I hadn’t heard of butterfly pea flowers until we moved to Thailand, where drinks come in a range of bright colors. Beverage menus here often look like rainbows, from brick-red Thai iced teas to verdant milky-green matchas.
However, the most vivid drink on anyone’s table is an electric blue or lurid purple. The key ingredient is butterfly pea flower, or Clitoria ternatea—sometimes just known as “blue tea” or, strangely, Asian pigeonwings. The flowers have virtually no taste; when sold as tea, the dried flowers are typically packaged with dried lemongrass to add flavor, then you might add honey or even passion fruit to the drink.
Inevitably, bartenders also use the teas for flashy cocktails. This can involve a show, since the tea has an unusual property: Originally a bright blue, the drink turns purple once you add an acid, such as a squeeze of lime juice.
The flowers themselves are a deep blue color, and people in South and Southeast Asia have long used them to dye fabrics as well as color drinks, rice, and other foods. Folk medicine has uses for it—as an aphrodisiac or anti-anxiety drug, among other things—but scientific research on the benefits is scanty. In 2021, the U.S. Food and Drug Administration deemed it harmless.
So, essentially, it’s just a natural and effective food coloring.
That color is unmistakable, so when I recently saw a particularly purple beer appear on my Twitter feed, my first thought was, “That has to be butterfly pea.” The beer in question was from Foam Brewers in Burlington, Vermont, and—sure enough—its ingredients include butterfly pea flowers.
Foam is best known for their hazy IPAs, but they also like to play around with mixed fermentation. Bob Grim, cofounder and head brewer, says this one started with banter at the Vermont Brewers Festival.
“I was chatting with a friend of Foam and faithful imbiber, Scott Moody, about saison,” Grim says. “He lightheartedly requested that we brew them more often. I invited him to hang for the next saison brew and asked if there were any special ingredients he would like to see in the beer; he suggested lavender. From there, I went down the proverbial rabbit hole of ‘purple’ and decided to incorporate plums and, of course, butterfly pea flowers.”
Grim knew that Brasserie Dunham in Quebec had recently used the fleur de pois to brew a plainly purple pale ale called Halochromie, and he was impressed by the beer. So he reached out to Dominic Malo, head of production at Dunham, to find out more.
Following Malo’s recommendation, Grim turned to a source that is using flowers hand-picked off wild vines in Thailand. “The flowers are sun-dried and ground into a fine powder, which is a nice format to work with in the brewery,” he says. “The powder form allows us to precisely meter it into a beer—which is important, since a small amount goes a long way.”
While flavor-neutral in the beer, Grim describes the color as “magical,” especially given how it reacts to acidity. An article he read online (“The Butterfly Pea Flower as a pH Indicator,” International Scholastic Journal of Science) helped him to understand how the color reacts to different pH levels.
“At a very low pH, around 2, the flower will contribute a vibrant pink hue,” Grim says. “Around 3, bright purple, and 4 to 5, a deep blue. At a more alkaline pH, between 8 and 13, the flower expresses different shades of green and eventually hits a bright yellow at 14. Surprisingly, a neutral pH seems to be a murky greenish-blue that quickly fades to gray.”
The Foam team chose to use the flowers at the end of fermentation. “We added the lavender, lemon zest, and butterfly pea flower together, a couple of days before cooling the beer. We tested different pea-flower ratios in the base beer before landing on six grams per gallon.”
Thus, at homebrew scale, just about 30 grams of the blue tea powder—or a bit more than an ounce—is all you’d need for that striking color effect.
Whether that’s enough to work as an aphrodisiac remains to be seen.