Walnut

This article is about walnuts as food. For the tree species, see Juglans regia. For the genus, see Juglans. For other meanings, see Walnut (disambiguation).

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Walnuts
Scientific classification
Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Magnoliopsida
Order:	Fagales
Family:	Juglandaceae
Genus:	Juglans
Species:	J. regia
Binomial name
Juglans regia L.

A walnut is an edible seed of any tree of the genus Juglans, especially the Persian walnut, Juglans regia. Broken nutmeats of the eastern black walnut, from the tree Juglans nigra, are also commercially available in small quantities, as are foods prepared with butternut nutmeats.

Walnut seeds are high density source of nutrients, particularly proteins and essential fatty acids. 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. Mold infested walnut seed batch should not be screened then consumed; the entire batch should be discarded.

In certain parts of the world, walnuts are locally known as walnüsse, noix, nuéz, nuc, akharōṭ (अखरोट), kurumi (胡桃) or hétáo (核桃).

Introduction

Walnut seed shell inside its green husk

Walnuts are rounded, single-seeded stone fruits of the walnut tree. The walnut fruit is enclosed in a green, leathery, fleshy husk. This husk is inedible. After harvest, the removal of the husk reveals the wrinkly walnut shell, which is in two halves. This shell is hard and encloses the kernel, which is also 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 so preventing rancidity.

There are two major varieties of walnuts grown for its seeds — the English walnut and the Black walnut. The English Walnut originated in Persia, and the Black walnut is native to the United States. 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. The commercially produced walnut varieties are nearly all hybrids of the English walnut.

Production

The worldwide production of walnut seeds has been increasing rapidly in recent years, with most increase coming from Asia. The world produced a total of 2.55 million metric tonnes of walnut seeds in 2010; China was the world's largest producer of walnut seeds, with a total harvest of 1.06 million metric tonnes. The other major producers of walnut seeds were (in the order of decreasing harvest): United States, Iran, Turkey, Ukraine, Mexico, Romania, India, France and Chile.

The average worldwide walnut seed 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 seed farms in 2010 were in Romania, with yields above 23 metric tonnes per hectare.

The United States is the world's largest exporter of walnut seeds. The Sacramento and San Joaquin valleys of California produce 99 percent of the nation’s commercial English walnut seeds.

Storage

Walnuts, like all seeds, are living organs in which respiration processes dominate. Once harvested, the seeds continually consume oxygen and release carbon dioxide. The storage life of seeds depends, in part, on the rate of this respiration.

The ideal temperature for longest possible storage of walnut seeds is in the -3 to 0 ^oC 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, walnut seeds are best stored below 25 ^oC and low humidity. Temperatures above 30 ^oC, 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.

Freshly harvested raw walnut seeds with water content between 2 to 8 percent offer the best color, flavor and nutrient density.

Nutritional Value

Walnut, English

Persian or English walnut, Juglans regia
Nutritional value per 100 grams
Energy	2,738 kJ (654 kcal)
Carbohydrates	13.71
Starch	0.06
Sugars lactose	2.61 0
Dietary fiber	6.7
Fat	65.21
Saturated	6.126
Monounsaturated	8.933
Polyunsaturated	47.174
Protein	15.23
Vitamins and minerals Vitamins Quantity %DV† Vitamin A equiv. beta-Carotene lutein zeaxanthin 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 D 0% 0 μg Vitamin D 0% 0 IU 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 constituents	Quantity
Water	4.07
Alcohol (ethanol)	0
Caffeine	0
†Percentages estimated using US recommendations for adults, except for potassium, which is estimated based on expert recommendation from the National Academies.

Walnuts are one of the several high nutrient density foods. 100 grams of walnuts contain 15.2 gram protein, 65.2 gram fat, and 6.7 gram dietary fiber. The protein in walnuts provides many essential amino acids.

While English walnut is the predominant commercially distributed nut because of the ease of its processing, its nutrient density and profile is significantly different than black walnut. The table below compares some of the major nutrients between English and Black walnuts.

Comparison of nutrient profile of English and Black walnuts

Nutrient (per 100 gram)	English walnut seed	Black walnut seed
Carbohydrates (g)	13.7	9.9
Protein (g)	15.2	24.1
Unsaturated fatty acids (g)	56.1	50.1
Poly to mono unsaturated fatty acids ratio	47:9	35:15
Fiber (g)	6.7	6.8
Calcium (mg)	98	61
Iron (mg)	2.9	3.1
Zinc (mg)	3.1	3.4
Vitamin B-6 (mg)	0.54	0.58

Unlike most nuts that are high in monounsaturated fatty acids, walnuts are composed largely of polyunsaturated fatty acids (47.2 grams), particularly alpha-linolenic acid (18:3n - 3; 9.1 gram) and linoleic acid (18:2n - 6; 38.1 gram). The beneficial effects of this unique fatty acid profile has been a subject of many studies and discussions. Banel and Hu conclude that while walnut-enhanced diets are promising in short term studies, longer term studies are needed to ascertain better insights.

Medical benefits and claims

A whole walnut kernel, with both halves unbroken.

Raw walnuts contain glyceryl triacylates of the n-3 fatty acid alpha-linolenic acid (ALA), which is not as effective in humans as long-chain n-3 fatty acids, and (mostly insoluble) antioxidants. Roasting reduces antioxidant quality. In 2010, a report published in the Journal of the American College of Nutrition said that walnuts and walnut oil improve reaction to stress.

A study has suggested that consumption of walnuts increases fat oxidation and reduces carbohydrate oxidation without affecting total consumption, suggesting that walnut consumption may improve the use of body fat in overweight adults. Walnuts have been shown to decrease the endothelial dysfunction associated with a high-fat meal. Aged rats fed diets containing 2% to 6% walnuts showed reversal of age-associated motor and cognitive function, but a 9% walnut diet impaired performance, suggesting a J curve.

On October 11, 2006, ScienceDaily published a report which stated "New research shows that consuming a handful of raw walnuts along with meals high in saturated fat appears to limit the ability of the harmful fat to damage arteries," and attributed the result to a 2006 article in the Journal of the American College of Cardiology. The lead researcher, Emilio Ros, MD, PhD, was quoted as saying "People would get the wrong message if they think that they can continue eating unhealthy fats provided they add walnuts to their meals." Funding for the study was provided by the California Walnut Commission, an industry marketing agency.

Medicinal uses

Scientists are not yet certain whether walnuts act as a cancer chemopreventive agent, an effect which may be a result of the fruit's high phenolic content, antioxidant activity, and potent in vitro antiproliferative activity. Walnuts contain the ellagitannin pedunculagin

Compared to certain other nuts, such as almonds, peanuts and hazelnuts, walnuts (especially in their raw form) contain the highest total level of antioxidants, including both free antioxidants and antioxidants bound to fiber.

Meaning and etymology

"Walnut" does not distinguish the tree from other species of Juglans. Other names include common Walnut in Britain; Persian walnut in South Africa and Australia; and English walnut in North America and Great Britain, New Zealand, and Australia, the latter name possibly because English sailors were prominent in Juglans regia nut distribution at one time. Alternatively, Walter Fox Allen stated in his 1912 treatise What You Need to Know About Planting, Cultivating and Harvesting this Most Delicious of Nuts: "In America, it has commonly been known as English walnut to distinguish it from our native species."

Etymologically, the word derives from the Germanic wal- and Old English wealhhnutu, literally "foreign nut", wealh meaning "foreign" (wealh is akin to the terms Welsh and Vlach; see Walha). For the walnut to be identified as a "foreign" nut by Anglo-Saxons arriving in the fifth century, native Britons must have passed to them the tradition that it had been introduced to England from Gaul and Italy.

See also

• Nut (fruit)