Walnuts

In Chinese cuisine, walnuts are believed to make a person smarter because of the nut's resemblance

to a brain.

What are they? The kernel of a fruit. The fruit surrounding the kernel is edible when fresh and green but it shrivels after being picked. Except in walnut-growing areas, the nuts are usually sold with the surrounding fruit removed. They're sold either in the shell or out, as walnut halves (because it's extremely difficult to extract the nut whole), chopped or ground.

What are the differences? There are many varieties of walnut but the most common is the Persian, which has a slightly wrinkled, brittle shell that's fairly easy to crack. Black walnuts have an extremely hard shell: people have been known to attempt to crack the shell by driving over it with a car. Black walnuts are more bitter than the Persian variety.

What else? The shells can be used to produce a brown dye. Walnuts have a high fat content and the nuts are sometimes pressed to extract the oil, which is a delicious alternative to olive oil in salad dressings. Don't cook with walnut oil because heat dissipates the delicate flavour. Green (unripe) walnuts can be preserved by pickling or candying. If you buy walnuts that are too bitter, blanch them in boiling water then drain and rinse. If you're grinding walnuts (or any nuts), take care they don't turn into an oily paste. If the recipe you're using is sweet, grind the nuts with some of the sugar, which will absorb the oil.

How to use? Walnuts play an important part in Eastern European desserts such as tortes, which are dense cakes that are traditionally flour-free. In pie or tart crusts, finely ground walnuts are delicious when substituted for part of the flour, but the crust will be more difficult to roll. This crust is especially good with a rich chocolate filling. Chopped walnuts can be added to streusel topping to use on fruit pies and crumbles.