Walnut

A walnut is the nut of any tree of the genus Juglans (Family Juglandaceae), particularly the Persian or English walnut, Juglans regia. It is used for food after being processed while green for pickled walnuts or after full ripening for its nutmeat. Nutmeat of the eastern black walnut from the Juglans nigra is less commercially available, as are butternut nutmeats from Juglans cinerea. The walnut is nutrient dense with protein and essential fatty acids.

Etymology

Etymologically, the word walnut derives from the Germanic wal- and Old English wealhhnutu, literally "foreign nut", wealh meaning "foreign" (wealh is akin to the terms Welsh and Vlach; see Walha).^[1]

Characteristics

Walnuts are rounded, single-seeded stone fruits of the walnut tree commonly used for the meat after fully ripening. Following full ripening, the removal of the husk reveals the wrinkly walnut shell, which is usually commercially found in two segments (three-segment shells can also form). During the ripening process, the husk will become brittle and the shell hard. The shell encloses the kernel or meat, which is usually 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.^[2]

Walnuts are late to grow leaves, typically not until more than halfway through the spring. They secrete chemicals into the soil to prevent competing vegetation from growing. Because of this, flowers or vegetable gardens should not be planted close to them.

Types

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. Numerous walnut cultivars have been developed commercially, which are nearly all hybrids of the English walnut.^[3]

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.

Production

Top 10 Walnut Producing Countries - 2012^[4]

Rank	Country	Production (Tonnes)
1	China	1,700,000
2	Iran	450,000
3	United States	425,820
4	Turkey	194,298
5	Mexico	110,605
6	Ukraine	96,900
7	India	40,000
8	Chile	38,000
9	France	36,425
10	Romania	30,546
—	World	3,282,398

The worldwide production of walnuts 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 walnuts in 2010; China was the world's largest producer of walnuts, with a total harvest of 1.06 million metric tonnes.^[5] The other major producers of walnuts were (in the order of decreasing harvest): Iran, United States, Turkey, Ukraine, Mexico, Romania, India, France and Chile.

The average worldwide walnut 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 farms in 2010 were in Romania, with yields above 23 metric tonnes per hectare.^[6]

The United States is the world's largest exporter of walnuts. The Sacramento and San Joaquin valleys of California produce 99 percent of the nation’s commercial English walnuts.

Storage

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 batch should be entirely discarded.^[2]

The ideal temperature for longest possible storage of walnuts 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, walnuts 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.^[2][7]

Freshly harvested raw walnuts offer the best color, flavor and nutrient density when with water content between 2 to 8 percent.

Food use

Walnut meats are available in two forms; in their shells or shelled. The meats can be as large as halves or any smaller portions that may happen during processing, candied or as an ingredient in other foodstuffs. Pickled walnuts that are the whole fruit can be savory or sweet depending on the preserving solution. Walnut butters can be homemade or purchased in both raw and roasted forms. All walnuts can be eaten on their own (raw, toasted or pickled) or as part of a mix such as muesli, or as an ingredient of a dish.

Walnut Whip, coffee and walnut cake, and pickled walnuts are three disparate examples.

Walnut oil is available commercially and is chiefly used as a food ingredient particularly in salad dressings. It has a low smoke point, which limits its use for frying.^[8][9]

Nutritional value

Walnut, English

Persian or English walnut, Juglans regia
Nutritional value per 100 grams
Energy	2,738 kJ (654 kcal)
Carbohydrates	13.71
Starch	0.06
Sugars lactose	2.61 0
Dietary fiber	6.7
Fat	65.21
Saturated	6.126
Monounsaturated	8.933
Polyunsaturated	47.174
Protein	15.23
Vitamins
Vitamin A equiv. beta-carotene lutein zeaxanthin	(0%) 1 μg (0%) 12 μg 9 μg
Vitamin A	20 IU
Thiamine (B1)	(30%) 0.341 mg
Riboflavin (B2)	(13%) 0.15 mg
Niacin (B3)	(8%) 1.125 mg
Pantothenic acid (B5)	(11%) 0.570 mg
Vitamin B6	(41%) 0.537 mg
Folate (B9)	(25%) 98 μg
Vitamin B12	(0%) 0 μg
Vitamin C	(2%) 1.3 mg
Vitamin D	(0%) 0 μg
Vitamin D	(0%) 0 IU
Vitamin E	(5%) 0.7 mg
Vitamin K	(3%) 2.7 μg
Trace metals
Calcium	(10%) 98 mg
Iron	(22%) 2.91 mg
Magnesium	(45%) 158 mg
Manganese	(163%) 3.414 mg
Phosphorus	(49%) 346 mg
Potassium	(9%) 441 mg
Sodium	(0%) 2 mg
Zinc	(33%) 3.09 mg
Other constituents
Water	4.07
Alcohol (ethanol)	0
Caffeine	0
Units μg = micrograms • mg = milligrams IU = International units
Percentages are roughly approximated using US recommendations for adults. Source: USDA Nutrient Database

Walnuts are a nutrient-dense food: 100 grams of walnuts contain 15.2 grams of protein, 65.2 grams of fat, and 6.7 grams of dietary fiber. The protein in walnuts provides many essential amino acids.

While English walnuts are the most common, their nutrient density and profile are significantly different from those of black walnuts. For example, the Omega-3 fatty acid content of English walnuts is approximately 4.5 times that of black walnuts.^[10][11]

Unlike most nuts that are high in monounsaturated fatty acids, walnut oil is composed largely of polyunsaturated fatty acids (47.2%), particularly alpha-linolenic acid (18:3n - 3; 9.1%) and linoleic acid (18:2n - 6; 38.1%). 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.^[12]

Non-food applications

Medicinal

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".^[13]

Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies,^[14] 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".^[15]

Medical benefits and claims

Raw walnuts contain (mostly insoluble) antioxidants.^[16][17][18] They also contain triglycerides of the n-3 fatty acid alpha-linolenic acid (ALA)^[19] which is effective in reducing heart risk.^[20]

Roasting reduces antioxidant quality.

Inks and dyes

The husks of the black walnut Juglans nigra are used to make an ink for writing and drawing. Walnut ink has good archival properties, and has been used by several great artists including Leonardo da Vinci and Rembrandt.^[21]

Walnut husks are used as a brown dye for fabric.^[22] Walnut dyes were used in classical Rome and in medieval Europe for dyeing hair.^[23]

Cleaning

The United States Army used ground walnut shells for the cleaning of aviation parts because it was inexpensive and non-abrasive. However, an investigation of a fatal Boeing CH-47 Chinook helicopter crash (September 11, 1982, in Mannheim, Germany) revealed that walnut grit clogged an oil port, leading to the accident and the discontinuation of walnut shells as a cleaning agent.^[24]

Chemical analysis

Compared to other tree nuts and ground nuts (almonds, peanuts and hazelnuts, pistachios, Brazil nuts, cashews, macadamias, and pecans), walnuts (particularly in their raw form) contain the highest level and potency of antioxidants, including free antioxidants and antioxidants bound to fiber.^[17]

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^[25] and regiolone^[26]) have been identified in walnut husks by using reverse-phase high-performance liquid chromatography or crystallography.

Walnuts also contain the ellagitannin pedunculagin.^[27]

(−)-Regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem-bark of J. regia.^[28]

Uses in Chinese culture

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.^[29] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du he tao.^[30]

Cultivars

See also

• List of ineffective cancer treatments
• Nut (fruit)
• Pickled walnuts
• Walnut ink

References

Further reading

• Ramos, David D. (1997). Walnut Production Manual. UCANR. ISBN 978-1-879906-27-3. 
• Smith, Jane S. (2009). The Garden of Invention Luther Burbank and the Business of Breeding Plants. Penguin Books. ISBN 978-0143116899. 

External links

Wikimedia Commons has media related to Walnuts.

• Walnut at the Encyclopedia of Life
• Database of walnut research reports, from UC Davis

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