This blog post was written by ScienceGrrl Glasgow’s own Gillian Borland.
Many of us have been using lockdown to do more baking. Maybe you, like me, have found yourself specifically baking more bread rather than buying it. But did you know that there’s a lot of fascinating science in bread-making?
At its most basic, bread is a baked mixture of flour and water, and humans have been eating this food for millennia: charred breadcrumbs dating from over 14000 years ago were found at an archaeological site in Jordan. Once humans domesticated various cereal plants and began farming, bread was often a staple food. The grains used to make it could be stored for a long time, allowing bread to be eaten throughout the year not only at harvest, and potentially helping communities survive times of poor harvests. Nutritionally, bread often has a bad reputation in modern times, but when made from whole grains, it’s a very useful source of complex carbohydrates and fibre.
We tend to think of flour as containing starch (a form of carbohydrate) and not much else. However, flour also contains a number of different proteins, and in wheat flour these interact to form a substance known as gluten. You may have heard of gluten in relation to coeliac disease, which is a condition affecting about 1% of people, where the immune system attacks the intestine when gluten is eaten; people with this disease must avoid gluten in their diet. However, gluten is very important in bread-making: it’s visco-elastic and helps the bread dough to rise. It’s produced from the wheat proteins as the dough is kneaded allowing the proteins to interact with each other and resulting in a mesh-like gluten network. As the air introduced during mixing and the carbon dioxide produced by the raising agent in the bread expand, this gluten mesh helps to contain these gas bubbles and lets the dough stretch smoothly, a bit like blowing up a balloon. Gluten-free bread, which is made without wheat flour, usually requires additives which perform the same function as gluten. Next time you’re in the supermarket, have a look at the different types of flour available: high-gluten flour is used for bread-making, while lower-gluten flours are used for cakes and pastry. The amount and strength of gluten in any type of flour, or under different processing conditions, can be measured using a device called a farinograph: this measures the amount of resistance in a flour when mixed with water. A flour with higher gluten will produce a far more resistant (tough) dough.
Most modern bread eaten in western countries is leavened, i.e. contains a raising agent. Several kinds of raising agents are commonly used. Soda breads use bicarbonate of soda (also known as baking soda and chemically known as sodium hydrogen carbonate or NaHCO3) and a source of acid (often sour milk or buttermilk, which contain lactic acid, but sometimes vinegar or lemon juice). Bicarbonate of soda reacts with the acid to produce carbon dioxide; this CO2, and the air trapped in the dough while mixing, expand when the dough is heated in the oven, though soda breads are still quite dense. One of my favourites, especially fresh from the oven and topped with butter!
Most leavened breads use microbes as a source of carbon dioxide. If bread dough is not baked right away, naturally occurring yeasts and bacteria present on the cereal grains and in the atmosphere will start to grow in it, especially if it’s kept warm. These microbes consume the sugars and starches in the flour and produce carbon dioxide; proving the bread dough (allowing it to sit in a warm atmosphere for a while) gives this process time to happen. Historically, a piece of the dough containing these microbes would be kept and added to the next dough made; this process evolved into the use of starter cultures that are used to make sourdough bread today. In this process, flour and water are mixed and left to become infected by natural microbes. Part of the starter culture is added to bread dough and the rest is “fed” with fresh flour and water to keep the culture alive. The bread dough is kneaded and allowed to rise (fermented), sometimes for 24 hours or more, before baking. Starter cultures can be kept alive for many years, and some are claimed to be over a century old. Scientific analysis of the microbes growing in these cultures has shown them to be mainly naturally-occurring yeasts and lactobacilli (lactic acid-producing bacteria) and the lactic acid is thought to be one of the main contributors to the sour taste of sourdough bread. Sourdough bread is now only produced commercially in limited quantities, as the long rising time makes it much more expensive to produce than standard sandwich loafs. However, a quick look at social media suggests that many people are using the extra time they have at home during lockdown (and, in some cases, the lack of available yeast in supermarkets!) to make their own starter cultures and sourdough bread, so perhaps we’ll develop a taste for more traditionally-made breads.
Despite the current surge in popularity of sourdough baking, most breads are still made with purified cultures of a specific species of yeast called Saccharomyces cerevisiae; you may have seen packets or tins containing dried granules of this yeast in the supermarket for making bread by hand or in a bread-maker. Although dried, the yeast cells are still alive and when rehydrated by warm water in a bread dough they’ll rapidly start to convert the sugars and starch to carbon dioxide gas. Using this type of yeast gives a much faster (1-2 hours) and more predictable rise in the bread dough but some people think that the short fermentation time results in a much less flavoursome bread.
Most of us buy our bread in the supermarket, and much of the bread you would find there will have been made in an industrial bakery. Industrial-scale bread making is based on the same principles as home baking, using flour, water and yeast, but has been optimised for rapid, reliable and cheap production. In the early years of the 20th century, the vast majority of wheat used to make flour for UK bread making was imported; attacks on merchant shipping led to a restricted supply of wheat and although bread wasn’t rationed during the war, only a single type of highly-nutritious wholemeal loaf was allowed to be sold; this was called the National Loaf and wasn’t very popular. Once the war was over, people wanted white bread that was light and soft – the opposite of wartime bread! Industrial bakers were happy to meet this demand and investigated new ways of making white bread at an affordable price.
In 1961, the British Baking Industries Research Association based in Chorleywood launched the Chorleywood Bread Process, which is still used for most commercial bread production today. This way of producing bread used the same basic ingredients of flour, water and yeast but the scientists had realised that they could replace the long proving (rising) time for the bread dough with a much shorter but more energetic and highly mechanised mixing process, and the use of additives such as vitamin C and high melting point fat. This produced a light, fluffy white bread in less than 4 hours and at a much reduced price to customers and is still used for most commercial bread production in the UK today. Although many critics say that bread made this way lacks both flavour and texture, it’s still very popular with the bread-buying public.
Do you prefer a light, fluffy commercially-produced bread for your sandwiches, or a more dense sourdough with lots of flavour? What microbes have been used to help it rise? Do you love a wheat-based bread or do you need to eat a gluten-free diet? The next time you’re making a sandwich or buttering your toast, have a think about the science behind your bread!
Science Grrl Glasgow would like to thank Mark Borland, Global Process Development Manager at AB Mauri for his input into this blog.