While cooking recently, I found out quite by accident that if you pour the juice from cooked red cabbage over a frying egg, the egg white turns green. Why is this?
鈥� I teach a Year 7 class of 11 and 12-year-olds at High Storrs School, Sheffield. The answer below is the result of experimentation by the class.
We thought we knew the answer, but we wanted to be sure. We knew that red-cabbage juice (which is actually purple) is a good indicator of whether a substance is an acid or an alkali. We then attempted to change the juice鈥檚 colour with citric acid, which turned it red, and with ammonia, which changed it to green because it is an alkali. When we added the cabbage juice to raw egg it turned green, so we established that egg white, or albumen, is alkaline. We checked this with universal indicator and discovered that the pH of egg white is about 9, meaning it is alkaline as we suspected. Cooking the egg after this made no difference to its colour.
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Philip Ward and Class 7C, High Storrs School, Sheffield, UK
鈥� There is a well-known high-school science experiment that involves creating a 鈥渞ed-cabbage indicator鈥� to measure the pH of acids and bases. Red-cabbage juice is added to solutions of known pH and a chart of the colour change observed for each pH is created.
The juice of a red cabbage contains a water-soluble pigment molecule called flavin, an anthocyanin, which is also present in plums, apple skins and grapes. Adding an acid (which has a pH value of less than 7) to the indicator will change this pigment to red, while a base, or alkali (which has a pH greater than 7), will change it to a green/yellow colour. In neutral solutions (pH 7) it is purple.
The colour changes in the juice are the result of a change in its hydrogen ion concentration after an acid or base is mixed with it. In an aqueous solution, acids donate hydrogen ions and bases receive them.
The colour change seen in the egg white shows that egg whites are basic and have a pH of approximately 10.
Emma Bland, East Malvern, Victoria, Australia
鈥� Anthocyanins are versatile pH indicators: they change colour depending on the hydrogen ion (proton) concentration or pH around them. They have a three-ringed structure whose light-absorbing properties vary according to how many protons are attached. In acidic conditions, the molecule acquires protons and turns red or pink.
鈥淩ed鈥� cabbage is only red after pickling in acidic vinegar. If conditions are neutral it is purple and if alkaline it loses protons, turning blue to green to yellow, depending on the strength of the alkali. The questioner has shown that the pH of egg white is alkaline, probably about pH 10.
鈥淐olour changes in cabbage are subtle, matching those of commercial pH indicators鈥�
The colour changes are quite subtle and match those of many commercially produced indicators. The only unfortunate side effects are the smell cabbage creates and the fact that the indicator goes off quickly. Nonetheless, you can have great fun extracting the indicator (boil shredded purple cabbage in a minimum of water and strain off the indicator liquid when cool) and using it to determine the pH of substances and conducting neutralisation experiments. Obviously, if you try this on strong solutions be very careful.
All this means hours of fun when you should be cooking, which is why I am now barred from buying the stuff.
Kate Johnston, The Centre for Plant Sciences, Faculty of Biological Sciences, University of Leeds, UK
鈥� You can create 鈥渕agic鈥� paper to amaze children by soaking reasonably low-quality paper in boiled red-cabbage water and leaving it to dry. Let the children paint on it with household substances such as vinegar and washing powder dissolved in water and they鈥檒l see a range of colours, including pink, blue, yellow and green, depending on the pH and strength of the substance.
Susan Crafer, Barnsley, South Yorkshire, UK
鈥� Anthocyanins are powerful antioxidants that are thought to protect the plant鈥檚 photosynthesising apparatus against free radicals produced under bright light, especially in cold weather. The red cabbage, therefore, is a green cabbage with a shield of anthocyanins above its chloroplasts. This shield absorbs the photo-oxidative and unusable wavelengths of green light, and cancels out the green of chlorophyll.
Boiling up the cabbage, like boiling any leaf vegetable, ruptures the leaf cells, allowing the chlorophyll and anthocyanin pigments to leach into the water. This becomes a mixture of green from the chlorophyll and purple-blue from the anthocyanins.
Caspar Chater, Centre for Economic Botany, Royal Botanic Gardens, Kew, Surrey, UK
鈥� The photo below illustrates the phenomenon well. It was supplied by Peter Scott of Hove, East Sussex, UK 鈥� Ed