Yummy Science – Designer Onions. Experiments you can try in the kitchen – Magical Water, Growing Crystals, Spud Guns, Custard Bouncing Balls, Honeycomb Toffee, Sticky Milk, Toast Busters, Toasty Science, Self Slicing Banana, Secret Messages

How it's done

We tried this experiment with Northsound1, listen to the results:

Photos

Science Links

Polyphenols are made by plants. There are many thousands of different types and they have been used as natural glues, varnishes, dyes and in the manufacture of leather. Some have also been used as natural medicines for thousands of years. Polyphenols are also are found in foods such as onions, grapes and blackcurrants and drinks such as tea, coffee and wine. Research at the Rowett Research Institute indicates that some polyphenols have important properties that may help to prevent cancer and heart disease. This suggests that regularly eating foods rich in these polyphenols may be good for your health. For more information visit The Rowett Research Insitute research page

Other links:

More fun nutrition activities at
the Rowett Research Institute, eduweb
Check out National Science & Engineering Week at:
The BA (British Association for the Advancement of Science)

Further food science ideas for the primary aged audience are presented in the ‘Food for Thought resource, downloadable at The BA

For secondary pupils, there are lots of great kitchen chemistry ideas available in the Salters Chemistry Club handbooks or in various publications produced by the Royal Society of Chemistry

Exploring the science:

It is the polyphenols in the onion skins that are the active dyeing agents. Polyphenols are a group of chemical substances found in lots of plants including onions, tea, coffee, grapes and blackcurrants.

Each time you wash the t-shirt, the colour gradually fades but you can help prevent this by applying a mordant to the fabric. A mordant is a substance that helps to set the dye to the fabric and is usually more effective if it is used before dyeing. A good mordant to use with onion skins is a mix of alum (potassium aluminium sulphate available at most chemists) and cream of tartar (used in baking). Use 7g alum and 8g cream of tartar for each 100g of fibre to be dyed. Dissolve them in warm water, add the fabric and bring to boil, simmer for one hour then leave overnight. Rinse in fresh water and then dye.

Taking it further:

If you put in more onion skins, does this make a deeper colour?
If you boil the cloth in the dye for longer, does this produce a better colour?
Try the skins of red onions what colour does this produce?
Try adding a couple of tablespoons of vinegar to the water before dyeing - does this affect the colour produced?
What other food stuffs could you use for dyeing? (tumeric, red cabbage, beetroot, blackcurrants, tea, coffee, and...)
There is a lot of information on the web and in books on natural plant dyes, mordants, methods of varying the colour obtained. If you have enjoyed this activity, you may like to do a bit of research and try some further dyeing activities.

And for something a little bit different….

Make some colourful hard boiled eggs(!) Add some of the outer skins of onions to water before boiling the eggs (the skins won’t affect the taste!). For a mottled effect, wrap the skins around the egg in layers and hold in place whilst boiling by tying a piece of cheesecloth around the egg. And of course for colours other than yellow try using some of the alternative food stuffs for dyeing as mentioned above.

Safety:

Boiling water will cause scalding! Adult supervision is recommended for this activity.

The brown onion skins, that you normally discard into the bin, can be used to make a wonderful designer t-shirt. The onion skins contain an amazing dye. To create a unique item of clothing to impress your friends, read on …

You’ll need:

White t-shirt (or strip of white cotton if you prefer to make a scarf)
The brown outer skins of some onions (from at least 3-4 large onions)
Large saucepan filled with water

For tie-dyeing: string and a few small pebbles or buttons

What to do:

Put the white t-shirt (or cotton strip) in the saucepan of water, add onion skins and bring slowly to the boil.
Stir gently with wooden spoon from time to time to ensure that all parts of the t-shirt are evenly dyed.
Simmer for 20 minutes then leave to cool, remove t-shirt and rinse in cold water to remove excess dye. Allow to dry and iron before putting on!

For ‘star burst’ patterns: before you put the t-shirt into the saucepan, take one of the small pebbles or buttons and put inside the t-shirt, tie in place with string. Repeat at lots of locations all over the t-shirt. Bunching up the t-shirt in this way means that the dye (from your boiled up onion skins) doesn’t get to colour the t-shirt evenly, the parts of the t-shirt under the string are not exposed to the dye, and you create some wonderful patterns. Remove the pebbles/buttons before the final rinse in cold water.

What happens:

The onion skins dye the t-shirt yellow.