Walnut

A walnut is the nut of any tree of the genus Juglans (Family Juglandaceae), particularly the Persian or English walnut, Juglans regia.

This article is about walnuts as food. For the genus, see Juglans. For walnut wood (timber), see Juglans § Wood. For other uses, see Walnut (disambiguation).
The common walnut in growth
California black walnut in growth.
Inside of a walnut in growth
Walnut shell inside its green husk

A walnut is the edible seed of a drupe, and thus not a true botanical nut. It is commonly consumed as a nut. After full ripening for its edible seed when the shell has been discarded, it is used as a garnish or a snack. Nuts of the eastern black walnut (Juglans nigra) and butternuts (Juglans cinerea) are less commonly consumed.

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 or four-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.[1]

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 Iran (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.[2]

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. Other sources list J. californica californica as native to southern California, and Juglans californica hindsii, or just J. hindsii, as native to northern California; in at least one case these are given as "geographic variants" instead of subspecies (Botanica).

Production
Walnuts – 2017
Country Production
(millions of tonnes)
 China
1.92
 United States
0.57
 Iran
0.35
 Turkey
0.21
 Mexico
0.15
 Ukraine
0.11
World
3.8
Source: FAOSTAT of the United Nations[3]

In 2017, world production of walnuts (in shell) was 3.8 million tonnes, with China contributing 51% of the total (table). Other major producers (in the order of decreasing harvest) were the United States, Iran, and Turkey.

In 1998, walnuts from California accounted for 99% of all walnuts grown in the United States, and for 38% of the world's production of walnuts.[4]

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.[1]

The ideal temperature for the longest possible storage of walnuts is −3 to 0 °C (27 to 32 °F) with 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 °C (77 °F) with low humidity. Temperatures above 30 °C (86 °F), 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.[1][5]

Food use
Walnuts in their shells available for sale in a supermarket in the United States.

Walnut meats are available in two forms; in their shells or shelled. The meats may be whole, halved, or in smaller portions due to processing. 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: e.g. walnut soup, walnut pie, walnut coffee cake, banana cake, brownie, fudge. Walnuts are often candied, or pickled. 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.

Walnuts may be used as an ingredient in other foodstuffs. Walnut is the main ingredient in baklava, Circassian chicken, chicken in walnut sauce, and poultry or meat ball stew from Iranian cuisine.

Walnuts are also popular as ice cream toppings, and walnut pieces are used as a garnish on some foods.[6]

Nocino is a liqueur made from unripe green walnuts steeped in alcohol with syrup added.

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.[7][8]

Nutritional value
Walnut, English
Walnut kernel, halves
Nutritional value per 100 grams
Energy2,738 kJ (654 kcal)
13.71
Starch0.06
Sugars2.61
Dietary fiber6.7
65.21
Saturated6.126
Monounsaturated8.933
Polyunsaturated47.174
15.23
VitaminsQuantity %DV†
Vitamin A equiv.
0%
1 μg
0%
12 μg
9 μg
Vitamin A20 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 E
5%
0.7 mg
Vitamin K
3%
2.7 μg
MineralsQuantity %DV†
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 constituentsQuantity
Water4.07 g
Link to Full USDA Database Entry
†Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database

Walnuts without shells are 4% water, 15% protein, 65% fat, and 14% carbohydrates, including 7% dietary fiber (table). In a 100-gram reference serving, walnuts provide 2,740 kilojoules (654 kcal) and rich content (20% or more of the Daily Value or DV) of several dietary minerals, particularly manganese at 163% DV, and B vitamins (table).

While English walnuts are the most commonly consumed, their nutrient density and profile are generally similar to those of black walnuts.[9][10]

Unlike most nuts that are high in monounsaturated fatty acids, walnut oil is composed largely of polyunsaturated fatty acids (72% of total fats), particularly alpha-linolenic acid (14%) and linoleic acid (58%), although it does contain oleic acid as 13% of total fats.[9]

Health claims

In 2016, the US Food and Drug Administration (FDA) provided a Qualified Health Claim allowing products containing walnuts to state: "Supportive but not conclusive research shows that eating 1.5 ounces (43 g) per day of walnuts, as part of a low saturated fat and low cholesterol diet and not resulting in increased caloric intake, may reduce the risk of coronary heart disease."[11] The FDA had, in 2004, refused to authorize the claim that "Diets including walnuts can reduce the risk of heart disease"[12] and had sent an FDA Warning Letter to Diamond Foods in 2010 stating there is "not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease."[13] A recent systematic review assessing the effect of walnut supplementation on blood pressure found insufficient evidence to support walnut consumption as a BP-lowering strategy.[14]

Non-food applications

Folk medicine

Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies,[15] a herbal remedy promoted in folk medicine practices for its supposed effect on health. 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".[16]

Inks and dyes
Further information: Walnut ink

Walnut husks can be used to make a durable ink for writing and drawing. It is thought to have been used by artists including Leonardo da Vinci and Rembrandt.[17]

Walnut husk pigments are used as a brown dye for fabric[18] as once applied in classical Rome and medieval Europe for dyeing hair.[19]

Cleaning

The United States Army once used ground walnut shells for abrasive blasting to clean aviation parts because of low cost and non-abrasive qualities. 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.[20] Commercially, crushed walnut shells are still used outside of aviation for low-abrasive, less-toxic cleaning and blasting applications.[21]

A three-segment shell which occurs rarely.

Phytochemicals

Walnut hulls contain diverse phytochemicals, such as polyphenols that stain hands and can cause skin irritation. Seven phenolic compounds, including ferulic acid, vanillic acid, coumaric acid, syringic acid, myricetin, and juglone were identified in walnut husks. Juglone, the predominant phenolic, was found in concentrations of 2-4% fresh weight.[22]

Walnuts also contain the ellagitannin pedunculagin.[23] Regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem bark of J. regia.[24]

Chinese culture

Large, symmetrically shaped, and sometimes intricately carved walnut shells (mainly from J. hopeiensis ) are valued collectibles in China where they are rotated in the hand as a plaything or as decoration. They are also an investment and status symbol, with some carvings having high monetary value if unique.[25] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du qing pi.[26]

Cultivars
  • Walnut in shell and a nutcracker utensil used to break the shell
  • Video of cracking a walnut
  • Walnuts as a snack
  • A large serving of shelled walnuts
  • Kuku sabzi garnished with a walnut, an Iranian snack
  • Applesauce coffee cake garnished with walnuts
  • Walnuts as collectibles
gollark: That's probably the main problem - nobody really wants a standard.
gollark: Ideally stackable, too.
gollark: Just standardize a form factor with however-many-volt terminals and probably a serial link for communicating with a control computer.
gollark: With decent interfaces.
gollark: I don't see why they couldn't be.

See also

References
  1. "Walnut; Agriculture – Transport Information Service". Association for German Insurance. 2010. Archived from the original on 2015-02-14.
  2. "Commodity Profile: English Walnuts" (PDF). AgMRC, University of California. 2006. Archived (PDF) from the original on 2012-03-12.
  3. "Walnut (in shell) production in 2017, Crops/Regions/World list/Production Quantity (pick lists)". UN Food and Agriculture Organization, Corporate Statistical Database (FAOSTAT). 2018. Retrieved 9 May 2019.
  4. Ramos, Dave, "Crop Profile for Walnuts in California" (PDF), ucanr.edu, University of California Agricultural and Natural Resources, p. 1, retrieved 14 December 2019
  5. "Food, Nutrition & Agriculture – Prevention of aflatoxin". FAO, United Nations. 1998. Archived from the original on 2011-03-07.
  6. Forsberg, B.; Joan Clark-Warner, M.S.R.D.C.D.E.; Beale, L. (2004). The Complete Idiot's Guide to Terrific Diabetic Meals. DK Publishing. p. 98. ISBN 978-1-61564-486-5. Archived from the original on March 31, 2018. Retrieved May 27, 2017.
  7. "Walnut oil recipes". BBC. Archived from the original on 17 February 2014. Retrieved 3 July 2014.
  8. Turner, Lisa. "Oil Change". Better Nutrition. Archived from the original on 6 July 2014. Retrieved 5 July 2014.
  9. "Nutrition facts: Nuts, walnuts, English dried per 100 g". Condé Nast. Archived from the original on 5 July 2014. Retrieved 4 July 2014.
  10. "Nutrition facts: Nuts, walnuts, black, dried per 100 g". Condé Nast. Archived from the original on 5 July 2014. Retrieved 4 July 2014.
  11. "Qualified Claims About Cardiovascular Disease Risk: Walnuts & Heart Disease Docket No. 02P-0292". US Food and Drug Administration. 9 March 2004. Archived from the original on 27 October 2016. Retrieved 1 November 2016.
  12. Laura M. Tarantino (9 March 2004). "Qualified Health Claims: Letter of Enforcement Discretion – Walnuts and Coronary Heart Disease (Docket No 02P-0292)". US Food and Drug Administration, Labeling and Nutrition. Archived from the original on 7 May 2017. Retrieved 30 April 2017.
  13. Wagner, Roberta (22 February 2010). "FDA Warning Letter to Diamond Food, Inc". US Food and Drug Administration, Inspections, Compliance, Enforcement, and Criminal Investigations. Archived from the original on 12 January 2017. Retrieved 1 November 2016. the evidence supporting a relationship between walnuts and coronary heart disease is related to the omega-3 fatty acid content of walnuts. There is not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease. Therefore, the above statement is an unauthorized health claimCS1 maint: BOT: original-url status unknown (link)
  14. Li J, Jiang B, O Santos H, Santos D, Singh A, Wang L. Effects of walnut intake on blood pressure: A systematic review and meta-analysis of randomized controlled trials. Phytother Res. 2020. doi: 10.1002/ptr.6740. Epub ahead of print. PMID: 32510725.
  15. D. S. Vohra (1 June 2004). Bach Flower Remedies: A Comprehensive Study. B. Jain Publishers. p. 3. ISBN 978-81-7021-271-3. Archived from the original on 31 December 2013. Retrieved 2 September 2013.
  16. "Flower remedies". Cancer Research UK. 26 January 2015. Archived from the original on 11 September 2013. Retrieved 1 September 2013.
  17. "Black Walnut Ink Workshop". Guild of Natural Science Illustrators. October 2002. Archived from the original on 9 October 2014. Retrieved 3 July 2014.
  18. "The Colors of Invention – How to Dye Fibers Naturally". Smithsonian Museum. November 13–16, 1997. Archived from the original on 21 October 2014. Retrieved 3 July 2014.
  19. Sherrow, Victoria (2006). Encyclopedia of Hair: A Cultural History. Greenwood Publishing Group. pp. 267, 355. ISBN 978-0-313-33145-9. Retrieved 3 July 2014. walnut.
  20. "In Re Air Crash Disaster at Mannheim Germany on 9/11/82. Ursula J. Schoenborn, As Executrix of the Estate of Leonedward Schoenborn, Deceased, v. the Boeing Company. Appeal of the Boeing Company. United States Court of Appeals, Third Circuit. 769 F.2d 115". Justia. 1985. Archived from the original on 17 May 2014. Retrieved 10 April 2014.
  21. "OSHA fact sheet addresses abrasive blasting hazards". Retrieved 2018-08-20.
  22. Cosmulescu, Sina Niculina; Trandafir, Ion; Achim, Gheorghe; Botu, Mihai; Baciu, Adrian; Gruia, Marius (15 June 2010). "Phenolics of Green Husk in Mature Walnut Fruits". Notulae Botanicae Horti Agrobotanici Cluj-Napoca. 38 (1): 53–56. doi:10.15835/nbha3814624 (inactive 2020-01-22). ISSN 1842-4309. Archived from the original on 29 July 2017. Retrieved 11 October 2016.
  23. Metabolism of Antioxidant and Chemopreventive Ellagitannins from Strawberries, Raspberries, Walnuts, and Oak-Aged Wine in Humans: Identification of Biomarkers and Individual Variability. Begoña Cerdá, Francisco A. Tomás-Barberán, and Juan Carlos Espín, J. Agric. Food Chem., 2005, 53 (2), pages 227–235, doi:10.1021/jf049144d
  24. (−)-Regiolone, an α-tetralone from Juglans regia: structure, stereochemistry and conformation. Sunil K. Talapatra, Bimala Karmacharya, Shambhu C. De and Bani Talapatra, Phytochemistry, Volume 27, Issue 12, 1988, pages 3929–3932, doi:10.1016/0031-9422(88)83047-4
  25. Jason Subler, Jane Lanhee Lee (Aug 28, 2012). "Status-conscious investors shell out on great walnuts of China". Reuters. Archived from the original on 2015-11-14.CS1 maint: uses authors parameter (link)
  26. "Game of clones". Global Times. 16 October 2012. Archived from the original on 27 October 2012.
  27. "Walnut Cultivar Table". Fruit and Nut Information Center, Department of Plant Sciences, University of California, Davis. 2018. Archived from the original on 8 February 2018. Retrieved 6 March 2018.
  28. Le Verger Francais tomme II Chapitre I Fruits Locaux & Regionaux 1948
  29. Gale McGranahan, Charles Leslie (8 December 2009). "Walnut tree named 'Ivanhoe'; Patent number USPP21718P2". US Patent and Trade Office (Google Patents). Archived from the original on 2 February 2017.CS1 maint: uses authors parameter (link)
  30. Le Verger Francais, Tome 1 Catalogue Descriptif des Fruits Adoptes 1947

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 978-0-14-311689-9.
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