Walnut

The common walnut in growth.

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.

California black walnut in growth.

Inside of a walnut in growth

Walnut shell inside its green husk

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-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 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).

Walnut Production – 2014^[3]

Country	Production (millions of tonnes)
China	1.60
United States	0.52
Iran	0.45
Turkey	0.18
Mexico	0.13
World	3.46

Production

In 2014, worldwide production of walnuts (in shell) was 3.46 million tonnes, with China contributing 46% of the world total (table).^[3] 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.^[3] Eastern European countries had the highest yield, with Slovenia and Romania each harvesting about 19 tonnes per hectare.^[3]

In 2014, the United States was the world's largest exporter of walnuts, followed by Turkey.^[4] The Central Valley of California produces 99 percent of total United States commerce in English walnuts.^[5]

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 longest possible storage of walnuts is in the −3 to 0 °C (27 to 32 °F) and 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) and 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]

Food use

Walnut meats are available in two forms; in their shells or shelled. The meats can be as large as halves or any smaller portions that may happen during processing, candied or as an ingredient in other foodstuffs. 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. 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. For example, walnut soup and walnut pie are prepared using walnuts as a main ingredient. Walnut Whip, coffee and walnut cake, and pickled walnuts are more examples. 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.^[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
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
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
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 Nutrient Database

Walnuts without shells are 4% water, 15% protein, 65% fat, and 14% carbohydrates, including 7% dietary fiber (table).

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.^[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]

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 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."^[12] The FDA had, in 2004, refused to authorise the claim that "Diets including walnuts can reduce the risk of heart disease"^[13] 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."^[14]

Non-food applications

Folk medicine

Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies,^[15] 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".^[16]

Inks and dyes

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 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.^[20]

Three-segment shell which occurs rarely.

Chemical analysis

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, juglone, were identified in walnut husks. Juglone, the predominant phenolic, was found in concentrations of 2-4% fresh weight.^[21]

Walnuts also contain the ellagitannin pedunculagin.^[22] Regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem bark of J. regia.^[23]

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.^[24] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du he tao.^[25]

Cultivars

See also

• Kurumi
• List of unproven and disproven cancer treatments
• Nut (fruit)
• Pickled walnuts
• Walnut ink
• Walnut pie

References

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 978-0143116899. 

External links

Wikimedia Commons has media related to Walnuts.

• Walnut at the Encyclopedia of Life