Did you know there’s a science lab in your kitchen?
Kitchen Chaos is your opportunity to get hands on and try some real science at home.
You’ll be able to have a go at extracting DNA from onions and make your own margarine. You’ll even be able to make your own pH indicator to test acids and alkalis around you’ll find under your kitchen sink.
We’ll also explain some of the science we see every day - from heat transfer to rising bread, you’ll be able to understand more about the science of everyday life.
Acids and alkalis
Acids and Alkalis (also known as ‘bases’) have the ability to change the colour of certain vegetable materials. One common vegetable whose colour is able to respond to acids and alkalis is the humble red cabbage. In fact, it makes a very effective pH indicator. Why don’t you have a go at testing some common chemicals around your home with your own red cabbage pH indicator?
Red cabbage as a pH indicator
Red cabbage contains a pigment called flavin, which is an anthocyanin. Anthocyanin is the red/purple pigment that is found in autumn leaves, red poppies, grapes, pea flowers and blackberries. Very acidic solutions will turn this pigment a red colour. Neutral solutions result in a purple colour and basic solutions appear greenish yellow. Indicators, such as the one we’re about to make, acquire a proton at low pH but lose it at higher pH. The structure of the anthocyanins changes on protonation such that the molecule absorbs light of a different frequency and thus reflects light of different frequency.
PH is a measure of how ‘acidic’ a solution is, so the lower the pH, the more acidic the solution. An acidic solution has an excess of protons (or H+), and a pH indicator is able to acquire these protons to affect the colour change.
In chemical terms, pH means “the negative log of the concentration of protons” in solution, which is written pH = -log[H+].
So, for example, if the concentration of H+ is .01M, the pH will be:
-log[.01] = -log[10 -2] = -(-2) = 2 (very acidic!)
Make your own red cabbage pH indicator
When juice is extracted from a red cabbage, it’s a very dark red/purplish colour.
You will need:
- red cabbage
- hot or boiling water
- strainer or filter paper
To test your pH indicator, try some of these:
- antacids (calcium carbonate, calcium hydroxide, magnesium hydroxide)
- baking soda (sodium bicarbonate NaHCO3)
- cream of tartar (potassium bitartrate KHC4H406)
- household ammonia (NH3)
- lemon juice (citric acid, C6H807)
- soda water (carbonic acid, H2CO3)
- vinegar (acetic acid, CH3C00H)
Chop the cabbage into small pieces. Place about 2 cups of cabbage in a blender, cover with boiling water and blend. Strain or filter out the cabbage to get the red/purplish liquid.
Alternatively, if you don’t have a blender, place the cabbage in a glass container and add enough boiling water to cover. Let it sit for about 10 minutes to allow the colour to leach out of the cabbage. Strain or filter to extract the liquid.
This liquid is about pH 7 (neutral) but its exact colour will depend on the pH of the water.
Pour some of the liquid into a number of different containers (beakers are ideal, but cups would be fine) Add to each container some of the test solutions and note the change in colour.
You can make your own litmus paper by taking either blotting paper or filter paper and soak it in a concentrated red cabbage juice solution.
After a few hours, remove from the solution and allow to dry. Cut into strips, and voilà - home made litmus paper!
Baking powder is commonly used in cooking. Muffins, cakes, biscuits and scones all use baking powder to help them to rise. The bubbles produced from baking-powder are carbon dioxide.
Baking-powder is made up of three different parts, in powder form; an acid, an alkali and some sort of filler. Most commonly the elements in baking-powder are baking soda (an alkali), cream of tartar (an acid) and corn starch (a filler). Add water, and voila! A chemical reaction! Why don’t you find out the pH of baking-powder, using a red cabbage pH indicator.
Did you know that vinegar is an all-purpose solution to cleaning and deodorising around your home? It’s also very environmentally friendly.
Vinegar was discovered quite by chance over 10,000 years ago, it’s quite literally ‘sour wine’ (vinaigre in French). It’s made by two processes, the first is fermentation (which uses yeasts to turn sugar into alcohol) and the second is acetic (or acid) fermentation which uses bacteria to convert the alcohol into acid. Why don’t you test some vinegar at home using a red cabbage pH indicator?
Versatile Vinegar -
Most bleaches use sodium hypochlorite. It’s important when using bleach around the home not to mix it with other household chemicals, especially those containing ammonia. This is because deadly chlorine gas can form.
Bleach is a great disinfectant, as it’s able to kill bacteria and algae. Our drinking water is treated with chlorine to eliminate water-borne bacteria. Millions of people have died from water-borne diseases, and the use of chlorine in modern water treatment plants have avoided this. Why don’t you test some bleach at home using a red cabbage pH indicator?
How Stuff Works
Extracting DNA from onions
Did you know that onions have more DNA that you do? In fact, an onion has 12 times more DNA than your average Cambridge professor. But why? It’s a question that has puzzled scientists for 50 years, ever since DNA was discovered. Not all DNA is involved in genetic processes. Quite a lot of it is what is known as “junk DNA”. This DNA is created when mistakes are made in DNA reproduction. Many organisms actually discard junk DNA at a very high rate. This is good, as it results in compact, junk-free genomes. But onions faithfully reproduce all their DNA, even the junk. Why don’t you try extracting some DNA from onions?
It was the early 1950’s, and the race was on to discover the structure of the building block of genes - deoxyribose nucleic acid (DNA). The two camps were: Cambridge University graduate student Francis Crick and research fellow James Watson; and Maurice Wilkins and Rosalind Franklin of King’s College, London. Crick and Watson decided to make physical models to narrow down the possibilities of the structure of DNA. The King’s team took an experimental approach, and examined x-ray diffraction images of DNA in order to establish its structure. History has Crick and Watson as having discovered the structure of DNA, but the truth is that shared knowledge from the King’s College team enabled them to make the leap forward.
Onions have a rather large amount of DNA, and here in Kitchen Chaos, we’re going to extract some of it.
Every living thing contains DNA so you can use just about anything, including spinach, kiwi fruit, wheat germ, green split peas, chicken liver or broccoli, if you don’t have any onions.
What you need:
- Onions (don’t use Spanish onions)
- Salt (non-iodized)
- blender or pestle and mortar
- Liquid dishwashing detergent
- Strainer or coffee filters
- Alcohol (isopropyl will do)
- Wooden stick
- Place about one-fifth of a diced onion into a cup.
- Add about 1 tsp of salt. The salt adds positively charged atoms to the normal negatively charged DNA. This allows the DNA strands to come together and helps them to dissolve.
- Blend (or mash) the onions. This breaks down the cell walls and releases the DNA. Try not to introduce too much air to the mash.
- Add about 1 tsp of liquid dishwashing detergent and about 40 ml water. This destroys fatty cell membranes and some proteins. If you’re using a pestle and mortar, continue to pound for a further 5 - 10 minutes. If using a blender, give it a really good whiz.
- Using a strainer or the coffee filters, strain the solution into a new container. Now very gently layer about 20ml of ice-cold alcohol on top of the mixture by pouring it slowly and gently down the side of the container. Make sure you don’t pour it too quickly - you don’t want the alcohol to mix. The DNA is attracted to the interface between your mixture and the alcohol.
- Gently put the wooden stick into the mixture and spin slowly for about a minute. You are winding the DNA onto the stick. It might be a bit hard to see. Now pull the stick slowly through the alcohol, and the DNA should become visible, looking a bit like slimey gloop on the end of the stick.
The gloop is actually DNA, and contains all of the genetic code for making an onion. Now why don’t you have a go extracting DNA from something else?
Why onions have more DNA than we do
DNA extraction from wheat germ
How to extract DNA from anything living
How many times have you heard that margarine is manufactured industrially and is full of chemicals? Ever wondered how it’s made? Well, here in Kitchen Chaos, we’re going to make our own margarine! But seeing as this is DIY Science, we’re not going to make it too easy for you.
You will need:
- Two measuring cups, 250ml
- Measuring cylinder, 100ml (or a yoghurt pot)
- Flat spatula
- Ice bath
- Hot plate
- A commercial emulsifier (trade name amidan SDM-T, which is essentially a monoglyceride; or alternatively mustard or lecithin)
- Iced water
- Sunflower oil
- Hardened fats - either lard, cocoa butter, ghee or copha
- Food colourant
- 50g of commercial margarine
How much water is in margarine?
First you have to do a small experiment to measure how much water is present in margarine. Get some commercial margarine, place 50g in a glass measuring cylinder and pop it in the oven on a low heat. You can use a yoghurt pot if you like, but only if you keep the temperature quite low. Once it’s melted, let it settle and see how much water there is. Some low-fat margarines contain so much water and emulsifier that the separation is not very clean, you may have to disturb the emulsion with a wire. You might want to write down how much water there is as a percentage value, it will help you in the next step.
- Measure the water into the measuring cylinder (or yoghurt pot), add 2-3 drops of food colourant and place in an ice bath.
- Warm the sunflower oil and hardened fat together on a hot plate until they melt (at about 40o C). Add the emulsifier and stir vigorously to dissolve. You may need to raise the temperature to 60 - 70o C for this to happen. DO NOT LET THE MIXTURE WARM BEYOND 80o C.
- Once the emulsifier has dissolved, cool the fat blend to 40o C, taking care to ensure that the fat does not crystallise on the surface of the beaker. Place the fat blend in the blender.
- Start the blender on a low-to-medium speed, then add the iced water at 2 - 4o C and increase the speed to medium. An emulsion should form immediately.
- You now need to cool the emulsion. You should place the mixture in a cup and into the ice bath. Using a flat spatula, keep moving the crystallising fat from the cold surface, using a vigorous scraping and folding motion in a figure of eight.
- When the mixture has visibly thickened and/or has cooled to below approximately 10o C, place it in the fridge. The spread should thicken further on standing.
Things to consider:
You’ll notice how we’ve carefully omitted to give you any measures of the ingredients. The experiment requires that you work out the amounts yourself.
- How much water content is there in the margarine?
- Is the margarine polyunsaturated?
- What makes up the unsaturated fats?
- What makes up the saturated fats?
- What proportions of the ingredients did you use?
Colloids and emulsions
Margarine and milk are both emulsions. An emulsion is a mixture of two liquids that aren’t normally compatible, such as oil and water. Emulsions contain a globular collection of fat/oil molecules.
One of the liquids forms small droplets (known as the discrete phase) that are surrounded by the other (called the continuous phase). The droplets are formed by whichever liquid is the lesser in volume.
Emulsifying is done by slowly adding one ingredient to another whilst stirring rapidly. This disperses and suspends tiny droplets of one liquid through the other. However, if you simply mixed oil and water together they would quickly separate into two layers, with the oil floating on top of the water. To produce a stable emulsion, you need a surfactant, otherwise known as an emulsifier.
Emulsifiers act as a liaison between the two liquids and stabilise the mixture. They do this by sitting at the interfaces between the oil molecules and the surrounding water. The water ‘sees’ only the ends that are like water and doesn’t ‘see’ any of the oil. Similarly, the oil doesn’t ‘see’ any of the water and only ‘sees’ the ends that are like oil. Egg yolk and gelatine both contain emulsifiers. In foods, the surfactant molecules are often proteins, or parts of proteins, that can be damaged by small changes in acidity and temperature. This is why when margarine melts, it often divides out into its constituent parts - water and oil.
Chemically, emulsions are colloids, which are mixtures composed of tiny particles suspended in another material that don’t mix. These particles are larger than molecules, but smaller than one-thousandth of a millimetre. The particles in a colloid can be solid, liquid and bubbles of gas. Similarly, the medium in which they are suspended can also be solid, liquid and gas (although gas colloids cannot be suspended in gas). Because the droplets in colloids are so small, they can easily pass through filter paper.
So why are pancakes as flat as a - well - pancake? It’s because pancakes (or more precisely, traditional crepes) don’t contain any sort of leavening agent.
The most common forms of leavening agents are yeast, baking-powder baking soda. Leavening agents create tiny bubbles of carbon dioxide, causing batter or dough to rise. In order for leavening agents to work, they need gluten to capture the carbon dioxide bubbles and hold them in the mixture. This is why it’s not a good idea to beat pancake mixture too much, as it creates too much gluten from the flour and makes your pancakes chewy.
Kitchen Science - gluten
Science of Cooking - Leaveners for baking
Proteins are one of those molecules that are essential for life. In order for us to make proteins in our bodies (in the form of muscles, haemoglobin and other cells) we need to eat proteins. That’s why no matter what sort of a diet you are on, it will contain proteins. They are found in meat, eggs, pulses, fish, nuts, beans, cheese, soya products and vegetarian foods.
Proteins are a special type of polymer made up of chains of small molecules called amino acids. Each protein is made up from about 20 different types of amino acid. Folds in the amino acid chain produce the shape of the protein, and this determines that particular protein’s chemical and biological properties. There are several types of internal bonds that form links between the amino acids, one of which is a hydrogen bond. These bonds can be made to break down, making the protein change shape in a process called denaturation. Many proteins are tightly coiled up (globular proteins) so when the internal bonds are broken down, they expand outwards, like a ball of string being unravelled.
So what causes denaturation? The most usual cause is heat - you guessed it - cooking! Most proteins can be denatured at temperatures of around 40OC. When more heat is applied, the proteins start to undergo chemical reactions that can cause them to break up or to join together into even larger molecules. And it is these chemical changes that are at the heart of cooking.
Eggs are mostly proteins dissolved in water, and some of these are globular proteins. These are the type of proteins that are folded into compact balls and they have a fixed sequence of amino acids. High temperatures make these proteins stretch out, and the long chains of amino acids tangle with each other and become cross-linked with hydrogen bonds. This leads to the formation of a solid, three-dimensional network of protein molecules, turning the liquid egg into a solid.
Eggs consist of two main parts – the white and the yolk, both of which contain proteins. These parts solidify when cooked, but at different rates. The white hardens at a lower temperature than the yolk - a soft-boiled egg will have a hard white but a runny yolk. Heating eggs with milk will lead to an even more solid product. Overheating eggs creates too many hydrogen bonds to form within the egg, and that’s what makes your custard go lumpy and your scrambled eggs rubbery!
Let’s see those whites!
Egg whites can also be denatured by beating them. And the secret is in the bubbles of air that are introduced into the egg whites. Some of the amino acids that make up the proteins are attracted to water, they’re hydrophilic, or water-loving. Other amino acids are water-fearing, or hydrophobic. Egg whites contain both types. In its natural state, the hydrophobic amino acids in the protein are packed near the centre, away from the water in the egg white, and the hydrophilic ones are on the outside of the protein, near the water (because they love it!).
When you whisk an egg white, you introduce air bubbles, and part of the protein is exposed to air, and part is still in water. But not all parts of the protein like air (or water!), so the protein starts to uncurl as the amino acids re-arrange themselves. The water-loving ones moving towards the water, and the water-hating ones moving towards the air bubble. Once the proteins uncurl, they start to form hydrogen bonds with each other, just like they do when they’re heated. This creates a 3D network that hold the air bubbles in place. If you then heat these captured air bubbles, they expand as the gas inside them heats up. If done correctly, the network solidifies and the structure hardens, just like you get when making a pavlova!
Learning to love the yellow
Because some of the proteins in egg yolks are water-loving, and others are water-loathing, egg yolk is an excellent emulsifier. Emulsifiers allow oil and water to mix, without separating, which is why they’re so important in making things like mayonnaise or margarine.
Mix egg yolks with oil and water, and one part of the protein will stick to the oil, and another part will stick to the water.
Another substance that’s found in egg yolks that acts as an emulsifier is lecithin. This is a phospholipid, which is a fat-like molecule that has a water-loving “head” and a water-loathing “tail”. This long molecule buries its tail in the oil and pokes its head out into the water. By doing so, it forms a barrier that prevents the surfaces of the oil droplets from touching, keeping them apart and stopping them from joining together and hence separating out from the water.
How bread works…
There’s nothing more delicious than a slice of freshly baked bread. The two most important things about bread are its tiny bubbles, and the gluten that binds them together.
Flour is made up of small starch molecules, and gluten is formed when water is added and the dough is kneaded. Adding water makes the proteins on the outside of the starch molecules become “sticky” in a process known as hydration. These sticky molecules bind together, and if they’re moved apart, the proteins between them become stretched - forming gluten. This happens when two different protein molecules (gliadin and glutenin) interact with each other to form a “protein complex”.
Gluten is very elastic, and forms thin sheets that behave a bit like rubber balloons. In bread, these balloons become filled with carbon dioxide gas generated by yeast as the bread rises. To make great bread, these gluten sheets have to be strong enough not to break as the carbon dioxide forms. They also have to be plentiful enough to be able to capture the gas in small bubbles.
Yeasts are single celled fungal micro-organisms. They metabolise sugars and create carbon dioxide and alcohol as a by-product. Although there are over 160 different types of yeast, in bread-making yeasts that make little alcohol are favoured. The species of yeast used in almost all baking is Saccharomyces cerevisiae, which converts sugar into either alcohol and carbon dioxide or, in an oxygen rich environment, into carbon dioxide and water. Yeasts are able to ferment at temperatures as low as 5oC, but their rate of gas production increases exponentially up to about 38oC. At 40oC and above, the yeast is slowly killed.
Glucose is the type of sugar that yeasts like to metabolise the most. But when you add sugar to dough, it’s usually sucrose and not glucose that you are adding. The yeast first converts it to glucose and fructose using an enzyme called sucrase. Yeasts also produce another enzyme called amylase, and this breaks down the starch molecules into another sugar called maltose, which is also metabolised by the yeast into carbon dioxide and alcohol.
Heat is energy which flows from hot to cold. It will always flow from an object with a higher temperature to a cooler one. This causes the hotter object’s temperature to decrease and the cooler’s to increase, until both reach the same temperature. When this equilibrium is found, heat will stop flowing.
Heat is transferred in cooking by one of three methods – convection, conduction or radiation.
Convection is the process of heat transfer by movement in a fluid or in air. It’s the main process of heat transfer in cooking.
Boiling, deep frying and baking are all examples of convection.
Both boiling and deep frying use liquid to heat the food, although oil boils at a much higher temperature than water. When boiling water in a pan, the water at the bottom is heated. This hot water becomes less dense than the cold water above it. The less dense liquid rises, transferring its heat to the cooler water as it goes. It draws the cooler water in behind it, which in turn is then heated up. As the water rises to the surface it cools and sinks, to be heated up again in a continuous cycle.
When food is baked it is cooked by means of convection through air in an oven. This works in exactly the same way as water in a pan, but uses air instead of water. The air is heated at the bottom of the oven by an electric element or a gas flame, the hot air rises, transferring its heat and cooling down on the way. The cool air sinks to the bottom to be heated up again. And so it goes on. Fans are often used in ovens. They push the air around, giving a more even temperature throughout the oven - not just relying on the natural circulation of the hot air rising and the cool air sinking, which causes the top of the oven to be hotter than the bottom.
Conduction is when heat is transferred inside a solid by the collision of atoms from a region of higher temperature to a region of lower temperature. As the atoms collide, they get hotter.
Unlike heating by convection it’s not the material itself that is moving, all movement is happening at an atomic level. When a metal pan is put onto a hob the heat is transferred from the hob through the metal in the bottom of the pan by conduction. Most metals are good conductors, but some metals are more conductive than others. Stainless steel isn’t a good conductor of heat, but copper is – that’s the reason why stainless steel pans often have copper bottoms.
Wood and plastic are not good conductors of heat, which is why spoons used to stir food during cooking tend to be made out of these materials. Heat is not conducted through the spoon to the handle, which makes it much easier to hold onto.
When hot, all objects radiate heat. Unlike convection and conduction, radiation doesn’t need a medium through which to carry heat – it can even carry heat through a vacuum. Radiation is used in two forms of cookery - grilling and microwaving.
Microwave ovens use high frequency electromagnetic waves which penetrate the food and are absorbed by the water molecules inside. The water molecules vibrate when hit by a microwave at exactly the right frequency and this vibration generates heat. The food is further heated by the energy from this molecule being transferred by conduction to neighbouring molecules. Microwaves can penetrate up to one centimetre, so the inside of the food as well as the surface is heated.
Strictly speaking, grilling food uses all three forms of heat transfer. The heat radiates from the grill and is absorbed through the surface of the food nearest to the grill. Conventional grills use infra-red radiation, which has a shorter wavelength than microwaves. The heat is then conducted from the surface to the inside of the food, which is then cooked by convection.