Walnut

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Walnuts Walnuts - whole and open with halved kernel.jpg
Walnuts
Inside of a walnut in growth Inside of a walnut in growth.jpg
Inside of a walnut in growth
Three-segment walnut Three-part Walnut-8283.jpg
Three-segment walnut
Walnut shell inside its green husk Juglans regia Echte Walnussfrucht 2.JPG
Walnut shell inside its green husk

A walnut is the nut of any tree of the genus Juglans (family Juglandaceae), particularly the Persian or English walnut, Juglans regia .

Contents

A walnut is the edible seed of a drupe, and thus not a true botanical nut. It is commonly consumed as a nut. After full ripening for its edible seed when the shell has been discarded, it is used as a garnish or a snack. Nuts of the eastern black walnut ( Juglans nigra ) and butternuts ( Juglans cinerea ) are less commonly consumed.

Characteristics

Walnuts are rounded, single-seeded stone fruits of the walnut tree commonly used for the meat after fully ripening between September and November, in which the removal of the husk at this stage reveals a browning wrinkly walnut shell, [1] 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 within a fibrous sheath that splits when ripened. [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, thereby preventing rancidity. [2]

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.[ citation needed ]

History and cultivation

During the Byzantine era, the walnut was also known by the name "royal nut". [3] An article on walnut tree cultivation in Spain is included in Ibn al-'Awwam's 12th-century Book on Agriculture. [4]

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 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 commercially cultivated in orchards.

Numerous walnut cultivars have been developed commercially, which are nearly all hybrids of the English walnut. [5]

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).[ citation needed ]

Production

Top 5 Walnut producing countries – 2019
CountryProduction
(millions of tonnes)
Flag of the People's Republic of China.svg  China 2.52
Flag of the United States.svg  United States 0.59
Flag of Iran.svg  Iran 0.32
Flag of Turkey.svg  Turkey 0.23
Flag of Mexico.svg  Mexico 0.17
World4.50
Source: FAOSTAT of the United Nations [6]

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. [6]

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. [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; 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. [2] [7]

Food use

Walnuts in their shells available for sale in a supermarket in the United States Walnuts in their shells available for sale.jpg
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.[ citation needed ]

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.[ citation needed ]

Walnuts are also popular as an ice cream topping, and walnut pieces are used as a garnish on some foods. [8]

Nocino is a liqueur made from unripe green walnuts steeped in alcohol with syrup added.[ citation needed ]

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. [9] [10]

Nutritional value

Walnut, English
Whole Walnut Kernel.jpg
Walnut kernel, halves
Nutritional value per 100 grams
Energy 2,738 kJ (654 kcal)
13.71 g
Starch 0.06 g
Sugars 2.61 g
Dietary fiber 6.7 g
Fat
65.21 g
Saturated 6.126 g
Monounsaturated 8.933 g
Polyunsaturated 47.174 g
9 g
38 g
15.23 g
Vitamins Quantity
%DV
Vitamin A equiv.
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 constituentsQuantity
Water4.07 g

Percentages are roughly approximated using US recommendations for adults.
Source: USDA FoodData Central

Walnuts with no 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. [11] [12]

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. [11]

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 (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." [13] The FDA had, in 2004, refused to authorize the claim that "Diets including walnuts can reduce the risk of heart disease" [14] and had sent an FDA Warning Letter to Diamond Foods in 2010 stating there is "not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease." [15] A recent systematic review assessing the effect of walnut supplementation on blood pressure found insufficient evidence to support walnut consumption as a BP-lowering strategy. [16]

As of 2021, the relationship between walnuts and cognitive health is inconclusive. [17]

Non-food applications

Folk medicine

Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies, [18] 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". [19]

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. [20]

Walnut husk pigments are used as a brown dye for fabric [21] as once applied in classical Rome and medieval Europe for dyeing hair. [22]

Woodworking

The fine, straight-grained wood of the black walnut is highly valued as furniture wood and for gunstocks. [23]

Cleaning

The United States Army once used ground walnut shells for abrasive blasting to clean 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. [24] Commercially, crushed walnut shells are still used outside of aviation for low-abrasive, less-toxic cleaning and blasting applications. [25]

Phytochemicals

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. [26]

Walnuts also contain the ellagitannin pedunculagin. [27] Regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem bark of J. regia. [28]

Chinese culture

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

Cultivars

See also

Related Research Articles

Pistachio Member of the cashew family

The pistachio, a member of the cashew family, is a small tree originating from Central Asia and the Middle East. The tree produces seeds that are widely consumed as food.

<i>Juglans</i> Genus of trees

Walnut trees are any species of tree in the plant genus Juglans, the type genus of the family Juglandaceae, the seeds of which are referred to as walnuts. All species are deciduous trees, 10–40 metres (33–131 ft) tall, with pinnate leaves 200–900 millimetres (7.9–35.4 in), with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts (Pterocarya), but not the hickories (Carya) in the same family.

Pecan Species of hickory native to the southern USA and northern Mexico

The pecan is a species of hickory native to the southern United States and northern Mexico in the region of the Mississippi River. The tree is cultivated for its seed in the southern United States, primarily in Georgia, New Mexico, Texas, and Mexico, which produces nearly half of the world total. The seed is an edible nut used as a snack and in various recipes, such as praline candy and pecan pie. The pecan, in various aspects, is included in state symbols of Alabama, Arkansas, California, Oklahoma, and Texas.

Nut (fruit) Dry and edible seed, that usually has a high fat content

A nut is a fruit consisting of a hard or tough nutshell protecting a kernel which is usually edible. In general usage and in a culinary sense, a wide variety of dry seeds are called nuts, but in a botanical context "nut" implies that the shell does not open to release the seed (indehiscent).

Juglandaceae Family of trees

The Juglandaceae are a plant family known as the walnut family. They are trees, or sometimes shrubs, in the order Fagales. Members of this family are native to the Americas, Eurasia, and Southeast Asia.

Brazil nut Species of flowering plant in the family Lecythidaceae

The Brazil nut is a South American tree in the family Lecythidaceae, and it is also the name of the tree's commercially harvested edible seeds. It is one of the largest and longest-lived trees in the Amazon rainforest. The fruit and its nutshell – containing the edible Brazil nut – are relatively large, possibly weighing as much as 2 kg in total weight. As food, Brazil nuts are notable for diverse content of micronutrients, especially a high amount of selenium. The wood of the Brazil nut tree is prized for its quality in carpentry, flooring, and heavy construction.

<i>Juglans nigra</i> Species of tree

Juglans nigra, the eastern American black walnut, is a species of deciduous tree in the walnut family, Juglandaceae, native to North America. It grows mostly in riparian zones, from southern Ontario, west to southeast South Dakota, south to Georgia, northern Florida and southwest to central Texas. Wild trees in the upper Ottawa Valley may be an isolated native population or may have derived from planted trees.

Walnut oil is oil extracted from walnuts, Juglans regia. The oil contains polyunsaturated fatty acids, monounsaturated fatty acids, and saturated fats.

<i>Juglans cinerea</i> Species of tree

Juglans cinerea, commonly known as butternut or white walnut, is a species of walnut native to the eastern United States and southeast Canada.

<i>Juglans regia</i> Species of tree (walnut)

Juglans regia, the Persian walnut, English walnut, Carpathian walnut, Madeira walnut, or especially in Great Britain, common walnut, is an Old World walnut tree species native to the region stretching from the Balkans eastward to the Himalayas and southwest China. It is widely cultivated across Europe.

<i>Juglans ailantifolia</i> Species of fruit and plant

Juglans ailantifolia, the Japanese walnut, is a species of walnut native to Japan and Sakhalin. It is a deciduous tree growing to 20 m (66 ft) tall, rarely 30 m (98 ft), and 40–80 cm stem diameter, with light grey bark. The leaves are pinnate, 50–90 cm long, with 11-17 leaflets, each leaflet 7–16 cm long and 3–5 cm broad. The whole leaf is downy-pubescent, and a somewhat brighter, yellower green than many other tree leaves. The male flowers are inconspicuous yellow-green catkins produced in spring at the same time as the new leaves appear. The female flowers have pink/ red pistils. The fruit is a nut, produced in bunches of 4-10 together; the nut is spherical, 3–5 cm long and broad, surrounded by a green husk before maturity in mid autumn.

Juglone Chemical produced by walnut trees

Juglone, also called 5-hydroxy-1,4-naphthalenedione (IUPAC) is an organic compound with the molecular formula C10H6O3. In the food industry, juglone is also known as C.I. Natural Brown 7 and C.I. 75500. It is insoluble in benzene but soluble in dioxane, from which it crystallizes as yellow needles. It is an isomer of lawsone, which is the staining compound in the henna leaf.

<i>Juglans californica</i> Species of tree

Juglans californica, the California black walnut, also called the California walnut, or the Southern California black walnut, is a large shrub or small tree of the walnut family, Juglandaceae, endemic to California.

<i>Juglans australis</i> Species of tree

Juglans australis, the nogal criollo, is a species of plant in the Juglandaceae family. This large, fast-growing tree can grow to 20 m (66 ft) tall at altitudes of 0.5—1.5 km in the "Yungas" or Montane Cloud Forest of Argentina and in Bolivia. It is more frost resistant than the Persian Walnut (J. regia). It is threatened by habitat loss.

<i>Juglans neotropica</i> Species of plant

Juglans neotropica is a species of plant in the Juglandaceae family. It is found in Colombia, Ecuador, and Peru. It is threatened by habitat loss. Common names include Colombian walnut, Ecuadorian walnut, Andean walnut, nogal, cedro negro, cedro nogal, and nogal Bogotano.

<i>Juglans mandshurica</i> Species of tree

Juglans mandshurica, also known as Manchurian walnut, or Tigernut, is a deciduous tree of the genus Juglans, native to the Eastern Asiatic Region. It grows to about 25 m.

<i>Macadamia tetraphylla</i> Species of tree in the family Proteaceae native to Queensland and New South Wales in Australia

Macadamia tetraphylla is a tree in the family Proteaceae, native to southern Queensland and northern New South Wales in Australia. Common names include macadamia nut, bauple nut, prickly macadamia, Queensland nut, rough-shelled bush nut and rough-shelled Queensland nut.

<i>Juglans hindsii</i> Species of tree

Juglans hindsii, commonly called the Northern California black walnut and Hinds's black walnut, is a species of walnut tree native to the western United States. It is commonly called claro walnut by the lumber industry and woodworkers, and is the subject of some confusion over its being the root stock for English walnut orchard stock.

<i>Curculio caryae</i> Species of beetle

The pecan weevil, Curculio caryae is an obligate feeder on the nuts of North American hickories and pecans, most widely recognized as an economically important pest of the pecan, Carya illinoinensis. It has also been observed to infest one Juglans species, the Persian walnut, Juglans regia.

References

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Further reading