Why does turmeric water mixed with quicklime turn red?
Chengam, Tamil Nadu
Though their taste and uses differ, their similarity of shape and other taxonomical aspects place both ginger and turmeric in the same family, zingiberaceae.
Turmeric, botanically, curcuma longa, is a traditional spice in both vegetarian and non-vegetarian Indian curries. It has been also used in traditional medicine.
As the binomial name indicates, the active ingredient of turmeric is curcumin which is chemically a biphenol and also a 1,3 diketone. Usually 1,3 diketones exhibit an isomerisation process, well known as, keto-enol tautomerism.
Further, curcumin also has an extended conjugation (meaning a repetitive alternate single and double bonds, as ….–C=C-C=….beyond each of its two aromatic phenolic rings.
The presence of tautomerism, extended conjugation and aromatic phenolic rings in it make curcumin appear yellow and it is this ingredient that renders turmeric also yellow.
Phenols are chemically acidic in nature and do readily lose their acidic proton in an alkaline medium and curcumin, being a biphenol, is no exception for this neutralization. However, the loss of proton at any of the phenolic sites converts the phenolate ion from benzenoid structure into a quinonoid one. This means a change at the extended conjugation which would further disturb the tautomerism.
Whenever a benzenoid form goes into a quinonoid form, the species suffers a bathochromic shift in its optical property which means that the quinonoid form appears in a colour that has a longer wavelength than its benzenoid form.
Quicklime is chemically a strong alkali (base). Hence, exposure of turmeric powder or turmeric water to quick lime neutralizes any of the two phenolic protons and triggers the conversion of the original benzenoid structure with yellow appearance into a quinonoid structure with red colour. Red colour has higher wavelength than yellow. That is why turmeric water, when mixed with quicklime, turns red.
PROF. A. RAMACHANDRAIAH
Department of Chemistry
National Institute of Technology Warangal
Warangal, Andhra Pradesh