A walnut is the edible seed of any tree of the genus Juglans particularly the Persian or English walnut. 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.
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 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. 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.
Walnut trees are late to grow leaves, typically not doing so until more than halfway through the spring.
Species
The three species of walnuts most commonly grown for their seeds are the Persian (or English) walnut, originating from Iran, the black walnut–native to eastern North America – and the Japanese walnut, also known as the heartnut. Other species include, the California black walnut (often used as a rootstock for commercial propagation of butternuts, and, the Arizona walnut. Other sources list J. californica as native to southern California, as native to northern California; in at least one case, these are given as “geographic variants” instead of subspecies (Botanica).
Numerous walnut cultivars have been developed commercially, which are nearly all hybrids of the English walnut.
Cultivation
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.
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.
Production
| Country | millions of tonnes |
|---|---|
 China | 1.40 |
 United States | 0.68 |
 Iran | 0.36 |
 Turkey | 0.34 |
 Mexico | 0.18 |
| World | 3.9 |
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.
Nutrition
English walnuts without shells are 4% water, 15% protein, 65% fat, and 14% carbohydrates, including 7% dietary fibre (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 monounsaturated fatty acids walnut oil is composed largely of polyunsaturated fatty acids (72% of total fats), particularly alpha linoleic acid (14%) and linoleic acids (58%), although it does contain oleic acids 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.” At the same time, the agency refused to authorize the claim that “Diets including walnuts can reduce the risk of heart disease” and in 2010, it sent a warning letter to diamond food there is “not sufficient evidence to identify a biologically active substance in walnuts that reduces the risk of coronary heart disease.”
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. The recommended health claim was later authorized by the European Commission.
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.
As of 2021, the relationship between walnut consumption and cognitive health is inconclusive.
Uses
Culinary
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, potica (a traditional festive pastry from slovenia), satsivi (chicken in walnut sauce), tarator (a summer soup in bulgarian cuisine, polutry 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.
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.
Inks and dyes
Further information: walnut ink
Walnut husk pigments are used as a brown dye for fabric and were used in classical Rome and medieval Europe for dyeing hair.
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.
Commercially, crushed walnut shells are still used outside of aviation for low-abrasive, less-toxic cleaning and blasting applications. In the oil and gas industry, deep bed filters of ground walnut shell are used for “polishing” (filtering) oily contaminates from water.
Cat litter
At least two companies, Litter Maid and Naturally Fresh, make cat litter from ground walnut shells. 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. Disadvantages include the possibility of allergic reactions among humans and cats.
Folk medicine
Walnuts have been listed as one of the 38 substances used to prepare Bach Flower Remedies 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”.
In culture
Large, symmetrically shaped, and sometimes intricately carved walnut shells (mainly 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.
Gallery
