Thought to have originated in ancient Persia, the English walnut is named after the merchants who subsequently shipped it around the world. Sweet and mild in flavor, it is the most commonly sold variety in the United States. The black walnut, which is native to North America, has a distinctly richer and more pungent taste. However, its exceedingly hard shell makes it difficult to process commercially. For optimal flavor and texture, toast walnuts on a baking sheet in a 350º oven for 10 minutes. Because of their high oil content, walnuts go rancid quickly. Store shelled nuts in an airtight container for up to four months in the refrigerator, or one year in the freezer. Store unshelled walnuts in a cool, dry place for up to three months.