Nothing more than simple grain, yeast and water combine to make bread but the chemical reactions that take place in the dough are numerous and complex. In his second article for Kitchen Geekery, Alex will look at the science behind the humble loaf.
The chemical building blocks of bread are proteins and starch. Starch molecules are long, chained polymers of simple sugars (such as glucose) joined end to end by chemical bonds. Proteins on the other hand are more complex, made up of varying combinations of different amino acids.
Two proteins in particular, giladin and glutenin, are extremely important in bread making. They are present in flour, and when combined together with water they form gluten.
Gluten forms naturally in a tangled bunch structure, but by adding more water and some yeast to the flour we add mass, and by kneading the resulting dough, we straighten the gluten bunches internal structure into lines which more effectively trap carbon dioxide inside the dough.
This is what gives bread its fluffy interior - it is full of tiny pockets of gas trapped by gluten. Poorly kneaded bread will have a weak gluten structure, and will remain heavy and dense when baked, as it was unable to trap enough carbon dioxide to form enough pockets of gas in the dough.
The majority of the carbon dioxide comes from anaerobic respiration using yeast. Yeast is a collection of tiny, one-celled fungi which when added to flour and water produces sticky dough. This dough seems perfectly placid but inside it, reactions are occurring.
Some yeast enzymes (chemical eating proteins) break down the long chains of starch into individual glucose molecules whilst other enzymes present in the yeast use the glucose molecules to produce CO2 and ethanol in a process known as Fermentation.
Now this process is not perfectly efficient. Not all of the glucose molecules are used to form carbon dioxide. Some molecules get used in other chemical processes and are converted into acids, esters and alcohols; substances which add to the flavour of the bread.
Some people choose to use baking powder instead of yeast to leaven the bread dough but this does alter the taste dramatically. As only carbon dioxide and salt forms in the reaction between baking powder, flour, and water – these alternate chemicals and their flavours are not created and added to the bread.
The dough produced is not usually strong enough on its own and gas can escape all too easily, this would still leave us with flat bread, but fortunately, adding salt will strengthen the dough.
Salt strengthens gluten by slowing down the other enzymes which speed up the breakdown of proteins. The addition of too little salt means the dough will be sticky and tough to knead. Too much salt means that water will flow out of yeast cells via osmosis, and slow down carbon dioxide production.
It is because of these essential these chemical reactions inside the dough that it is important to let the enzymes do their work on breaking the gluten down. Enzymes are sensitive to heat and pH, which is why warm, dry conditions with a covering are required for the bread dough to rise.
After the bread has risen, it is time for the dough to go into the oven. For a short period, the heat causes the yeast to multiply even faster, and produce more carbon dioxide making the dough rise. Eventually though the high temperatures in the oven will kill the yeast cells, any water trapped inside those cells becomes steam and escapes. This causes the dough exterior to harden - creating the familiar bread loaf texture.
Sources: Kitchen Chemistry RSC
Special thanks to Jill Matsuyama for the gorgeous photo.
If you are making bread with yeast, it is very simple to speed up the reactions to get the dough to rise faster.
Place the dough in a glass bowl, sprinkle it with sugar water, and cover it with plastic film (fermentation needs anaerobic conditions to produce carbon dioxide). Place it in the microwave or oven at a very low temperature for short bursts of time. This will increase the reaction in the enzymes without being too high to destroy them.