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Colorful science: purple bean pods and dazzling white paint that cools

by Cheryl Hogue
October 17, 2021 | A version of this story appeared in Volume 99, Issue 38


Purple pod pigments

Photo shows a container of beans in their pods.
Credit: Cheryl Hogue/C&EN
Colorful crop: Purple beans contain pigments found in some blue and purple flowers.

It started with a packet of heirloom pole bean seeds—a gift to this Newscriptster for the holidays. Planted in a no-till suburban garden in the spring, the seedlings shot up as if they had jumped from the pages of the “Jack and the Beanstalk” fairy tale, quickly growing over a trellis. By summer, the fast-growing vines wound their way over tall, nearby tomato plants and had to be whacked back.

The pruning didn’t stop the legumes from bearing plenty of beans with gorgeous deep-reddish-purple pods. But the purple hue disappeared when the beans were steamed, rendering the pods green. The Newscripts gang wanted to know why that happened. What is behind the beans’ color, and why is it fickle?

The likely answer to the color question is anthocyanins—a broad class of plant pigments that impart shades of blue, purple, and red.Food chemist Cordelia A. Running, a professor of nutrition and food sciences at Purdue University, identifies two likely suspects responsible for the bean pods’ purple pigmentation, both anthocyanins. Either or both could be present in any purple bean, depending on the variety, she tells Newscripts.

One is delphinidin, the chemical that gives the flowers of delphiniums and North American violets their blue or purple colors. Delphinidin is an antioxidant and also occurs in blueberries, cranberries, and concord grapes.

The second anthocyanin that may lurk in the bean pods is cyanidin, which differs from delphinidin by having one less hydroxyl group. In addition to the bean pods, cyanidin occurs in blackberries, blueberries, cherries, and red cabbage.

“These typically have glycosides or acetyl groups on them,” Running says of delphinidin and cyanidin inside growing plants. “Those side groups seem to be part of what makes them more—or less—resistant to losing the color when heated.” This Newscriptster’s purple beans apparently have pigments sporting such functional groups that make beans go green when cooked.

Nevertheless, there’s hope for cooks who want to gussy up a Thanksgiving bean casserole with purple pods. Plant breeders are working to develop beans and other purple veggies that don’t lose their color when heated, Running says.


Dazzling, cooling white paint

Photo shows Xiulin Ruan, a Purdue University professor of mechanical engineering, wearing safety goggles and gloves and holding a brush full of paint.
Credit: Purdue University photo/John Underwood
Paint it white: Xiulin Ruan and his students developed a paint that can cool the surface it coats.

Plant pigments aren’t the only colors that Purdue scientists work with. Researchers at the university have concocted the world’s whitest paint, according to Guinness World Records 2022.

The researchers’ goal wasn’t to set a world record. Xiulin Ruan, a professor of mechanical engineering at Purdue who invented the paint with his team, sought to create a paint that would reflect sunlight from buildings. They want to help save energy and fight climate change.

Ruan’s team used barium sulfate nanoparticles, which are highly reflective, to make the white acrylic paint. The reflective ability of a BaSO4 nanoparticle depends on its size: each size scatters a different part of the light spectrum. The researchers used a variety of particle sizes, allowing the paint to reflect a broad spectral range of light.

In a paper published in April, they report that the paint reflects 98.1% of the solar radiation that strikes it. Remarkably, the paint emits more infrared light—heat—than it absorbs, meaning this coating cools the surface it’s applied to (ACS Appl. Mater. Interfaces 2021, DOI: 10.1021/acsami.1c02368). In contrast, commercial white paints get warmer in the presence of sunlight.

Ruan’s team estimates that using the new paint to cover a roof area of about 93 m2 could result in a cooling power of 10 kW. “That’s more powerful than the air conditioners used by most houses,” Ruan says in a news release.

The researchers are working with a company to manufacture and market the paint.

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