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).
(thousands of tonnes)
In 2016, worldwide production of walnuts (in shell) was 3.7 million tonnes, with China contributing 48% of the world total (table). Other major producers were (in the order of decreasing harvest): United States, Iran, Turkey, Mexico, Ukraine and Chile.
The average worldwide walnut yield was about 3.5 tonnes per hectare in 2014. Eastern European countries had the highest yield, with Slovenia and Romania each harvesting about 19 tonnes per hectare.
In 2014, the United States was the world's largest exporter of walnuts, followed by Turkey. The Central Valley of California produces 99 percent of total United States commerce in English walnuts. It has been introduces and been found naturalized in England.
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 may be whole, halved, or in smaller portions due to processing. Walnuts are often candied, and may be used 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.Nocino is a liqueur made from unripe green walnuts steeped in alcohol with syrup added.
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.
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." The FDA had, in 2004, refused to authorize the claim that "Diets including walnuts can reduce the risk of heart disease" 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."
Walnuts have been listed as one of the 38 substances used to prepare Bach flower remedies, 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".
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. Commercially, crushed walnut shells are still used outside of aviation for low-abrasive, less-toxic cleaning and blasting applications.
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, and juglone were identified in walnut husks. Juglone, the predominant phenolic, was found in concentrations of 2-4% fresh weight.
Large, symmetrically shaped, and sometimes intricately carved walnuts are valued highly in China where they are rotated in the hand as a plaything, and used as an investment and status symbol, with some fetching tens of thousands of dollars. Pairs of walnuts are sometimes sold in their green husks for a form of gambling known as du qing pi.
the evidence supporting a relationship between walnuts and coronary heart disease is related to the omega-3 fatty acid content of walnuts. There is not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease. Therefore, the above statement is an unauthorized health claim