Walnut

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.

During the Byzantine era, the Walnut was also known by the name "royal nut."[2] An article on Walnut tree cultivation in Spain is brought down in Ibn al-'Awwam's 12th-century agricultural work, Book on Agriculture.[3]

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.[4]

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).

Walnut production – 2019

Country	Production (millions of tonnes)
China	2.52
United States	0.59
Iran	0.32
Turkey	0.23
World	4.50
Source: FAOSTAT of the United Nations[5]

In 2019, world production of walnuts (in shell) was 4.5 million tonnes, with China contributing 56% of the total (table). Other major producers (in the order of decreasing harvest) were the United States, Iran, and Turkey.[5]

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][6]

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. 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.[7]

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.[8][9]

Nutritional value

Walnut, English

Walnut kernel, halves
Nutritional value per 100 grams
Energy	2,738 kJ (654 kcal)
Carbohydrates	13.71
Starch	0.06
Sugars	2.61
Dietary fiber	6.7
Fat	65.21
Saturated	6.126
Monounsaturated	8.933
Polyunsaturated	47.174
Protein	15.23
Vitamins	Quantity %DV†
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 E	5% 0.7 mg
Vitamin K	3% 2.7 μg
Minerals	Quantity %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 constituents	Quantity
Water	4.07 g
Link to Full USDA Database Entry
Units μg = micrograms • mg = milligrams IU = International units
†Percentages are roughly approximated using US recommendations for adults. Source: USDA FoodData Central

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.[10][11]

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.[10]

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.[23]

Walnuts also contain the ellagitannin pedunculagin.[24] Regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem bark of J. regia.[25]

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.[26] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du qing pi.

• 
  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
• 
  Georgian snack Gozinaki made from roasted walnuts and honey
• 
  Applesauce coffee cake garnished with walnuts
• 
  Walnuts as collectibles
• 
  Muraba made from young walnuts

• Kurumi
• List of unproven and disproven cancer treatments

• 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.

Wikimedia Commons has media related to Walnuts.

• "Walnut" at the Encyclopedia of Life