Ready, set, bake – physicsworld.com

Baking has always fascinated me. Not only because I love to eat, but also because I love to prepare food for others. Generations of people have put a lot of dedication, love and passion into making something delicious – when people gather for a celebration, the centre of attraction is almost always food.

My other love is science. That comes from my questioning nature, which, combined with my potential for talking constantly for hours, always kept my parents busy. As a little boy, one thing that intrigued me was rain. Rain is a big thing in India, especially during monsoon season, which brings a sigh of relief after the scorching heat of summer. It always made me wonder where the rain came from. Little did I know that the same water that turns into vapour and then condenses to create clouds also creates the all-important steam during baking, helping cakes and breads to rise and pastries to puff.

For me, baking is a perfect combination of chemistry, biology and physics. Chemistry, as you mix different edible chemicals to create dough or batter, with tiny air bubbles trapped inside. Biology, as the culture of yeast provides carbon dioxide to make your bread rise. And physics, as those trapped gases expand and give the rise to the bakes.

Being part of The Great British Bake Off 2018 was a great experience. Apart from being in “the tent”, the other exciting part was the opportunity to do research on the science of baking and do some live experiments in front of millions of people. Just like any other experiment, understanding how baking works is the key to ensure the quality of the results. Designing and planning a bake is crucial – and is similar to setting up a laboratory experiment. You have to carefully consider each individual variable: from the proportion of ingredients, to the atmosphere inside the tent, to the structural stability of the bakes. All play important roles.

You could even say the kitchen is the oldest laboratory known to humankind.

Let’s talk about bakes

Baked goods are eaten all over the world, the most common being various kinds of cake, bread and biscuit. These are part of our daily life, whatever our country or culture. Their precise form will vary depending on the availability of grains – wheat, corn, barley, oat, rice and a few other cereals – as well as the local climate and the lifestyle of the people. But all start with grinding grains into flour and mixing it with liquid and fat to form some type of paste, dough or batter. Each will create a different type and texture of bread, cake or biscuit.

In my role as a STEM ambassador, I regularly visit schools and events to encourage young people to develop skills and careers in science, technology, engineering and mathematics. The way I do that is by talking about the science of baking, giving students a variety of baked foods and asking them how they think the same ingredients can give such different results.

The kids will quickly tell you that cake is airy, soft, moist, spongy and squeezy. Bread is soft and squeezy too, but also chewy. And biscuits are solid, hard, maybe chewy, and snappy.

Same ingredients, different results

Victoria sandwich cake	Brioche bread	Biscuit
225 g flour (plain)	500 g flour (strong)	275 g flour
225 g sugar	50 g sugar	100 g sugar
225 g butter	250 g butter	150 g butter
4 eggs	5 eggs	1 egg
1 tbsp milk (optional)	150 ml milk	—
1 tsp baking powder	10 g yeast	—

Even though they contain the same basic set of ingredients (see table above), the textures and flavours of Victoria sandwich cake, brioche bread and biscuits are completely different.

Cake (top) is the most decadent of all with equal amounts of fat, flour, sugar and eggs, which contributes to the soft and moist texture.

Brioche bread (centre), on the other hand, is more chewy and bouncy in texture thanks to the gluten that forms during kneading.

The snappy structure of the biscuits (bottom) comes from the comparative deficiency of hydration in the biscuit ingredients.

They are all delicious to eat, but the specific proportions (and some added ingredients like baking powder and yeast) make all the difference.

Each contains flour, fat, sugar and eggs in different proportions (see box above), which is one of the reasons for the differences in structure. The other reason is the additional ingredients – baking powder in cake, and yeast in bread – that make the mix rise and give you that airy spongy texture. Both play the crucial role of introducing carbon dioxide into the batter or dough.

Rise to the occasion

Baking powder is a combination of sodium bicarbonate and a dried acid, such as cream of tartar (potassium bitartrate). In its dry condition, it is inert and doesn’t react. But when added to the cake batter, the liquid makes it active, creating lots of tiny carbon dioxide bubbles that fill the mixture. To give maximum flexibility, commercial baking powders often have a two-stage rise – once during mixing, and again during oven baking. (Some recipes call for self-raising flour instead of baking powder – it simply contains added baking powder to provide the leavening during baking.)

The other way of introducing gases into cake batter is much more laborious. Known as the creaming method, it requires beating the butter with the sugar. You can observe this process by noticing the mixture change colour, from a buttery golden yellow hue to a creamy white, as you beat more air bubbles into the mix.

For bread, it’s slightly different. The gas bubbles are created by yeast, a common type of fungus that is readily found in the atmosphere – it’s the same type of fungus that is often seen as a white misty coating on grapes. Only a few varieties are safe for human consumption, and these are mass produced for the bread and beer industries. When added to a dough, the yeast feasts on the carbohydrate in the flour to create carbon dioxide as a byproduct, which makes the dough rise as it rests or “proves”.

So the important aspect for the soft fluffy baked goods is that they incorporate air before they are placed in the oven. It in this hot environment that the magic (or the physics) happens, and the dough or batter is transformed into a soft bread or spongy cake.

Into the oven

We can break down the many heat-induced reactions that occur in the oven into three generalized stages: expansion or rising; setting the structure; and colouring.

During expansion, the batter or dough reaches its full volume. The heat from the oven makes the trapped gases (air or carbon dioxide) expand in volume, as described by Charles’ law, which states that the volume of an ideal gas at constant pressure increases in direct proportion to absolute temperature. This is what gives us the familiar aerated texture. Alongside this expansion, the water molecules start to evaporate and create steam, which also expands and increases the total volume of the bake.

During the setting stage, the structural elements of egg and flour start to form the framework of the bake. The coiled protein molecules start to denature or unwind, and coagulate or firm up. The starch molecules absorb water and swell up until they disrupt and gelate. Along with the coagulated protein, this is what gives the cake or bread its final spongy structure.

In the final, colouring stage, the batter shape becomes stable. The proteins and sugars react on the surface of the bake, to create a beautifully coloured crust. This is the Maillard reaction, which can also cause caramelization at higher temperatures. The Maillard reaction can create hundreds of different flavour compounds depending on the conditions and ingredients – the same basic reaction also gives roasted coffee and seared meat their distinctive flavours.

Make it snappy

Now let’s talk about the harder, snappier bakes – pastry and biscuits. They are a bit different from bread and cakes, but the basic ingredients are still very similar: flour, fat and water, sometimes with added eggs or sugar.

In the case of pastry, the all-important rise comes from steam generated from the water and fat during baking in the oven. Fats used in baking generally include some water – butter is about 18–20% water, while baking spread is a kind of margarine with around 25% water. During baking, the water vaporizes to create steam, which increases in volume as the heat rises, pushing the flour molecules or layers apart. The proportion of butter or fat gives different kinds of pastry their distinctive short, flaky or puffed texture (see box).

Shortcrust pastry, as the name suggests, is very short or crumbly. Recipes for shortcrust typically involve rubbing the flour and butter together until it resembles bread crumbs. A bit of water (and in some recipes, egg or yolk) is then added to bind the dough together. When everything is combined, pockets of fat are separated and surrounded by dough. During baking, the butter melts and its water vaporizes, creating pockets of steam that expand and push the particles of dough apart. This creates the short texture of this pastry.

Flaky pastry uses more butter than shortcrust. Frozen or very cold butter is grated and folded into the pastry dough – taking care not to let it melt through the dough, otherwise something more like shortcrust is produced. The steam from these little grated chunks of fat pushes the layers of dough apart during baking, creating the distinctive flaky structure.

Puff pastry needs a lot of work to prepare (there’s no shame in buying ready-to-roll pastry – many professional cooks do). Layers of butter are folded between layers of dough, to create a structure of many thin sheets. During baking, the butter melts and creates steam, which pushes the sheets of dough apart, resulting in the typical puff pastry structure.

Choux is the most complex type of pastry. When baked, it creates a thin crisp shell of pastry encasing a hollow centre that can then be stuffed with a delicious filling. It takes a bit of effort and practice to master this. The trick to choux is that it is cooked twice. The flour is cooked first during the preparation of the dough, and then again during baking. If you look at the ingredients, you will see that it takes a huge amount of water compared to other pastries, almost double the volume of flour, as well as eggs. The water evaporates during baking and, as the temperature increases, its volume increases according to the gas laws, pushing the pastry out and creating the hollow cavity with a crisp outside.

Have your cake and eat it

I have found that talking about the science of baking is a great way to get young students thinking about how we can explain everyday things that we might otherwise take for granted. It’s particularly engaging if there’s something tasty to eat alongside the science.

A lot of the time in schools, we forget to link science with its applications in an engaging way. That can make study much drier and more difficult than it needs to be, which is completely the opposite of what science is intended for.

Think about how you were taught addition when you were little – if you have two sweets and your friend gives you two more sweets, how many sweets do you have? The thought of sweets probably helped you learn your lesson a lot quicker.

In the same way, if you want to get kids interested in science, it helps to link it to real life in ways they might not have considered before. That can be as simple as thinking about what happens to the water in your kettle when you make a cup of tea. Or it can be more complex, like chocolate tempering, which is a remarkably similar process to steel tempering. In recent STEM events at the University of Sheffield’s Nuclear Advanced Manufacturing Research Centre where I work, we’ve let children try their hand at “welding” with chocolate, using molten chocolate to join slabs together into box structures, and then testing them to destruction. Bringing together the fun of food and baking with science and engineering really does give you the best of both worlds.