WalnutsInside of a walnut in growthThree-segment walnutWalnut shell inside its green huskArtistic depiction of two walnuts (Adriaen Coorte, 1702)
Awalnut is the edibleseed of any tree of the genusJuglans (familyJuglandaceae), particularly the Persian or English walnut,Juglans regia. They areaccessory fruit because the outer covering of the fruit is technically aninvolucre and thus not morphologically part of the carpel; this means it cannot be adrupe but is instead a drupe-like nut.
After full ripening, theshell is discarded, and the kernel is eaten. Nuts of the eastern black walnut (Juglans nigra) and butternuts (Juglans cinerea) are less commonly consumed.
Walnuts are the round, single-seedstone fruits of the walnut tree. They ripen between September and November in the northern hemisphere. The brown, wrinkly walnut shell is enclosed in ahusk.[1] Shells of walnuts commercially available 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 brownseed coat which contains antioxidants. The antioxidants protect the oil-rich seed from atmospheric oxygen, preventingrancidity.[2]
Walnut trees are late to growleaves, 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 includeJ. californica, the California black walnut (often used as arootstock for commercial propagation ofJ. regia),J. cinerea (butternuts), andJ. major, the Arizona walnut. Other sources listJ. californica californica as native to southern California, andJuglans californica hindsii, or justJ. hindsii, as native to northern California; in at least one case, these are given as "geographic variants" instead of subspecies (Botanica).[citation needed]
Numerous walnutcultivars have been developed commercially, which are nearly all hybrids of the Persian walnut.[7]
During theByzantine era, the walnut was also known by the name "royal nut".[8] An article on walnut tree cultivation in theIberian Peninsula is included inIbn al-'Awwam's 12th-centuryBook on Agriculture.[9]
Thewal element in the name is Germanic and meansforeign, especially in the sense ofLatin ornon-Germanic. Compare, for example,Wales,Walloons,Wallachia. Thewal element is present in other Germanic-language words for the same nut, such as: GermanWalnuss, Dutchwalnoot, Danishvalnød, and Swedishvalnöt.
Walnuts, like other tree nuts, must be processed and stored properly. Poor storage makes walnuts susceptible to insect and fungal mold infestations; the latter producesaflatoxin – a potentcarcinogen. 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 aflatoxin can form.[2][10]
In 2023, world production of walnuts (in shell) was four milliontonnes, with China contributing 35% of the total (table). Other significant producers (in the order of decreasing harvest) were the United States,Iran, andTurkey.[15]
English (or Persian) 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 654 kilocalories (2,740 kJ) and rich contents (20% or more of theDaily Value, DV) of severaldietary minerals, particularlymanganese at 148% DV, along with significant amounts ofB vitamins (table).
In 2004, the USFood and Drug Administration (FDA) provided a qualifiedhealth 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 lowsaturated fat and lowcholesterol diet and not resulting in increased caloric intake, may reduce the risk ofcoronary 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 awarning letter toDiamond Foods stating there is "not sufficient evidence to identify abiologically active substance in walnuts that reduces the risk of coronary heart disease."[20]
Walnuts are among several tree nuts having potential to causeallergic reactions when eaten or if their oils contact the skin.[25][26] Lifetime walnut allergies occur in about 0.4% of the population in Europe and the United States,[26] although regional prevalence may vary according to availability of walnut products and consumption amounts.[26] The prevalence of walnut and other nut allergies may be as high as 5% in American children.[25]
Symptoms of a walnut allergic reaction are abdominal pain, cramps, nausea, vomiting, difficulty swallowing, itching, shortness of breath, and potentiallyanaphylaxis, which may be fatal.[25][29] Walnuts are considered to be a priority food allergen in Canada.[29] Emergency treatment for anaphylaxis includes an injection ofepinephrine, while milder symtoms are treated with anantihistamine.[29] An allergic reaction to walnuts is best avoided by not eating or touching walnut products.[25][29] Walnut allergy is usually lifelong and there is no cure.[25][29]
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 asmuesli, or as an ingredient of adish: 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 includingLeonardo da Vinci andRembrandt.[33]
TheUS Army once used ground walnut shells forabrasive blasting to clean aviation parts because of low cost and low abrasive qualities. However, an investigation of a fatalBoeing 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.[36]
Commercially, crushed walnut shells are still used outside of aviation for low-abrasive, less-toxic cleaning and blasting applications.[37] In the oil and gas industry,deep bed filters of ground walnut shell are used for "polishing" (filtering) oily contaminates from water.[38]
At least two companies, LitterMaid and Naturally Fresh, makecat litter from ground walnut shells.[39][40] 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.[41] Disadvantages include the possibility of allergic reactions among humans and cats.[42]
Walnuts have been listed as one of the 38 substances used to prepareBach flower remedies,[43] aherbal remedy promoted infolk medicine practices for its supposed effect on health. According toCancer Research UK, "there is no scientific evidence to prove that flower remedies can control, cure or prevent any type of disease, including cancer".[44]
Large, symmetrically shaped, and sometimes intricately carved walnut shells (mainly fromJ. hopeiensis) are valued collectibles in China where they are rotated in hand as a plaything or as decoration. They are also an investment andstatus symbol, with some carvings having highmonetary value if unique.[45] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known asdu qing pi.[46]
^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. Retrieved6 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 22 February 2011, issued 22 February 2011, assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
^abLe Verger Francais, Tome 1 Catalogue Descriptif des Fruits Adoptes 1947
^Wagner, Roberta (22 February 2010)."FDA Warning Letter to Diamond Food, Inc". US Food and Drug Administration, Inspections, Compliance, Enforcement, and Criminal Investigations. Archived fromthe original on 12 January 2017. Retrieved1 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
