WalnutsInside of a walnut in growthThree-segment walnutWalnut shell inside its green huskArtistic depiction of two walnuts (Adriaen Coorte, 1702)
A walnut is the edible seed of any tree of the genus Juglans (family Juglandaceae), particularly the Persian or English walnut, Juglans regia. They are accessory fruit because the outer covering of the fruit is technically an involucre and thus not morphologically part of the carpel; this means it cannot be a drupe but is instead a drupe-like nut.
After full ripening, the shell is discarded, and the kernel is eaten. Nuts of the eastern black walnut (Juglans nigra) and butternuts (Juglans cinerea) are less commonly consumed.
Description
Walnuts are the round, single-seed stone fruits of the walnut tree. They ripen between September and November in the northern hemisphere. The brown, wrinkly walnut shell is enclosed in a husk.[1] Shells of walnuts available in commerce usually have two segments (but three or four-segment shells can also form). During the ripening process, the husk becomes brittle and the shell hard. The shell encloses the kernel or meat, which is usually in two halves separated by a membranous partition.[1] 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, preventing rancidity.[2]
Walnut trees are late to grow leaves, typically not doing so until more than halfway through the spring.
The three species of walnuts most commonly grown for their seeds are the Persian (or English) walnut (J. regia), originating from Iran, the black walnut (J. nigra) – native to eastern North America – and the Japanese walnut, also known as the heartnut (J. ailantifolia).[6] Other species include J. californica, the California black walnut (often used as a rootstock for commercial propagation 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).[citation needed]
Numerous walnut cultivars have been developed commercially, which are nearly all hybrids of the English walnut.[7]
The black walnut is of strong flavor, but due to its hard shell and poor hulling characteristics, it is not commercially cultivated in orchards.[citation needed]
Cultivation
History
During the Byzantine era, the walnut was also known by the name "royal nut".[8] An article on walnut tree cultivation in Spain is included in Ibn al-'Awwam's 12th-century Book on Agriculture.[9] The wal element in the name is Germanic and means foreign, especially in the sense of Latin or non-Germanic. Compare, for example, Wales, Walloons, Wallachia. The wal element is present in other Germanic-language words for the same nut, such as: German Walnuss, Dutch walnoot, Danish valnød, and Swedish valnöt.
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 batch that contains mold-infested walnuts should be entirely discarded.[2]
The ideal temperature for the extended 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; walnuts are best stored below 25 °C (77 °F) with low humidity. Temperatures above 30 °C (86 °F) and humidity levels 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][10]
In 2022, world production of walnuts (in shell) was 3.9 million tonnes, with China contributing 36% of the total (table). Other significant producers (in the order of decreasing harvest) were the United States, Iran, and Turkey.[15]
English walnuts without shells are 4% water, 15% protein, 65% fat, and 14% carbohydrates, including 7% dietary fiber (table). In a reference amount of 100 g (3.5 oz), walnuts provide 2,740 kilojoules (654 kcal) and "rich" amounts (20% or more of the Daily Value or DV) of several dietary minerals, particularly manganese at 163% DV, along with significant amounts of B vitamins (table).
In 2004, 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."[18] At the same time, the agency refused to authorize the claim that "Diets including walnuts can reduce the risk of heart disease"[19] and in 2010, it sent a warning letter to Diamond Foods stating there is "not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease."[20]
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 de-shelled. Due to processing, the meats may be whole, halved, or in smaller portions. 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 husks can be used to make durable ink for writing and drawing. It is thought to have been used by artists including Leonardo da Vinci and Rembrandt.[28]
The fine, straight-grained wood of the black walnut is highly valued for furniture, wall paneling, automobile interiors, and gunstocks.[31]
Cleaning
The US Army once used ground walnut shells for abrasive blasting to clean aviation parts because of low cost and low abrasive qualities. However, an investigation of a fatal Boeing CH-47 Chinook helicopter crash (11 September 1982, in Mannheim, Germany) revealed that walnut shell grit had clogged an oil port, leading to the accident and the discontinuation of walnut shells as a cleaning agent.[32]
Commercially, crushed walnut shells are still used outside of aviation for low-abrasive, less-toxic cleaning and blasting applications.[33] In the oil and gas industry, deep bed filters of ground walnut shell are used for "polishing" (filtering) oily contaminates from water.[34]
Cat litter
At least two companies, LitterMaid and Naturally Fresh, make cat litter from ground walnut shells.[35][36] Advantages cited over conventional clay litter include environmental sustainability of using what would otherwise be a waste product, superior natural biodegradability, and odor control as good or better than clay litter.[37] Disadvantages include the possibility of allergic reactions among humans and cats.[38]
Folk medicine
Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies,[39] 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".[40]
In culture
Large, symmetrically shaped, and sometimes intricately carved walnut shells (mainly from J. hopeiensis) are valued collectibles in China where they are rotated in hand as a plaything or as decoration. They are also an investment and status symbol, with some carvings having high monetary value if unique.[41] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du qing pi.[42]
^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. ISSN1842-4309. Archived from the original on 29 July 2017.
^ abcdefghijklmnopqrst"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.
^fr:Le Verger Francais tomme II Chapitre I Fruits Locaux & Regionaux 1948 The French en:Orchard Book II Chapter I Local & Regional Fruits 1948
^US active USPP21718P2, Gale McGranahan & Charles Leslie, "Walnut tree named 'Ivanhoe' (patent)", published 2011-02-22, issued 2011-02-22, assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
^ abLe Verger Francais, Tome 1 Catalogue Descriptif des Fruits Adoptes 1947
^National Academies of Sciences, Engineering, and Medicine; Health and Medicine Division; Food and Nutrition Board; Committee to Review the Dietary Reference Intakes for Sodium and Potassium (2019). Oria, Maria; Harrison, Meghan; Stallings, Virginia A. (eds.). Dietary Reference Intakes for Sodium and Potassium. The National Academies Collection: Reports funded by National Institutes of Health. Washington, DC: National Academies Press (US). ISBN978-0-309-48834-1. PMID30844154. Archived from the original on 9 May 2024. Retrieved 21 June 2024.
^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 claim
^Turner, Lisa. "Oil Change". Better Nutrition. Archived from the original on 6 July 2014. Retrieved 5 July 2014.
^"Black Walnut Ink Workshop". Guild of Natural Science Illustrators. October 2002. Archived from the original on 9 October 2014. Retrieved 3 July 2014.
^Cheremisinoff, Nicholas P. (2017). Perfluorinated chemicals (PFCs) : contaminants of concern. Hoboken, New Jersey: Wiley. p. 194. ISBN978-1-119-36353-8.