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.
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.
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.
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).
The worldwide production of walnuts has been increasing rapidly in recent years, with the largest increase coming from Asia. The world produced a total of 2.55 million metric tonnes of walnuts in 2010; China was the world's largest producer of walnuts, with a total harvest of 1.06 million metric tonnes. The other major producers of walnuts were (in the order of decreasing harvest): Iran, United States, Turkey, Ukraine, Mexico, Romania, India, France and Chile.
The average worldwide walnut yield was about 3 metric tonnes per hectare, in 2010. Among the major producers, eastern European countries have the highest yield. According to the FAO, the most productive walnut farms in 2010 were in Romania, with yields above 23 metric tonnes per hectare.
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.
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.
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 pie is prepared using walnuts as a main ingredient. Walnut Whip, coffee and walnut cake, and pickled walnuts are more examples. Walnuts are also popular in brownie recipes and as ice cream toppings.
Walnut kernel, halves
|Nutritional value per 100 grams|
|Energy||2,738 kJ (654 kcal)|
|Vitamin A equiv.||
|Vitamin A||20 IU|
|Pantothenic acid (B5)||
|Percentages are roughly approximated using US recommendations for adults.
Source: USDA Nutrient Database
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).
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.
Black walnut has been promoted as a potential cancer cure, on the basis that it kills a "parasite" responsible for the disease. However, according to the American Cancer Society, "available scientific evidence does not support claims that hulls from black walnuts remove parasites from the intestinal tract or that they are effective in treating cancer or any other disease".
Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies, a kind of alternative medicine promoted for its effect on health. However, 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".
Inks and dyes
The husks of the black walnut Juglans nigra are used to make an ink for writing and drawing. Walnut ink has good archival properties, and has been used by several great artists including Leonardo da Vinci and Rembrandt.
The United States Army used ground walnut shells for the cleaning of aviation parts because it was inexpensive and non-abrasive. 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.
Walnut hulls contain phenolics that stain hands and can cause skin irritation. Seven phenolic compounds (ferulic acid, vanillic acid, coumaric acid, syringic acid, myricetin, juglone and regiolone) have been identified in walnut husks by using reverse-phase high-performance liquid chromatography or crystallography.
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. Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du he tao.
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