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
Artistic depiction of two walnuts (Adriaen Coorte, 1702) Adriaen Coorte - Two Walnuts.jpg
Artistic depiction of two walnuts (Adriaen Coorte, 1702)

A walnut is the edible seed of any tree of the genus Juglans (family Juglandaceae), particularly the Persian or English walnut, Juglans regia . They are accessory fruit because the outer covering of the fruit is technically an involucre and thus not morphologically part of the carpel; this means it cannot be a drupe but is instead a drupe-like nut.

Contents

After full ripening, the shell is discarded, and the kernel is eaten. Nuts of the eastern black walnut ( Juglans nigra ) and butternuts ( Juglans cinerea ) are less commonly consumed.

Description

Walnuts are the round, single-seed stone fruits of the walnut tree. They ripen between September and November in the northern hemisphere. The brown, wrinkly walnut shell is enclosed in a husk. [1] Shells of walnuts available in commerce 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 brown seed coat which contains antioxidants. The antioxidants protect the oil-rich seed from atmospheric oxygen, preventing rancidity. [2]

Walnut trees are late to grow leaves, typically not doing so until more than halfway through the spring.

Chemistry

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 had concentrations of 2-4% fresh weight. [3]

Walnuts also contain the ellagitannin, pedunculagin. [4] Regiolone has been isolated with juglone, betulinic acid and sitosterol from the stem bark of J. regia. [5]

Species

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 include J. californica , the California black walnut (often used as a rootstock for commercial propagation 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 ]

Numerous walnut cultivars have been developed commercially, which are nearly all hybrids of the English walnut. [7]

Cultivation

History

During the Byzantine era, the walnut was also known by the name "royal nut". [8] An article on walnut tree cultivation in Spain is included in Ibn al-'Awwam's 12th-century Book on Agriculture. [9] The wal element in the name is Germanic and means foreign, especially in the sense of Latin or non-Germanic. Compare, for example, Wales, Walloons, Wallachia. The wal element is present in other Germanic-language words for the same nut, such as: German Walnuss, Dutch walnoot, Danish valnød, and Swedish valnöt.

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 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]

Cultivars

Production

Walnut production – 2022
Countrymillions of tonnes
Flag of the People's Republic of China.svg  China 1.40
Flag of the United States (23px).png  United States 0.68
Flag of Iran.svg  Iran 0.36
Flag of Turkey.svg  Turkey 0.34
Flag of Mexico.svg  Mexico 0.18
World3.9
Source: FAOSTAT of the United Nations [15]

In 2022, world production of walnuts (in shell) was 3.9 million tonnes, with China contributing 36% of the total (table). Other significant producers (in the order of decreasing harvest) were the United States, Iran, and Turkey. [15]

Nutrition

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 and minerals
Vitamins Quantity
%DV
Vitamin A equiv.
0%
1 μg
0%
12 μg
9 μg
Vitamin A 20 IU
Thiamine (B1)
28%
0.341 mg
Riboflavin (B2)
12%
0.15 mg
Niacin (B3)
7%
1.125 mg
Pantothenic acid (B5)
11%
0.570 mg
Vitamin B6
32%
0.537 mg
Folate (B9)
25%
98 μg
Vitamin B12
0%
0 μg
Vitamin C
1%
1.3 mg
Vitamin E
5%
0.7 mg
Vitamin K
2%
2.7 μg
Minerals Quantity
%DV
Calcium
8%
98 mg
Iron
16%
2.91 mg
Magnesium
38%
158 mg
Manganese
148%
3.414 mg
Phosphorus
28%
346 mg
Potassium
15%
441 mg
Sodium
0%
2 mg
Zinc
28%
3.09 mg
Other constituentsQuantity
Water4.07 g

Percentages estimated using US recommendations for adults, [16] except for potassium, which is estimated based on expert recommendation from the National Academies. [17]

English 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 the Daily Value, DV) of several dietary minerals, particularly manganese at 148% DV, along with significant amounts of B vitamins (table).

Unlike most nuts, which 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 (table source).

Health claims

In 2004, 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." [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 a warning letter to Diamond Foods stating there is "not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease." [20]

In 2011, a scientific panel for the European Food Safety Authority recommended a health claim that "Walnuts contribute to the improvement of endothelium-dependent vasodilation" at a daily intake of 30 grams (1.1 oz); it also found that a cause and effect relationship did not exist between consuming walnuts and reduction of blood LDL-cholesterol levels. [21] The recommended health claim was later authorized by the European Commission. [22]

Research

A 2020 systematic review assessing the effect of walnut supplementation on blood pressure found insufficient evidence to support walnut consumption as a blood pressure-lowering strategy. [23]

As of 2021, the relationship between walnut consumption and cognitive health is inconclusive. [24]

Uses

Culinary

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. 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 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.

Walnuts may be used as an ingredient in other foodstuffs. Walnut is an important ingredient in baklava, Circassian chicken, potica (a traditional festive pastry from Slovenia), satsivi (chicken in walnut sauce), tarator (a summer soup in Bulgarian cuisine), and poultry or meat ball stew from Iranian cuisine.

Walnuts are also popular as an ice cream topping, and walnut pieces are used as a garnish on some foods. [25]

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. [26] [27]

Inks and dyes

Walnut husks can be used to make durable ink for writing and drawing. It is thought to have been used by artists including Leonardo da Vinci and Rembrandt. [28]

Walnut husk pigments are used as a brown dye for fabric [29] and were used in classical Rome and medieval Europe for dyeing hair. [30]

Cleaning

The US Army once used ground walnut shells for abrasive blasting to clean aviation parts because of low cost and low abrasive qualities. However, an investigation of a fatal Boeing 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. [31]

Commercially, crushed walnut shells are still used outside of aviation for low-abrasive, less-toxic cleaning and blasting applications. [32] In the oil and gas industry, deep bed filters of ground walnut shell are used for "polishing" (filtering) oily contaminates from water. [33]

Cat litter

At least two companies, LitterMaid and Naturally Fresh, make cat litter from ground walnut shells. [34] [35] 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. [36] Disadvantages include the possibility of allergic reactions among humans and cats. [37]

Folk medicine

Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies, [38] 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". [39]

In culture

Large, symmetrically shaped, and sometimes intricately carved walnut shells (mainly from J. hopeiensis ) are valued collectibles in China where they are rotated in hand as a plaything or as decoration. They are also an investment and status symbol, with some carvings having high monetary value if unique. [40] Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du qing pi. [41]

See also

Related Research Articles

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<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.

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<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.

<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 Caucasus eastward to the Kashmir region. It is widely cultivated across Asia, Northern America and Europe.

<span class="mw-page-title-main">Docosahexaenoic acid</span> Chemical compound

Docosahexaenoic acid (DHA) is an omega−3 fatty acid that is an important component of the human brain, cerebral cortex, skin, and retina. It is given the fatty acid notation 22:6(n−3). It can be synthesized from alpha-linolenic acid or obtained directly from maternal milk (breast milk), fatty fish, fish oil, or algae oil. The consumption of DHA (e.g., from fatty fish such as salmon, herring, mackerel and sardines) contributes to numerous physiological benefits, including cognition. As a component of neuronal membranes, the function of DHA is to support neuronal conduction and to allow the optimal functioning of neuronal membrane proteins (such as receptors and enzymes).

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<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 elevations of 0.5—1.5 km in the Southern Andean Yungas, montane cloud forests on the eastern slopes of the Andes in Tucumán, Salta, and Jujuy provinces of Argentina and Tarija and Chuquisaca departments of Bolivia. It is threatened by habitat loss.

<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.

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<span class="mw-page-title-main">Husk</span> Outer shell or coating of a seed

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Further reading