A walnut is an edible seed of any tree of the genus Juglans, especially the Persian or English 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 from Juglans cinerea.
Walnut seeds are a 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. A mold infested walnut seed batch should not be screened and then consumed; the entire batch should be discarded.
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. 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. The commercially produced walnut varieties are nearly all hybrids of the English walnut.
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.
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.Read More