Why butterfly pea tea changes colour: anthocyanin, lime and baking soda

Butterfly pea tea changes colour because the pigment in the petals, anthocyanin, changes its molecular form with the acidity, or pH, of the liquid. In acid it turns pink-red, near neutral it is blue, and in alkali it turns green. Your kitchen already has an acid, lime juice, and a mild alkali, baking soda, so three glasses are all you need to see it.

The end of the lesson covers growing butterfly pea from seed so it sprouts well and gives you fresh flowers through the year.

Where the blue comes from

University of Florida IFAS Extension explains that the main anthocyanin giving butterfly pea its blue-to-purple colour is of the delphinidin type. It exists in several forms, each a different colour:

• Flavylium form: red, dominant in acid
• Quinoidal form: blue
• Chalcone form: yellow
• Carbinol form: colourless

In the flower itself, the blue-purple comes from a near-equal mix of the red and blue forms. Lower the pH with a mild acid such as lime juice and the red form increases, so the tea turns pink or light purple. Raise the pH and the blue and yellow forms are present together, which the eye sees as green.

Colours measured at each pH

A 2014 study from the Faculty of Agro-Industry at Chiang Mai University adjusted the pH of butterfly pea extract with acid and alkali and measured the colour with an instrument. pH was the main factor setting the colour; pigment concentration had only a small effect.

pH	Colour	Close to, in the kitchen
1.76	Bright red-pink	Lots of lime juice
3	Purple	A good squeeze of lime
6	Blue	Tea made with plain water
9	Green	A little baking soda

UF/IFAS gives the extract’s normal pH as 6.0-8.0, with a deep blue-to-purple colour, which matches tea brewed with ordinary tap or drinking water. The Chiang Mai study also found that antioxidant activity measured in the test tube was highest at acidic pH and fell in alkali. That is a laboratory measurement, not evidence that lime-and-butterfly-pea tea treats anything. Where food ends and medicine begins is covered in Herbs as food vs herbs as medicine.

Example: the three-glass test

Making the tea

The Chiang Mai study extracted 10-40 g of fresh petals per 100 mL of water: soak 30 minutes, blend 30 seconds, strain through cheesecloth. It gave a rough equation:

Anthocyanin (mg per 100 mL) ≈ 3.92 + 0.587 × grams of petals

1. Use 20 g of fresh petals with 100 mL of water.
2. Substitute: 3.92 + 0.587 × 20 = 3.92 + 11.74 = 15.66.
3. You get about 15.7 mg of anthocyanin per 100 mL, inside the 10-20 mg range the study used for its colour tests.

The equation comes from flowers bought at a Chiang Mai market, so yours may differ. Treat it as a starting point; if the tea is so dark it looks black, dilute it with water.

Three glasses

1. Pour the tea into 3 clear glasses, about 30 mL each.
2. Glass 1: add lime juice 2-3 drops at a time and stir. The colour moves from blue to purple; add more and it goes pink.
3. Glass 2: add nothing. It is your blue reference.
4. Glass 3: stir in a pinch of baking soda, about the tip of a teaspoon. The colour shifts toward blue-green.
5. Reverse it: drip lime into glass 3 and the colour moves back toward blue and purple, because the anthocyanin forms switch back and forth with pH.

A colour change is not spoilage. It is the same pigment in a different form for a different pH.

For guests, serve the blue tea with lime juice in a separate cup and let them pour it in and watch the colour change.

Keeping the colour

• Gentle heat, short time. UF/IFAS found extraction rises with temperature up to about 80 °C, above which the pigments start to break down. Its home method is about 77 °C for 60 minutes at 1 g of flowers per 20 mL of liquid (that recipe uses 40% spirit as the solvent and says plain water can be used instead). In an ordinary kitchen, steeping in hot, not boiling, water gives good colour.
• Store it in the fridge. Heat during storage fades the colour toward brown.
• Sugar helps a little. UF/IFAS explains that sugar protects the colour against heat, but it cannot stop the shift to pink or green when pH changes.

Growing butterfly pea for flowers

Butterfly pea is a perennial twining vine that grows fast, tolerates drought and can flower nearly year-round, so it needs a trellis or fence to climb. Singapore’s NParks describes the double-flowered variety as climbing about 2-5 m, preferring full sun while tolerating slight semi-shade, with moderate water needs and fertile, well-drained soil. The flowers, leaves, young shoots and tender pods are edible.

The seeds have a waxy coat that keeps water out, so they sprout slowly and unevenly. A 2022 University of Florida study tested three treatments:

Seed treatment	Germination	Mean time to germinate
Nothing	26.8%	2.52 days
Small nick in the coat with a razor blade	45.8%	2.48 days
Nick, then soak in water 24 hours	80.9%	2.06 days

At home you can follow the study: nick the seed coat just through the shell, not into the inside, then soak 24 hours before sowing. Mind your fingers; if sowing with children, an adult should do the nicking. The same paper notes that plants produce anywhere from 6 to 3,400 seeds each depending on the line, so keep dry pods for the next sowing.

Before picking a spot, see how to count your sun hours in Count your sun hours, then choose what to grow.

Common mistakes

• Boiling the flowers hard for a long time. The colour fades and browns. Use hot water, not a long simmer.
• Adding lime to the whole pot at the start. The whole batch turns purple-pink. If you want blue for something else, set some aside first.
• Dumping in baking soda. It tastes soapy. Add a little at a time.
• Sowing seeds without nicking or soaking. Fewer sprout and they come up unevenly.
• Reading the colour change as a health benefit. Colour tells you the pH, not what it cures.

Sources

• Effects of pH and anthocyanin concentration on color and antioxidant activity of Clitoria ternatea extract — Food and Applied Bioscience Journal, Chiang Mai University
• Butterfly Pea (Clitoria ternatea) Flower Extract and Its Use as a pH-Dependent Natural Colorant — UF/IFAS Extension EDIS
• Substrate Temperature and Seed Scarification on Germination Parameters of Butterfly Pea — bioRxiv, University of Florida
• Clitoria ternatea var. pleniflora (Double Butterfly Pea) — NParks Flora & Fauna Web