For the next instalment in Science of Baking series, just in time for the Bake-Off
Final, Charlie Wilkinson has looked into the science of making the perfect cake.
Cake is a wonderful thing, there’s nothing quite like the first slice of homemade cake to cheer you up. We use it to celebrate birthdays for a reason! There is science in baking a cake, even if you don’t realise it.
The basic ingredients for cake include the use of flour, eggs, sugar and butter. The flour and eggs are strengthening ingredients for building structure in the cake while the sugar and butter are structure weakening. A good cake feels light in texture, this lightness is due to air bubbles formed throughout the batter which creates a structure of thin layers of cake separated by those air bubbles.
Baking a cake starts with creaming your fat and sugar, this action incorporates all that air which is required to form the light texture of cake. At this point eggs are added to the mixture, beaten egg essentially protects the air bubbles in the cake from collapsing during the baking process. Flour is then gently added into the mixture, gently to protect those precious air bubbles. The addition of flour is essential for the structure of the cake, forming gluten to add structure. This is a delicate process, however – too much gluten creates a heavy consistency like bread. This is why the type of flour used is important, with cake flours with lower protein content and heavy strong bread flours with higher.
As the cake bakes air expands as water vapour and carbon dioxide is released, the egg cooks and coagulates forming a permanent risen form of the cake. Browning reactions take place on the cake surface which enhance the flavour of the cake, creating a final form of browned, risen, light, airy, delicious cake.
This weeks Try This Tuesday takes a while, but you end up with a tasty treat!
You will need:
- A wooden skewer or chopstick
- 1 cup of water
- 2-3 cups of sugar
- A narrow glass or jar
Clip the wooden skewer into the peg so that it hangs down inside the glass and is a couple of centimetres off the bottom.
Put the water into a pan and bring it to the boil. Pour about a quarter of a cup of the sugar into the boiling water and stir until it dissolves.
Keep adding more and more sugar, each time stirring it until it dissolves, until no more will dissolve. This might take quite a while!
When no more sugar will dissolve remove it from the heat and leave it to cool for about 20 minutes.
Pour the sugar solution into the glass or jar almost to the top. Then put your skewer back into the glass so it hangs down and doesn’t touch the sides.
Leave your glass in somewhere it won’t be disturbed. The sugar crystals will grow over 3-7 days. Once these have grown you can eat them!
By mixing the sugar and water together when they were really hot, you have created a super saturated solution. This means that the water contains much more sugar than in could in normal circumstances. As the water cools back down the sugar leaves the solution (mixture) and becomes sugar crystals again, forming on the skewer.
Supersaturated solutions are used in real life. In a sealed fizzy drink the drink is saturated (full) with carbon dioxide, as the carbon dioxide is put in using pressure. When you open the drink, the pressure of the carbon dioxide is decreased, which causes your drink to be supersaturated as there is much more carbon dioxide dissolved than there would be at normal pressure. The excess carbon dioxide is given off as bubbles.
For this week’s experiment you will need to raid your fridge and kitchen cupboards to get some milk, food colouring and washing up liquid.
Pour some milk into a dish or bowl, this works better with full fat milk (we’ll tell you why later!). Add small drops of your food colouring wherever you like in the milk.
Get some washing up liquid on the end of a spoon or cotton bud and gently tap the spots of food colouring with it.
The food colouring should burst out into colourful stars and wavy shapes. This happens because the washing up liquid molecules have a hydrophobic tail, these means that they don’t like water so try to get away from it by seeking fat molecules. The milk (especially if it is full fat milk) contains lots of fat molecules. So the washing up liquid moves around in the milk seeking out this fat and takes the food colouring along with it, creating these funky patterns.
This is why we use washing up liquid to clean our dishes. The hydrophobic, fat-loving parts cling to grease and fat. The head of the washing up molecules are hydrophilic, meaning they love water. The heads cling to the water and the tails cling to the grease, this pulls the grease and dirt from your plates and washes them away with the water, giving you sparkly clean dishes.
Today we are looking at the science behind curly potato fries. First, let’s talk about how we make them.
- Carefully chop up a potato into straight thick chips.
- Boil around 250ml of water and stir salt into this water until no more salt will dissolve.
- Fill a bowl with tap water and place half of your chips into this bowl.
- When the salty water has cooled pour it into another bowl and add the rest of your chips to this.
- Leave both bowls of chips out overnight.
- The next day you should have one bowl of chips that are still hard and straight and the other bowl (with salty water in) will be full of chips that are more flexible, that you can shape into curls.
The addition of salt to the water allows you to make curly fries due to osmosis. Osmosis is the movement of water from an area that has few molecules in the water to an area that has more molecules in it to try to even things out and create a balance.
Plants like our potato here are made up of millions of cells that have a cell membrane around its edge which allows some things in and not others. Water can easily flow through this but the salt we dissolved in it can’t. Cells are filled with lots of little molecules so water usually flows into the cells and fills them to dilute the liquid. But when we have lots of salt in the water, there are more particles in the water outside of the potato cells than inside so the water leaves the cells.
When cells are filled with water they are quite rigid and packed closely together making a fairly sturdy chip. When the cells are dehydrated, they are smaller leaving space between cells, allowing the chip to bend without snapping.
Osmosis is used in all plants – not just when you cut them up and put them in a bowl of water! Plants use osmosis in their roots to allow water to move from the soil into their roots.
This week we’re making ice cream but instead of using an ice cream machine, we’re going to make it using science!
You will need:
- Two Ziploc bags – one small, one large
- 100ml double cream
- 50ml milk
- 40g sugar
- Vanilla extract
- Measure out the milk, cream and sugar and place them into the smaller Ziploc bag.
- Add a dash of vanilla extract then zip up the bag.
- Fill the larger bag 2/3 full with ice.
- Pour a generous amount of salt onto the ice.
- Making sure the small bag is tightly zipped up, place it inside the bigger bag with the salt and ice.
- Gently shake the bag for 5-10 minutes, be careful not to rip the bag!
- Leave the ice cream to sit inside the ice and salt bag for another 10 minutes
- Open up your bag and enjoy!
Try making different flavours of ice cream by swapping the vanilla extract for strawberry or mint extract or even cocoa powder for chocolate ice cream. You could also try adding chocolate chips.
How does this work?
Water, as I’m sure you know, freezes to make ice at 0oC. But your freezer at home is around -18oC, so how are we making the ice cold enough to freeze your creamy mixture? The secret is in the salt.
Ice is in a constant state of melting and refreezing and melting and refreezing. When we add salt, the salt particles block the path of the melted ice, stopping it from freezing back on to the rest of the ice but ice can still melt. Therefore more ice is melting that freezing.
Now you may be thinking that surely if the ice is melting that means it is getting warmer? It’s actually the opposite. For ice to melt it needs to break the bonds that are formed between the H2O molecules. This breaking requires energy which it gets in the form of heat. When a molecule melts away a bond is broken, taking heat away from the surrounding, causing the temperature to drop.
This is also the reason that salt is put on icy roads – it stops water forming ice.
We’re feeling very festive this Tuesday so we thought it was the perfect time to make snow with science. All you need for this one is some shaving foam and bicarbonate of soda.
Simply mix the bicarbonate of soda and shaving foam together in a bowl until you get a powdery consistency.
Pick it up and have a play – you might notice that your fake snow actually feels cold too. This is due to the reaction between the bicarbonate of soda and the shaving foam. The reaction is endothermic meaning that it requires heat to occur, it takes this from the environment and so decreases the temperature around it.
The Science of Shaving Foam
Do you think shaving foam is a liquid or a solid? It’s actually a colloid. A colloid is a substance which has droplets of one state surrounded by another state. There are lots of different types of colloids with different combinations of states making up the droplets and the surrounding. In the case of shaving foam, the droplets are gas and the surrounding is liquid making it a foam colloid.
Honeycomb or Cinder Toffee not only makes a great Bonfire Night snack, it’s also a fun and quick science experiment! Here’s our simple recipe for the honeycomb reaction:
1. Grease a baking tray with butter and set aside.
2. Mix 100g sugar with 2.5 tablespoons of golden syrup in a pan. Mix the two well before you heat the pan.
3. Gently heat the pan, try not to stir the mixture at this point just let it gently begin to melt.
4. Once you can see the sugar start to melt you can push the sugar around to ensure in melts evenly and doesn’t burn.
5. When all the sugar has melted turn up the heat so the sugar begins to boil and forms an amber coloured caramel
6. Turn off the heat and add one teaspoon of bicarbonate of soda, beat the mixture quickly as it begins to bubble up to incorporated all the bicarb then tip onto the greased baking tray.
7. Leave to set for 30-60 minutes then enjoy!
The heat causes the bicarbonate of soda (NaHCO3) to break down and release the gas, carbon dioxide (CO2). The gas gets trapped within the sugar, this results in the bubbles in your honeycomb.
With Halloween coming up, what better time to make some of your very own slime?
It’s super easy and quick to make – you just need to mix water and cornflour! Start with a little bit of both, if it seems too runny you can add more cornflour and if it becomes a solid then add more water.
You can also add food colouring and glitter if you want to add some sparkle to your slime.
The slime should become a consistency that appears to be a liquid but if you hit it or try to stir it quickly it becomes a solid – so which is it?
Liquid or Solid?
Slime isn’t actually a solid or a liquid – it is a non-Newtonian fluid, this is a fluid that changes its properties when a stress or force is applied.
The slime we’ve made is a particular non-Newtonian fluid called oobleck (yes it’s a funny sounding word – that’s because it is derived from a Dr. Seuss book). The particles of cornflour don’t dissolve in the water, they become suspended in the water and repel each other. Mechanical stress, such as stirring quickly provides energy that overwhelms the repulsive forces, causing the particles of cornflour to temporarily stick together. When the stress is removed, the repulsion returns and the slime becomes liquidy again.
More Non-Newtonian Fluids
1. Custard behaves just like oobleck, in fact if you filled an entire swimming pool with custard, you would be able to walk across it!
2. Ketchup is almost the opposite of oobleck – it become thinner and runnier under impact, that’s why it helps to bang the end of a ketchup bottle when you’re struggling the get some out.
3. Whipping cream acts differently when under a constant and prolonged stress, such as whipping. If you whip cream for long enough it will appear to go from liquid to solid as it becomes whipped cream.
4. Honey similarly needs prolonged stress to change it’s properties. When you stir honey, it will become more like a liquid than a solid.