A walnut is the nut of any tree of the genus Juglans (Family Juglandaceae), particularly the Persian or English walnut, Juglans regia . Technically a walnut is the seed of a drupe or drupaceous nut, and thus not a true botanical nut. 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.
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. [[CITE|undefined|http://tis-gdv.de/tis_e/ware/nuesse/walnuss/walnuss.htm#toxizitaet]]
Walnuts are late to grow leaves, typically not until more than halfway through the spring.
The two most common major species of walnuts are grown for their seeds – the Persian or English walnut and the black 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. 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).
Walnuts are harvested from the gound where they have fallen.
In 2014, worldwide production of walnuts (in shell) was 3.46 million tonnes, with China contributing 46% of the world total (table). [[CITE|undefined|http://faostat3.fao.org/browse/Q/QC/E]] Other major producers were (in the order of decreasing harvest): United States, Iran, Turkey and Mexico.
The average worldwide walnut yield was about 3.5 tonnes per hectare in 2014.
In 2014, the United States was the world's largest exporter of walnuts, followed by Turkey.
Walnuts, like other tree nuts, must be processed and stored properly.
The ideal temperature for longest possible storage of walnuts is in the −3 to 0 °C (27 to 32 °F) and low humidity – for industrial and home storage.
Walnut meats are available in two forms; in their shells or shelled.
Walnut is the main ingredient of Fesenjan, a khoresh (stew) in Iranian cuisine. Walnuts are also popular in brownie recipes, as ice cream toppings, and walnut pieces are used as a garnish on some foods. [[CITE|undefined|https://books.google.com/books?id=i-VkDQAAQBAJ&pg=PA98]] 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. [[CITE|undefined|http://bbc.co.uk/food/walnut_oil]] [[CITE|undefined|http://betternutrition.com/five-essential-oils/food/offtheshelf/963]]
In a 100-gram serving, walnuts provide 2,740 kilojoules (654 kcal) and rich content (more than 19% 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.
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. [[CITE|undefined|http://nutritiondata.self.com/facts/nut-and-seed-products/3138/2]]
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 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." [[CITE|undefined|http://fda.gov/Food/IngredientsPackagingLabeling/LabelingNutrition/ucm073992.htm#cardio]] The FDA had, in 2004, refused to authorise the claim that "Diets including walnuts can reduce the risk of heart disease"[[CITE|undefined|https://fda.gov/Food/IngredientsPackagingLabeling/LabelingNutrition/ucm072910.htm]] and had sent an FDA Warning Letter to Diamond Foods in 2010 stating "not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease." [[CITE|undefined|https://wayback.archive-it.org/7993/20170112194904/http://www.fda.gov/ICECI/EnforcementActions/WarningLetters/2010/ucm202825.htm]]
Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies, [[CITE|undefined|https://books.google.com/books?id=icG8onA0ys8C&pg=PR3]] a pseudomedicine 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". [[CITE|undefined|http://cancerresearchuk.org/cancer-help/about-cancer/treatment/complementary-alternative/therapies/flower-remedies]]
Walnut husks can be used to make a durable ink for writing and drawing.
Walnut husk pigments are used as a brown dye for fabric [[CITE|undefined|http://invention.smithsonian.org/centerpieces/whole_cloth/u3tc/u3materials/dyenatural.html]] as once applied in classical Rome and medieval Europe for dyeing hair. [[CITE|undefined|https://books.google.com/books?dq=walnut+hair+dye+history&ei=RNi1U_u4JMmZPeLBgPAI&hl=en&id=9Z6vCGbf66YC&lpg=PA267&ots=YL_azTp8ld&pg=PA267&sa=X&sig=9Cog5RD6zDZgPUMgdM-YzeWRP1o&source=bl&ved=0CDMQ6AEwAzgK#v=onepage&q=walnut&f=false]]
The United States Army once used ground walnut shells for the cleaning of 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. [[CITE|undefined|http://aic.ucdavis.edu/profiles/Walnut-2006.pdf]]
Walnut hulls contain 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. [[CITE|undefined|http://aic.ucdavis.edu/profiles/Walnut-2006.pdf]]
Walnuts also contain the ellagitannin pedunculagin. [[CITE|undefined|http://aic.ucdavis.edu/profiles/Walnut-2006.pdf]] Regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem bark of J. regia. [[CITE|undefined|http://aic.ucdavis.edu/profiles/Walnut-2006.pdf]]
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. Individual and pairs of large, old, symmetrically shaped, and sometimes intricately carved walnuts are valued highly and have recently been used as an investment, with some of them fetching tens of thousands of dollars. [[CITE|undefined|http://aic.ucdavis.edu/profiles/Walnut-2006.pdf]] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du he tao. [[CITE|undefined|http://aic.ucdavis.edu/profiles/Walnut-2006.pdf]]