Cranberries

Large, very tart North American berries – fresh in autumn, otherwise dried, frozen or as juice. Cook for sauces and bakes; dried ones are usually sweetened.

What are cranberries? A floating red carpet

Many people assume the big red American cranberry (large cranberry; *Vaccinium macrocarpon*) is the same thing as the tiny, rather pricey Nordic lingonberry. It isn't. The cranberry is a tough plant of the cold, wet bogs of North America. At harvest time, farmers flood the boggy fields with water. Tiny air pockets inside the firm, bright red berries mean that, once they're shaken loose by machine, millions of them float to the surface like a red carpet and can be skimmed off. Raw, unsweetened cranberries are not something you snack on for fun: they're very sharp and sour, and their tannins make your mouth feel dry and furry straight away.

Beyond vitamin C: proanthocyanidins (PACs)

Forget vitamin C for a moment – sour lemons have more of it. The more interesting part is a small, complex group of plant compounds:

• A-type proanthocyanidins (A-type PACs): almost every red fruit contains B-type PACs. Cranberries are unusual in containing large amounts of A-type PACs, whose building blocks are linked together in a more complex way. In lab studies, these compounds made it harder for certain bacteria to stick to surfaces.

What researchers are looking at: the anti-stick effect

Urinary tract infections (UTIs)

Urinary tract infections such as cystitis are common, especially in women. The most frequent cause is E. coli, a bacterium from the gut. E. coli has tiny hair-like structures (fimbriae) that it uses to attach itself to the inner lining of the bladder. Researchers have studied whether the A-type PACs from concentrated cranberry juice or powder wrap around these fimbriae, so the bacteria can't attach as easily. That's the idea behind cranberry products. If you have symptoms of a urinary tract infection, talk to your GP or pharmacist.

Plaque on teeth

The same anti-stick property is also being studied in the mouth: researchers are looking at whether PACs make it harder for plaque-forming bacteria to settle on tooth enamel or below the gum line.

Important: watch out for added sugar on the shelf

A common mistake is to reach for a sweet, bright red "cranberry juice drink" or nectar in a carton. These drinks are mostly water with a lot of added sugar. If you want cranberry juice, look for 100% pure, unsweetened cranberry juice, often sold in dark glass bottles. It's very tart and astringent, so dilute it with plenty of cold water.