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The secret behind that yellow dal, yellow rice, or beautifully colored curry? Chances are, it’s turmeric! A mere teaspoon can color a sauce or dish. Aside from those ‘typical’ dishes, you might also find it added to cheese, butter, or even popcorn! But, turmeric doesn’t have to be yellow. On the contrary, with a slight change in its environment, it changes into a bright red.
To understand, we need to have a closer look at the molecule responsible for these two colors: curcumin.
Turmeric starts out in the ground
If you use turmeric as a spice, you’ve likely bought it as a bright yellow powder. Turmeric starts out as a mass of roots (rhizome), growing underground, just like ginger and lotus root. The outside of the tuber is yellow/brown and rough, but upon cutting it open though you will find an orange-ish color, some brighter than others. It’s the inside you’re after when harvesting and processing turmeric root for making turmeric powder.
This further processing starts by boiling or steaming the ‘rhizome’. Once cooked sufficiently, they’re dried until they contain about 8-10% moisture. Traditionally this was done using just the sun, though nowadays it is also done industrially. Once cooked and dried, a manufacturer will remove the rough outside of the tuber (polishing). Then it’s a matter of grinding the remainder of the root into a powder. The resulting powder doesn’t just have a bright color but also a distinct flavor and smell.
Color comes from curcumin
So what makes the resulting powder bright yellow? It’s a group of molecules called curcuminoids which all have a very similar chemical structure. The three most common curcuminoids in turmeric are curcumin, demethoxycurcumin, and bisdemethoxycurcumin. Up to about 6,5% of turmeric is made of curcuminoids and of that, more than half is curcumin. You may sometimes find that this mixture of curcuminoids is sometimes referred to as simply ‘curcumin’.
Curcumin is the main ‘culprit’ for the yellow color of turmeric and is quite a stable molecule in its dry form. As is typical for colored molecules, the curcumin molecule contains several rings and double bonds which are used to ‘capture’ light.
If you’ve stored turmeric powder for a long (that is, years) period of time, you may have noticed that it barely changes color. And that is correct, dry curcumin, kept out of light, is very stable. Unfortunately though, if you use turmeric for both its color and flavor properties, that might fool you. The color is a lot more stable than the aroma which does decrease over time. Some of those typical turmeric aroma molecules are turmerone, ar-turmerone, and zingiberene (the last is also common in ginger).
Turning turmeric red!
Just like a lot of other natural food colors (e.g. the anthocyanins in red cabbage) curcumin is sensitive to the pH-value of its surroundings. At an alkaline pH (above 7, roughly within the 7-9.5 range) curcumin will turn a dark red color. Even though literature describes the process as ‘spontaneous’, in some of our experiments we did also have to add a little heat to the mix to actually initiate the color change.
This color change is due to a slight change of configuration of the molecule in the 2 oxygen molecules in the middle of the molecule. That slight change will change just how light is absorbed and reflected, causing the powder to change color!
You can make this red turmeric powder at home, we’ve described the process at the bottom of this post.
To strengthen the power of curcumin, you can extract it from the turmeric. In this form, curcumin is used as a colorant in a wide range of applications, not just food. It can be used to naturally dye clothing for instance (though it’s not too stable over time under the influence of sun light). Also, in India turmeric can be used to make sindoor, the bright red powder worn by married women on their forehead. By mixing slaked lime (which is very alkaline, so not the fruit called lime which is acidic) with the turmeric, it will turn the typical dark red color.
What else is turmeric made of?
Curcuminoids only make up about 5-6,5% of turmeric powder, despite the whole powder seeming yellow. Actually, about 70% of turmeric is made up of carbohydrates. Most plants, and turmeric is simply a ground root, contain a large amount of carbohydrates which make up the structure of the plant. Turmeric powder also still contains water, some fats and proteins and volatile oils.
That turmeric is made up of a mix of components shows well when you try to dissolve turmeric in water. First of all, the majority of the turmeric won’t dissolve. Instead, it will slowly sediment to the bottom of the liquid. What’s interesting though is that several separate layers of sediment form (especially under alkaline conditions). Some components will float, whereas others sink, demonstrating the difference in density.
Nelson, Kathryn M et al. “The Essential Medicinal Chemistry of Curcumin.” Journal of medicinal chemistry vol. 60,5 (2017): 1620-1637. doi:10.1021/acs.jmedchem.6b00975, link
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Priyadarsini, Kavirayani I. 2014. “The Chemistry of Curcumin: From Extraction to Therapeutic Agent” Molecules 19, no. 12: 20091-20112. https://doi.org/10.3390/molecules191220091, link
Sahdeo Prasad and Bharat B. Aggarwal, Herbal Medicine, Biomolecular and Clinical Aspects, 2nd edition, 2011, Taylor & Francis Group, Chapter 13 Turmeric, the Golden Spice, link
Wikipedia, Curcuminoid, link
Wikipedia, Rhizome, link
Wild Turmeric, DIY: How to Make Sindoor/ Kungumam /KumKum at Home, June-28, 2014, link