Three Turmeric Treats

Dean Campbell*, Ali Patel, Kathryn Campbell

*Bradley University, Peoria, Illinois

Curcumin and related compounds are found in the spice turmeric, which is produced from the rhizomes of plants in the Curcuma genus.¹The rod-like molecules of curcumin contain delocalized carbon-carbon bonds, can exhibit keto-enol tautomerization, and have limited solubility in water. Curcumin is well known for its ability to change color with changes in pH, being yellow in neutral and acidic conditions and orange-red in basic conditions.^1,2 This pH color sensitivity enables curcumin to be used as an indicator in many STEM-related demonstrations.² Three demonstrations are described in the paragraphs below.

Revealing Hidden Messages with Curcumin

Some forms of goldenrod-colored paper have been used to convey hidden messages. At our STEM outreach events, we often welcome the audience by spraying aqueous ammonia solution onto panels containing goldenrod paper. The paper had been previously marked with candle wax in the shape of large letters. The wax slows the rate of ammonia solution diffusion into the paper. The result is that the sign changes colors from all goldenrod to goldenrod letters on a red background (spelling the word WELCOME at our events).² Spraying the sign again with an acidic vinegar solution causes the goldenrod paper to transition back to its previous yellow color. Paper treated with turmeric exhibits similar color changes. Another way to reveal hidden messages is to mark white paper with a colorless base. For example, Milk of Magnesia (a magnesium hydroxide suspension) or water saturated with baking soda (sodium hydrogen carbonate) can be applied to paper with a small brush and then dried. Dry, powdered turmeric can be sprinkled onto the paper, followed by a drop or two of hand sanitizer. When the two components are mixed with a finger or a piece of paper towel, the alcohol in the hand sanitizer helps to dissolve the curcumin. When this yellow mixture is smeared onto the marked paper, the paper is typically stained yellow, and the markings turn to a shade of red, revealing the hidden message (Figure 1).

Figure 1. Writing composed of the dried bases magnesium hydroxide and sodium hydrogen carbonate on printer paper, (TOP) before and (BOTTOM) after exposure to turmeric mixed with hand sanitizer.

Blushing Pumpkin

The addition of a base, such as ammonia solution, to a suspension of powdered turmeric in water will turn the suspension from yellow to red. At our STEM outreach events, we have done this demonstration in a transparent round container with a face drawn on it and called it a blushing pumpkin.³ More recently, we have designed using Tinkercad software and printed with a Flashforge 3D printer, a hollow, translucent pumpkin-shaped flask using natural polylactic acid (PLA) filament. The STL design file for the pumpkin is provided in the Supporting Information. The wall of the printed pumpkin model has been made as thin as possible to show the colors of the contents of the flask. The printed flask should be checked for potential water leaks before use. If there is a small leak, it might be able to be sealed with a bit of silicone-based adhesive. Figure 2 shows the blushing pumpkin flask in use.

Figure 2. The blushing pumpkin flask containing powdered turmeric in water (LEFT) before, and (RIGHT) after the base aqueous ammonia is added.

Acid-Base Mechanochemistry

In mechanochemistry, solvent-free reactions are aided by grinding reactants together. Solid, colorless bases such as magnesium hydroxide have been shown to react with solid indicator species such as phenolphthalein and anthocyanins. As the reactions take place, the colors of the mixtures change. Grinding together turmeric powder and bases with a mortar and pestle will shift the color of the mixture from more yellow to more reddish. Figure 3 (top) shows the color of turmeric powder mixed with white buffer tablets with various pH values, as well as solid magnesium hydroxide and solid sodium hydrogen carbonate. The solid mixtures at higher pH values are a little more red in color. Figure 3 (middle) shows the same mixtures after the addition of 3 mL of 2-propanol, followed by 5 mL of water. The darkening/reddening of the mixtures at higher pH values is more obvious. Figure 3 (bottom) shows the mixtures from the middle image under illumination with 395 nm UV light. The yellow acid form of curcumin fluoresces much more brightly than the red base form.

Figure 3. Turmeric powder mixed with white buffer tablets with various pH values, magnesium hydroxide, and sodium hydrogen carbonate: (top) dry mixture, (middle) with 2-propanol and water added, and (bottom) with irradiation by UV light.

Conclusion

Turmeric and its curcumin have a number of interesting applications beyond their use as a pH indicator. Since turmeric is a spice, these demonstrations can be connected to National Chemistry Week 2025, with the theme The Hidden Life of Spices.⁴ Turmeric paper (paper containing curcumin) can be used to detect boric acid and borates.⁵ Perhaps the curcumin-stained paper from the hidden message demo could work for this, though it is unknown if the calcium carbonate found in many papers would interfere with this analysis. Curcumin is used as a food dye (E100) and is also used as the dye Natural Yellow 3.¹ Curcumin and related compounds have been studied for a wide range of possible health-related impacts.¹ With these many possible connections, curcumin is a compound worth mentioning in STEM educational settings, and its widespread availability helps to make it readily used for chemistry demonstrations.

Safety

If demonstrations are to be done in-person, then instructors must identify and respond to potential hazards, personal protective equipment, and disposal issues associated with these examples. Wear goggles and gloves. Chemical species involved include curcumin [CAS# 458-37-7] (in turmeric) and bases such as magnesium hydroxide [CAS# 1309-42-8], sodium hydrogen carbonate [CAS# 144-55-8], and aqueous ammonia [CAS# 1336-21-6]. 3D printers involve moving parts and heating elements. Please use caution to avoid burns and pinches.

Acknowledgements This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society. The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA. This work was supported through a NASA grant awarded to the Illinois/NASA Space Grant Consortium.

References

1.   Ciuca, M. D.; Racovita, R. C. “Curcumin: Overview of Extraction Methods, Health Benefits, and Encapsulation and Delivery Using Microemulsions and Nanoemulsions.” Int. J. Mol. Sci., 2023, 24, 8874.

2.   Schorr, D. K.; Campbell, D. J. “Demonstration Extensions Based on Color-Changing Goldenrod Paper.” J. Chem. Educ., 2019, 96, 308-312.  

3.   YouTube Chem Demos channel. Pumpkin blush (curcumin pH indicator). https://www.youtube.com/watch?v=CFn1_aKXVlc (accessed July, 2025).

4.   American Chemical Society. About National Chemistry Week. https://www.acs.org/education/national-chemistry-week/about.html (accessed July, 2025).

5.   CTL Scientific Supply Corp. Turmeric paper. https://www.ctlscientific.com/cgi/display.cgi?item_num=90747 (accessed July, 2025).

Supporting Information

We have attached a link to a file that we have used to produce a 3D model of a pumpkin flask.