April/May 2022 Issue
Hail to Kale
By Laura M. Ali, MS, RDN, LDN
Vol. 24, No. 4, P. 42
Learn about kale’s history, health benefits, and culinary uses.
If you asked clients when they first became familiar with kale, many might say they remember seeing it as a decoration on salad bars or as a garnish on catering platters. Kale has been available for purchase in the produce section of supermarkets for years, but not too many people bought it. If they did, most would describe it as a bitter green and not one of their favorite vegetables. But that all changed about 10 years ago when kale seemingly reached superstar status.
Bon Appétit magazine proclaimed 2012 “The Year of Kale,” and the vegetable was suddenly on the lips—and in the mouths of—consumers. It didn’t hurt that the owner of the public relations firm My Young Auntie launched the fictitious American Kale Association, which started promoting the virtues of kale. By the end of the year, the kale chips craze was well underway, enabling kale to claim its status as a so-called “superfood.”1
Today, kale still is a popular vegetable with more than 89 million lbs sold in 2020, an 11% increase from 2019, according to the latest data from the Produce Marketing Guide.2
As a nod to kale, this article reviews its history, varieties, nutrient profile, and culinary uses, as well as provides recommendations for RDs when counseling clients.
Kale is a cultivar of the Brassica oleracea species, which includes cabbage, Brussels sprouts, broccoli, cauliflower, and kohlrabi. Within the species is the acephala (“without a head”) group; unlike cabbage, broccoli, and other vegetables that have a center head or clusters of flowers, kale plants are composed of large stalks and leaves but no head, placing them in the acephala group.
Having been cultivated for more than 2,000 years, kale is one of the oldest forms of cabbage and is native to the region surrounding the eastern Mediterranean and modern-day Turkey. Ancient Greeks and Romans grew and ate kale. Its popularity increased throughout Europe during the Middle Ages, making it one of the most widely consumed vegetables in the continent.3
It’s uncertain when kale debuted in the United States, but credit is given to David Fairchild, an agricultural explorer for the USDA, for introducing it to farmers in the 19th century—although many believe the early settlers grew it long before then.3
Because kale is an easy plant to grow and can flourish in a variety of climates, Europeans have been eating it for centuries, especially in the Mediterranean region and northern European countries, including Scotland and Germany. In fact, kale is such an important part of Scottish cuisine that the word for kale is kali, which also means “food.” In Scotland, a common saying describing someone who isn’t feeling well enough to eat is that they’re “off their kale.”3,4
Some of the common kale dishes throughout Europe and Asia include the following:
• rumbledethumps, a traditional Scottish dish, similar to the Irish colcannon, combining mashed potatoes and turnips with finely sliced kale and baked with cheese on top;
• Grünkohl mit Pinkel, a German kale and sausage dish common in the northern region, served after a winter hike known as a Kohlfahrt (“kale tour” or “cabbage walk”);
• caldo verde, a Portuguese soup combining kale with potatoes and Portuguese sausage;
• Tuscan kale, an Italian favorite, sautéed with pancetta and olive oil and served as a side dish or used to make the Tuscan stew ribollita; and
• stir-fried kale with chopped garlic and oyster sauce, a popular side dish in Asia.
Botany and Agriculture
Kale’s diverse culinary uses spring from its many varieties. Cornell University lists more than 50 types of kale, but five are among the most commonly found in US supermarkets: curly or dwarf blue Scotch kale, Tuscan or lacinato kale, red or red Russian kale, Redbor kale, and baby kale.5 Most of the latter are cultivars of the Brassica oleracea species, while red Russian kale derives from the Brassica napus species.
Most Americans are familiar with curly kale, which is probably best known for garnishing plates and catering platters. It’s often considered tough and bitter tasting; however, if massaged and rubbed with oil and seasonings, it becomes milder and somewhat sweet.
Curly kale originated in central and northern Europe but is grown throughout the United States. It requires full to partial sunlight and well-drained soil, and grows 1 to 2 feet in height. It’s considered a winter vegetable and typically is harvested in late fall/early winter or spring. Curly kale primarily is used in soups, stews, and casseroles but is becoming more common in salads and other raw dishes.
Tuscan or lacinato kale, which has a dark green color with smooth, flat leaves, likely originated in the Mediterranean. It has a scalelike appearance, giving it the nickname “dinosaur kale.” More heat, drought, and cold tolerant than other types of kale, this variety grows best when planted in an area with full sunlight and well-drained soil. It’s considered a low-maintenance vegetable and can grow up to 3 ft in height.
Red Russian Kale
Red Russian kale, characterized by its smooth, dark leaves with purple veins and a purple stem, has a sweeter flavor than other varieties and often is used in salads and other raw dishes. It’s a versatile plant that thrives in cold weather but with care and proper watering also can grow in more temperate climates. It matures 50 to 60 days after planting and can grow 1.5 to 3 feet in height.
Redbor kale is similar in appearance to curly kale, with curly, frilly leaves, but carries a signature deep purple color throughout its body. It grows straight on tall stems that can reach 4 feet in height and thrives in cooler temperatures. Redbor kale typically is used to make soups or is massaged with olive oil and vinegar to add color to salads.
With immature leaves, baby kale is the younger version of kale. The most common variety found in supermarkets is young curly kale, but consumers can find other varieties such as baby lacinato kale in specialty markets. While kale plants are harvested when they reach full maturity in 50 to 70 days, baby kale is harvested earlier, as soon as 20 to 30 days after planting. Because the leaves are young when picked, they yield a tenderer, sweeter product that’s more suitable for smoothies and salads.
As mentioned, kale is simple to grow and can be planted in traditional garden beds, raised beds, or container gardens. The conditions under which kale is grown influence the flavor and texture of the harvested plant.
Most kale plants are grown from seed and produce a strong taproot. Nitrogen-rich, slightly acidic, and loamy soil will yield the best color and a nice tender leaf. Using compost to improve the soil’s nitrogen content and placing a layer of mulch on top will keep the soil moist, control weeds, and keep the plant cool.6,7
The Old Farmer’s Almanac recommends planting kale three months before the region’s first fall frost date. Kale is best grown in cooler temperatures with at least six hours of direct sunlight, but it can survive in warmer weather with enough water and slight shade. Cooler temperatures produce a sweeter-flavored, less bitter leaf; it’s recommended to pick the leaves after the first frost or a light snowfall if planted in the fall. The exception is Tuscan or lacinato kale, which is tolerant of heat and drought conditions as well as cooler weather and is inherently less bitter than other varieties regardless of where it’s grown. Mature kale is ready for harvesting when the leaf is the size of a human hand. It’s recommended to pick the outer leaves—no more than one-third of the plant—and allow the center leaves to continue to grow, which enables multiple harvests.7
After the harvest, it’s best to store kale in a loose plastic bag in the refrigerator for no more than one week. Once the leaves begin to turn yellow, the flavor begins to deteriorate and become bitter.7 To prevent waste and extend its life, blanch kale by submerging it in boiling water for two minutes and then in an ice bath, and dry it. Afterward, freeze it in portions for later use in soups, stews, and casseroles.
Nutrition and Health Benefits
Choosing the variety of kale to suit one’s taste buds and learning how to prepare it is a good way for clients to get many of the vitamins and minerals they need. A 1-cup serving of raw kale contains approximately 9 kcal, 0.6 g protein, less than 0.5 g fat, 0.8 g fiber, and 52 mg calcium. Plus, raw kale is an excellent source of vitamin C and vitamin K, and is one of the richest sources of the carotenoids beta-carotene, lutein, and zeaxanthin.8 Kale contains 40 mg of lutein and zeaxanthin per 100 g, compared with only 1 g per 100 g of yellow and orange fruits and vegetables.8,9
Cooking kale provides an opportunity for even greater nutrient intake; a 1-cup serving of cooked kale contains approximately 50 kcal, 3.5 g protein, 1.5 g fat, and 177 mg calcium. At this volume, cooked kale is a good source of vitamin A and an excellent source of vitamin C, providing more than 400% of the DV for vitamin K.8 It also includes most B vitamins, except vitamin B12, and has among the highest density of zinc, iron, potassium, and phosphorus of all vegetables in the Brassica genus, including broccoli, cauliflower, arugula, Swiss chard, cabbage, and radishes.9
Calcium from leafy green vegetables usually isn’t highly absorbable, but the calcium in kale is more bioavailable due to its low oxalate content and high ratio of calcium to oxalate compared with vegetables such as spinach.9
Limited research is available on the vitamin and mineral content of kale at various maturity levels, eg, baby vs mature kale. However, evidence suggests there are few differences apart from vitamin C content, which appears to be higher in more mature plants. Conversely, protein levels seem to be slightly higher in younger plants.6
Of note, kale’s high vitamin K content can pose a concern for individuals taking anticoagulant (blood-thinning) medications, since vitamin K plays a role in helping the blood to clot. General recommendations for those taking anticoagulants are to eat a consistent amount of leafy greens daily and have their physicians adjust their medication dose in accordance with their intake of vitamin K–rich foods.10
Another concern about kale has been its content of goitrogen, a type of antinutrient. Goitrogens prevent iodine from entering the thyroid gland and are a concern only for people with an iodine deficiency. This can be remedied by cooking kale, which seems to destroy the enzyme responsible for its goitrogenic activity.11
Cooking With Kale
Much research has shown that diets rich in cruciferous vegetables, including kale, may help reduce the risk of cancer and heart disease as well as support brain health.12,13 This may be attributed to different cooking methods that impact the bioavailability of certain compounds and the loss or preservation of antioxidants and vitamins.14 It’s also important to keep in mind how the various cooking methods affect the flavor of the final product.
Boiling has been the primary preparation method for kale throughout history, which may explain why many traditional kale recipes involve boiling the vegetable and combining it with other foods, such as potatoes and sausage. Boiling breaks down the fibrous tissues, tenderizing the product, but it results in the loss of vitamin C and, to a much lesser extent, beta-carotene.15
The good news is that kale is versatile, allowing it to be prepared in a variety of ways, including the following:
• Steaming kale leaves better preserves the antioxidants they contain compared with boiling and has the added benefit of enhancing the flavor profile.14 Steaming similar vegetables such as broccoli and cabbage has been shown to retain more of the water-soluble vitamins and antioxidants in the product while producing a tender vegetable.16
• Microwaving kale with a little water has many of the same effects as steaming. It preserves the nutrient profile and produces a tenderer, sweeter product.14
• Stir-frying, during which kale has minimal contact with the heat source, appears to be the best cooking method to retain the most antioxidants and bioactive compounds. Stir-frying kale also releases more of the sweet notes, providing a more pleasant eating experience.14
• Chopping, cutting, or chewing breaks open the cell walls of raw kale, activating and making bioavailable the phytochemical sulforaphane, which research suggests may have anticarcinogenic effects.17 It’s often recommended to massage raw kale leaves with olive oil or a dressing until they begin to turn a deep green before chopping or slicing. This tenderizes and infuses the leaves with the flavors of the oil or dressing, replaces some of the bitter-tasting compounds with sweetness, and improves absorption of carotenoids and other fat-soluble compounds.18,19
Recommendations for RDs
Kale is one of many cruciferous vegetables with health benefits, and there are many varieties from which to choose that have different flavor profiles. Clients can enjoy kale in a variety of ways, from raw to cooked to dried. While it’s often thought of as a bitter vegetable, learning about the different varieties available and preparation methods to glean the best flavor will help RDs encourage clients to try it.
For individuals who aren’t familiar with preparing this vegetable, suggest they start with baby kale, which has a sweeter flavor. They also can shred mature kale and add it to stews, soups, or omelets to boost nutrition and experiment with a myriad of easy recipes. The bottom line is that kale is a nutritious cruciferous vegetable with many health benefits that clients can enjoy.
— Laura Ali, MS, RDN, LDN, is a culinary nutritionist and food and nutrition communications consultant in Pittsburgh, and author of the new book The MIND Diet for Two. She works with consumers and health-focused companies to develop simple, delicious ways to incorporate healthful foods into everyday meals. Follow her on Instagram and Twitter @LauraAli_RD and at her blog at lauramali.com.
1. Mendoza R. Here’s why kale became so popular. 365 Days of Kale website. http://www.365daysofkale.com/heres-why-kale-became-so-popular. Published May 9, 2020. Accessed January 12, 2022.
2. Commodity: kale. Produce Marketing Guide website. https://www.producemarketguide.com/produce/kale. Accessed January 17, 2022.
3. Kale. Specialty Produce website. https://www.specialtyproduce.com/produce/Kale_7631.php. Accessed January 13, 2022.
4. Kale: the fascinating culinary history of today’s trendiest vegetable. University of Vermont Medical Center website. https://medcenterblog.uvmhealth.org/wellness/recipes-wellness/kale-history-recipe/. Published October 27, 2017. Accessed January 12, 2022.
5. Browse varieties. Cornell University website. http://vegvariety.cce.cornell.edu/main/showVarieties.php?searchCriteria=kale&searchIn=1&crop_id=0&sortBy=overallrating&order=DESC. Accessed January 14, 2022.
6. Šamec D, Urlić B, Salopek-Sondi B. Kale (Brassica oleracea var. acephala) as a superfood: review of the scientific evidence behind the statement. Crit Rev Food Sci Nutr. 2019;59(15):2411-2422.
7. Boeckmann C. Planting, growing, and harvesting kale. Almanac website. https://www.almanac.com/plant/kale. Accessed January 17, 2022.
8. FoodData Central. US Department of Agriculture, Agricultural Research Service website. https://fdc.nal.usda.gov/index.html. Updated April 1, 2019.
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15. Sikora E, Bodziarczyk I. Composition and antioxidant activity of kale (Brassica oleracea L. var. acephala) raw and cooked. Acta Sci Pol Technol Aliment. 2012;11(3):239-248.
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17. Bouranis JA, Beaver LM, Ho E. Metabolic fate of dietary glucosinolates and their metabolites: a role for the microbiome. Front Nutr. 2021;8:748433.
18. Loh A. How to massage kale. Eating Well website. https://www.eatingwell.com/article/7928293/how-to-massage-kale/. Published November 15, 2021. Accessed January 12, 2022.
19. White WS, Zhou Y, Crane A, Dixon P, Quadt F, Flendrig LM. Modeling the dose effects of soybean oil in salad dressing on carotenoid and fat-soluble vitamin bioavailability in salad vegetables. Am J Clin Nutr. 2017;106(4):1041-1051.