July 2016 Issue
By Judith C. Thalheimer, RD, LDN
Vol. 18 No. 7 P. 18
Fruits and vegetables that feature a violet hue are loaded with antioxidants that studies show can improve cardiovascular disease risk factors.
Purple produce is packed with beneficial nutrients and compounds, but this rare food color isn't limited to blueberries and grapes. A wide range of beautiful purple fruits and vegetables, some newly bred and some heirloom, are making a colorful appearance.
The rich, royal color that makes purple produce stand out in the marketplace also makes it a nutritional standout. The purple hue is a result of anthocyanins (or anthocyanidins), phytochemicals that are part of the flavonoid family.1 The name comes from the Greek words for flower (anthos) and blue (kuanos).2 More than 500 anthocyanins have been isolated from foods, all variations on a single core structure. If one or more sugar molecules are attached to this core, the molecule is referred to as an anthocyanin. If there are no sugar molecules, it's called an anthocyanidin. The terms often are used interchangeably, and most studies look at either one or the other, making comparison difficult, but both are linked to the same color and health properties.1 These powerful antioxidants could potentially lower cardiovascular disease (CVD) risk.1 They have been shown to lower cholesterol levels and reduce blood clotting,3 and analysis of the Framingham Offspring Cohort study found that people who consumed more anthocyanin-rich foods had lower concentrations of inflammatory chemicals and fewer markers of oxidative stress.1 Anthocyanin-rich foods and supplements also have been linked to lower LDL cholesterol, improved insulin sensitivity, and lower blood pressure. They also have been studied with regard to obesity control, diabetes management, and improvement of visual and brain functions.4 Currently, there's little evidence that diets rich in flavonoids like anthocyanins protect against cancer.1
While the impact of anthocyanins on health often are attributed to their antioxidant effects, that may be only part of the picture. Anthocyanins are thought to have chemical properties and physiological effects not yet understood.4 For example, recent research suggests that anthocyanins may protect the heart not only by lowering CVD risk factors but also by supporting mitochondrial functions.5
Many familiar fruits are great sources of these powerful phytochemicals. Blueberries often come to mind when people think about purple foods. There are plenty of anthocyanins in these deep-purple nutritional gems. Weighing in at 100 mg per 100 g serving, blackberries are a top anthocyanidin source, and they're packed with other nutrients as well.6 "Blackberries have so many seeds in them that one cup actually has 2 g of protein," says Sarah Krieger, MPH, RDN, a private practice dietitian based in St. Petersburg, Florida. "There are 7.6 g of fiber in a cup too, compared with 3.6 for a cup of blueberries, and blackberries also have significantly more potassium than blueberries." But topping listings of both anthocyanidins and anthocyanins is the elderberry.1,7 At more than 480 mg per 100 g, elderberries have nearly five times as much anthocyanidins as blackberries.6 Other lesser-known purple berries such as mulberries, black currants, and the trendy açaí berries share the benefits of these more familiar fruits.
Berries aren't the only purple standouts. Grapes are easily available in many shades of purple, from reddish-purple to deep black. "Many Americans have a love/hate relationship with grapes," Krieger says. "They think they're high in sugar. There's even a rumor that grapes cause miscarriages. This is absolutely false. In fact, grapes are high in water, so they can help pregnant women stay hydrated. And they have a little bit of calcium and some potassium, both nutrients of concern for Americans."
Plums and figs are other delicious, nutrient-packed purple fruits. Since both of these fruits have most of their purple color in their skins, they're less anthocyanin-rich than berries, which are purple all the way through. "Plums are such a juicy fruit," Krieger says, "and they're neck-and-neck with blackberries for potassium." Figs, which have been cultivated for thousands of years, are rich and sweet fresh or dried, and very nutritious. "Figs pack a powerful nutritional punch in a little package," she says.
The purple signature of anthocyanins isn't limited to fruits. Vegetables like eggplant, purple onions, and red cabbage are all excellent ways to add purple to a plate.
The name "eggplant" came from the small round white version popular in some regions of India during the British occupation.8 "Eggplant is considered a vegetable, but it's really a fruit," Krieger says. "It's extremely low calorie, at 20 calories per cup, and has some protein. A lot of people peel eggplant, but then you're just getting water and seeds. There's no need to remove that anthocyanin-rich skin. It softens up pretty quickly if you bake or sauté it."
Red onion is another common purple vegetable. According to the National Onion Association, about 8% of the US onion crop is red, but their appealing color has led to growing popularity in recent years.9 Often sweeter and more mild than their yellow and white cousins, red onions are a natural color variant of this popular ancient crop. While the outer skin of the onion is a rich reddish-purple, the inner layers are paler. A study published in the Journal of Agriculture and Food Chemistry found that when the outer layers are peeled before eating, all but 27% of the anthocyanins are removed. Storage also affects anthocyanin content, which was decreased between 64% and 73% in six weeks. The study recommends storing the bulbs at low temperatures to help preserve anthocyanin levels.10
For a vegetable that has deep purple in every layer, look no further than purple cabbage. "Purple cabbage is too often thought of as just a condiment," Krieger says. "Really, it's an amazing vegetable: high in fiber, high in water, with 216 mg potassium, and only 20 calories per cup." With over 200 mg of anthocyanidins in 100 g, purple cabbage is a top source for this heart-healthy flavonoid. "Purple cabbage is delicious raw or sautéed," Krieger says. "Just don't boil it (unless you want to dye Easter eggs blue). Cooking with dry heat helps retain color and nutrients."
Rising Purple Stars
While an analysis by the Produce for Better Health Foundation reportedly found that only 3% of the fruits and vegetables consumed in the United States in 2012 were from the purple or blue category, the Specialty Food Association identified purple as one of the top five food trends spotted at the 2016 Winter Fancy Food show.11,12
All kinds of seemingly unusual purple produce, such as purple corn, potatoes, carrots, tomatoes, peppers, cauliflower, asparagus, and Brussels sprouts, are showing up at farmers' markets, grocery stores, and on restaurant menus. "Some people think these are so-called Franken-foods," Krieger says, "but many of them have been grown and eaten in other countries for centuries. They may be new to us, but they may not be new to the rest of the world."
While it may be unfamiliar to North American consumers, purple corn has been cultivated in Latin America for hundreds of years.13 A 2003 study by Tsuda and colleagues found that mice fed anthocyanins from purple corn along with a high-fat diet for 12 weeks didn't gain as much weight as their counterparts fed only the high-fat diet. They also didn't develop the insulin resistance seen in the mice on the unsupplemented diet. The authors found that the anthocyanins affected mechanisms involved in fat synthesis.14 While this research helped elucidate how anthocyanins work, much more research is necessary to understand the roles anthocyanins play in the health and functioning of the human body.4
Like purple corn, purple potatoes have been around for centuries. "Back when they were growing wild in the Andes Mountains, potatoes had these colors naturally," says William J. Lamont, Jr, PhD, a professor of vegetable crops in Pennsylvania State University's department of plant science. "There's a wide array of shapes and colors in the wild types in Peru." But the purple potatoes showing up in stores and restaurants are not exactly wild. "Wild potatoes can look really strange," Lamont says. "Getting these potatoes to a point where they're ready for market isn't a fast undertaking. Breeders work on them and make them more acceptable by making crosses between two potato lines. They select out the seedlings with the best characteristics and cross those. It takes about 11 years to get a new line of potatoes marketable." However, new technologies in genetics can help speed up the process. "Now they have identified genetic markers for different things like color, shape, and disease resistance," Lamont says. "That can speed up the process. Breeders can select seedlings with desired genetic markers or based on visual traits." (It should be noted that doing genetic testing to determine which plants carry a particular trait is different from genetically modifying an organism by inserting the genes for a particular trait into its DNA.) A small study presented at a meeting of the American Chemical Society in 2011 found that overweight hypertensive participants had a drop in both systolic and diastolic blood pressure after eating purple potatoes twice a day for one month.15
Purple carrots also are a natural color variant. In fact, purple was one of the original colors that spread from the Afghanistan region to the Mediterranean and Western Europe in the 11th to 14th centuries. The Dutch carrots that became the basis for the orange cultivars we typically eat today weren't bred until the 1700s.16
Not all of these less familiar purple foods are heirloom or wild types. "Sometimes there's a quirk that shows up in a line," Lamont says. "An accidental genetic mutation may give you a purple bean, for example. Or you make a cross between two plants and end up with a whole variety of colors. If you're able to propagate this new trait, you might be able to bring a new product to the marketplace." Orange cauliflower, for example, was discovered growing in a field of traditional white plants in Canada in the 1970s. It was crossbred with other varieties to create a larger, more flavorful orange cauliflower. It's widely accepted that purple cauliflower arrived on the shelves in much the same way.17 And while the Stokes purple sweet potato was discovered in the United States when an unidentified woman gave them to a farmer at a state fair in Stokes County, North Carolina, purple sweet potatoes had been grown in Japan for decades.18 Purple asparagus was propagated from seeds by farmers in Italy, but purple Brussels sprouts were purposefully developed by a Dutch botanist who crossed purple cabbage with green Brussels sprouts in the 1940s to create a hybrid.19
While purposeful crossbreeding and hybridization are considered by some to be genetic modification, very few of these trendy purple products have been created in a lab. One exception is a type of purple tomato: While heirloom tomatoes do come in many different shades, including purple, researchers in Great Britain spliced genes from snapdragons into tomatoes to get intensely purple fruits with anthocyanin levels comparable to blackberries and blueberries.11 Genetically modified tomatoes like this one aren't currently for sale in the United States.20
Putting Purple on Our Plates
Anthocyanins have health benefits that go beyond their powerful antioxidant capacity. Like other colorful fruits and vegetables, purple produce should be a regular part of a healthful dietary pattern. "We should encourage our clients to consume a variety of these purple foods," Krieger says. "It doesn't have to be just blueberries and blackberries."
Getting consumers to accept unfamiliar food colors may be a challenge. "Our minds play a big role in what foods we'll accept," Lamont says. "Purple mashed potatoes can be a turn-off for some. These foods have a place for the public to eat, but not everyone will be ready for purple French fries and green ketchup." On the other hand, "consumers do seem to be willing to try different things, and boy is there a lot of variety in the vegetable world," Lamont says. "Color and texture in vegetable crops are the big thing. These new products make a beautiful display in stores and on plates. Chefs are artists. They like to create things, and these products give them a greater palette."
Getting home cooks to experiment could be even more challenging. "We might get people to buy more of these vegetables if we let them know how to use them," Lamont says.
Krieger suggests starting with familiar recipes and simply changing the color. "I encourage people to try making purple stuffed cabbage for a change of pace," Krieger says. All forms of purple produce can be encouraged. A 2004 study found no significant difference in the antioxidant activity of anthocyanin extracts from fresh, dried, and frozen blueberries.21 But bear in mind that flavonoids are water soluble, so boiling purple produce will result in losses.
"Encourage people to mix and match and eat a variety of beautiful, healthful purple fruits and vegetables," Krieger says.
Lamont adds, "I don't know if people are ready for purple French fries, but how great would it be next Fourth of July to serve red, white, and blue fries?"
— Judith C. Thalheimer, RD, LDN, is a freelance nutrition writer, a community educator, and the principal of JTRD Nutrition Education Services.
1. Higdon J. Flavonoids. Linus Pauling Institute Micronutrient Information Center website. http://lpi.oregonstate.edu/mic/dietary-factors/phytochemicals/flavonoids. Updated February 2016. Accessed April 28, 2016.
2. Anthocyanin. Memidex website. http://www.memidex.com/anthocyanin. Updated June 26, 2013.
3. Thompson K, Pederick W, Santhakumar AB. Anthocyanins in obesity-associated thrombogenesis: a review of the potential mechanism of action. Food Funct. 2016;7(5):2169-2178.
4. Tsuda T. Dietary anthocyanin-rich plants: biochemical basis and recent progress in health benefits studies. Mol Nutr Food Res. 2012;56(1):159-170.
5. Liobikas J, Skemiene K, Trumbeckaite S, Borutaite V. Anthocyanins in cardioprotection: a path through mitochondria [published online March 30, 2016]. Pharmacol Res. doi: 10.1016/j.phrs.2016.03.036.
6. United States Department of Agriculture. National Nutrient Database for Standard Reference, Release 28. https://ndb.nal.usda.gov/. Updated May 17, 2016.
7. Anthocyanins and anthocyanidins. Food-info website. http://www.food-info.net/uk/colour/anthocyanin.htm. Updated August 14, 2014. Accessed April 28, 2016.
8. Daunay MC, Janick J. History and iconography of eggplant. Chronica Horticulturae. 2007;47(3):16-22.
9. Colors, flavors, seasons & sizes. National Onion Association website. https://www.onions-usa.org/all-about-onions/colors-flavor-availability-and-sizes-of-onions
10. Gennaro L, Leonardi C, Esposito F, et al. Flavonoid and carbohydrate contents in Tropea red onions: effects of homelike peeling and storage. J Agric Food Chem. 2002;50(7):1904-1910.
11. Helm J. The year of purple: purple produce gains recognition from chefs, scientists and supermarkets. Nutritrion Unplugged website. www.nutritionunplugged.com/2013/05/the-year-of-purple-purple-produce-gains-recognition-from-chefs-scientists-and-supermarkets/. Published May 27, 2013. Accessed April 29, 2016.
12. The Top 5 specialty food trends at the 2016 Winter Fancy Food Show. Specialty Food Association website. www.specialtyfood.com/news/article/top-5-specialty-food-trends-2016-winter-fancy-food-show/. Published January 19, 2016.
13. Aoki H, Kuze N, Kato Y. Anthocyanins isolated from purple corn (Zea mays L.). Japan Food Chemical Research Foundation website. www.ffcr.or.jp/zaidan/ffcrhome.nsf/7bd44c20b0dc562649256502001b65e9/
14. Tsuda T, Horio F, Uchida K, Aoki H, Osawa T. Dietary cyanidin 3-O-beta-D-glucoside-rich purple corn color prevents obesity and ameliorates hyperglycemia in mice. J Nutr. 2003;133(7):2125-2130.
15. Potatoes reduce blood pressure in people with obesity and high blood pressure. American Chemical Society website. www.acs.org/content/acs/en/pressroom/newsreleases/2011/august/potatoes-reduce-blood-pressure-in-people-with-obesity-and-high-blood-pressure.html. Published August 31, 2011. Accessed April 30, 2016.
16. History of carrots — a brief summary & timeline. World Carrot Museum website. http://www.carrotmuseum.co.uk/history.html
17. Multi-colored cauliflower. Colorful Harvest website. http://www.colorfulharvest.com/Cauliflower.php
18. Karp D. Farmers markets: Stokes Purple is a sweet potato of mystery. Los Angeles Times. November 2, 2012. http://articles.latimes.com/2012/nov/02/food/la-fo-marketnews-online-20121102
19. Friesema F. What's in season at the farmers market: (the end of) purple brussels sprouts at Weiser Family Farms. LA Weekly. February 8, 2013. http://www.laweekly.com/restaurants/whats-in-season-at-the-farmers-market-the-end-of-purple-brussels-sprouts-at-weiser-family-farms-2894859#
20. What is GMO? Agricultural crops that have a risk of being GMO. The Non-GMO Project website. http://www.nongmoproject.org/learn-more/what-is-gmo/
21. Lohachoompol V, Srzednicki G, Craske J. The change of total anthocyanins in blueberries and their antioxidant effect after drying and freezing. J Biomed Biotechnol. 2004;2004(5):248-252.