of baking – Shari’s Berries