Although there are many different varieties of walnuts, they all fall into three basic categories: English (or Persian), Black and White (or Butternut). All three kinds of walnuts are known to be a wealthy source of omega-3 essential fatty acids and antioxidants, and vary in physical attributes, region and taste.
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English / Persian (Juglans regia)
As the most ubiquitous and commonly consumed walnut in the United States, the English walnut is characterized by a thin, gnarled shell enclosing a smooth, ivory-colored nut. English walnuts have a mild taste and are often roasted for many recipes that require them. This kind of walnut originated in India near the Caspian Sea -- hence the alternate moniker, Persian -- where ancient Romans discovered them in 4th Century A.D. and shipped the walnuts to Europe, where they continue to flourish. Centuries later, the English walnut was introduced to North America due to British mercantilism, and since gained the name "English walnut."
Black (Juglan nigra)
Less common than the English nut, black nuts are characterized by a thick, hard shell with sharp, jagged edges and a darker color. Contrary to the English nut's milder flavor, black nuts are known for their pungent aroma and robust flavor, and can replace English nuts in many recipes. Black nuts are native to North America, specifically the Appalachian area and the Mississippi Valley.
White / Butternut (Juglan cinerea)
The white nut is the rarest variety of walnut and considered an endangered species by the the United States and Canadian governments. Similar in origin to black nuts, white nuts can be found in the Mississippi Valley and the Appalachian area, as well as Canada. A sweet, oily flavor characterizes these nuts, which are covered with a green, fuzzy husk and protected by a light-colored shell with jagged edges.