A walnut is an edible seed of any tree of the genus Juglans, especially the Persian or English walnut, Juglans regia. Broken nutmeats of the eastern black walnut from the tree Juglans nigra are also commercially available in small quantities, as are foods prepared with butternut nutmeats from Juglans cinerea.
Walnut seeds are a high density source of nutrients, particularly proteins and essential fatty acids. Walnuts, like other tree nuts, must be processed and stored properly. Poor storage makes walnuts susceptible to insect and fungal mold infestations; the latter produces aflatoxin—a potent carcinogen. A mold infested walnut seed batch should not be screened and then consumed; the entire batch should be discarded.
Walnuts are rounded, single-seeded stone fruits of the walnut tree. The walnut fruit is enclosed in a green, leathery, fleshy husk. This husk is inedible. After harvest, the removal of the husk reveals the wrinkly walnut shell, which is in two halves. This shell is hard and encloses the kernel, which is also made up of two halves separated by a partition. The seed kernels — commonly available as shelled walnuts — are enclosed in a brown seed coat which contains antioxidants. The antioxidants protect the oil-rich seed from atmospheric oxygen thereby preventing rancidity.
The two most common major species of walnuts are grown for their seeds — the Persian or English Walnut and the Black Walnut. The English Walnut (J. regia) originated in Persia, and the Black Walnut (J. nigra) is native to eastern North America. The Black walnut is of high flavor, but due to its hard shell and poor hulling characteristics it is not grown commercially for nut production. The commercially produced walnut varieties are nearly all hybrids of the English walnut.
Other species include J. californica, the California Black Walnut (often used as a root stock for commercial breeding of J. regia), J. cinerea (butternuts), and J. major, the Arizona Walnut.
Walnuts are late to grow leaves, typically not until more than halfway through the spring. They also secrete chemicals into the soil to prevent competing vegetation from growing. Because of this, flowers or vegetable gardens should not be planted too close to them.
The husks of walnut contain a juice that will readily stain anything it comes into contact with. It has been used as a dye for cloth.
The worldwide production of walnut seeds has been increasing rapidly in recent years, with the largest increase coming from Asia. The world produced a total of 2.55 million metric tonnes of walnut seeds in 2010; China was the world's largest producer of walnut seeds, with a total harvest of 1.06 million metric tonnes. The other major producers of walnut seeds were (in the order of decreasing harvest): Iran, United States, Turkey, Ukraine, Mexico, Romania, India, France and Chile.
The average worldwide walnut seed yield was about 3 metric tonnes per hectare, in 2010. Among the major producers, eastern European countries have the highest yield. According to the FAO, the most productive walnut seed farms in 2010 were in Romania, with yields above 23 metric tonnes per hectare.
The ideal temperature for longest possible storage of walnut seeds is in the -3 to 0 oC and low humidity — for industrial and home storage. However, such refrigeration technologies are unavailable in developing countries where walnuts are produced in large quantities; there, walnut seeds are best stored below 25 oC and low humidity. Temperatures above 30 oC, and humidities above 70 percent can lead to rapid and high spoilage losses. Above 75 percent humidity threshold, fungal molds that release dangerous aflatoxin can form.
Freshly harvested raw walnut seeds with water content between 2 to 8 percent offer the best color, flavor and nutrient density.
|Persian or English walnut, Juglans regia|
|Nutritional value per serving|
|Serving size||100 grams|
|Energy||2,738 kJ (654 kcal)|
|- Dietary fiber||6.7|
|Vitamin A equiv.||1 μg (0%)|
|Vitamin A||20 IU|
|- beta-carotene||12 μg (0%)|
|- lutein and zeaxanthin||9 μg|
|Thiamine (vit. B1)||0.341 mg (30%)|
|Riboflavin (vit. B2)||0.15 mg (13%)|
|Niacin (vit. B3)||1.125 mg (8%)|
|Pantothenic acid (B5)||0.570 mg (11%)|
|Vitamin B6||0.537 mg (41%)|
|Folate (vit. B9)||98 μg (25%)|
|Vitamin B12||0 μg (0%)|
|Vitamin C||1.3 mg (2%)|
|Vitamin D||0 μg (0%)|
|Vitamin D||0 IU (0%)|
|Vitamin E||0.7 mg (5%)|
|Vitamin K||2.7 μg (3%)|
|Calcium||98 mg (10%)|
|Iron||2.91 mg (22%)|
|Magnesium||158 mg (45%)|
|Manganese||3.414 mg (163%)|
|Phosphorus||346 mg (49%)|
|Potassium||441 mg (9%)|
|Sodium||2 mg (0%)|
|Zinc||3.09 mg (33%)|
|Percentages are roughly approximated
using US recommendations for adults.
Source: USDA Nutrient Database
While English walnut is the predominant commercially distributed nut because of the ease of its processing, its nutrient density and profile is significantly different from black walnut. The table below compares some of the major nutrients between English and Black walnuts.
|Nutrient (per 100 gram)||English walnut seed||Black walnut seed|
|Unsaturated fatty acids (g)||56.1||50.1|
|Poly to mono unsaturated
fatty acids ratio
|Vitamin B-6 (mg)||0.54||0.58|
Unlike most nuts that are high in monounsaturated fatty acids, walnut oil is composed largely of polyunsaturated fatty acids (47.2 grams), particularly alpha-linolenic acid (18:3n - 3; 9.1 gram) and linoleic acid (18:2n - 6; 38.1 gram). The beneficial effects of this unique fatty acid profile have been a subject of many studies and discussions. Banel and Hu concluded in 2009 that while walnut-enhanced diets are promising in short term studies, longer term studies are needed to ascertain better insights.
Medical benefits and claims
Raw walnuts contain glyceryl triacylates of the n-3 fatty acid alpha-linolenic acid (ALA), which is not as effective in humans as long-chain n-3 fatty acids, and (mostly insoluble) antioxidants. Roasting reduces antioxidant quality.
Black walnut has been promoted as a potential cancer cure, on the basis it kills a "parasite" responsible for the disease. However, according to the American Cancer Society, "available scientific evidence does not support claims that hulls from black walnuts remove parasites from the intestinal tract or that they are effective in treating cancer or any other disease".
Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies, a kind of alternative medicine promoted for its effect on health. However according to Cancer Research UK, "there is no scientific evidence to prove that flower remedies can control, cure or prevent any type of disease, including cancer".
Compared to certain other nuts, such as almonds, peanuts and hazelnuts, walnuts (especially in their raw form) contain the highest total level of antioxidants, including both free antioxidants and antioxidants bound to fiber.
To remove the husk from kernel can lead to hand staining. Walnut hulls contain phenolics that stain hands and can cause skin irritation. Seven phenolic compounds (ferulic acid, vanillic acid, coumaric acid, syringic acid, myricetin, juglone and regiolone) have been identified in walnut husks by using reverse-phase high-performance liquid chromatography or crystallography.
The United States Air Force used to rely on ground walnut shells for cleaning aviation parts because it was an inexpensive and non-abrasive material. However, an investigation of a fatal Boeing CH-47 Chinook crash which occurred on September 11, 1982 in Mannheim, Germany, revealed that the accident was caused by an oil port clogged with walnut grit, leading to the discontinuation of walnuts shells as a cleaning agent.
Investment in China
In China, pairs of walnuts have traditionally been rotated and played with in the palm of the hand, both as a means to stimulate blood circulation and as a status symbol. Pairs of large, old and symmetrically shaped walnuts are valued highly and have recently been used as an investment, with some of them fetching tens of thousands of dollars. Pairs of walnuts are also sometimes sold still in their green skin, as a form of gambling known as du he tao.
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- "Status-conscious investors shell out on great walnuts of China". Reuters. Aug 28, 2012.
- "Game of clones". Global Times. 16 October 2012.
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