Jan Bilton: Cultivate a zest for citrus

Citrus fruits full of vitamin C are easy to get your hands on in the flu season.

Luckily, vitamin C-rich citrus fruit are abundant at this time of year. Sneezes and colds are rife and citrus offer a zesty boost to health, providing high levels of vitamin C, potassium, fibre plus folate - also known as folic acid - a B group vitamin important for cell growth and development.

Botanists have calculated that the history of citrus fruit - native to China and Southeast Asia - goes back 20 million years. Early explorers to the East discovered the tangy, juicy, bright orange that quickly gained popularity in Europe, and - by way of Christopher Columbus - in the New World.

Mandarins, oranges, grapefruit, sanguinelli (blood oranges), ugli fruit, lemons, limes and tangelos are but a few of the citrus varieties now cultivated worldwide. New varieties are constantly being discovered and developed.

I am hooked on New Zealand navel oranges - they have such great colour, flavour and juice that they make other varieties of orange look pale in comparison. They are also easy to peel and segment.

In general, about three oranges will yield one cup of juice. Two oranges will provide a cup of diced fruit. One orange has about 10 segments and will yield about four teaspoons of grated rind.

Limes grow in tropical and subtropical climates. Most New Zealand limes are still imported from the Pacific Islands. However, many of us now grow our own. Lime juice and lime zest is addictive and is a great - and healthy - substitute for salt in dishes. Limes are also an integral ingredient of appetising Asian recipes.

The kaffir lime, common in Thai and Vietnamese cooking, is small with a skin that's yellow-green, knobbly and wrinkled. The flesh is usually dry: it's the rind that is used in cooking plus the glossy, dark green leave, which have a unique double shape and look like two leaves joined end to end.

The kaffir limes I grew in Auckland had dry, pale, useless flesh. The kaffir limes I'm now cultivating in the "top of the south" (sheltered from frosts) are very juicy - and this juice is excellent in dressings and marinades. However, the flesh is still bitter and unusable. I store my surplus crop of fruit in the freezer ready to put extra zest into soups, meatballs and sauces.