Cracked the case: counterfeit nuts in China.

An unusual counterfeit crime wave has hit China as fraudsters target the country's walnut-loving population.

Walnuts are being taken apart, their insides removed and then refilled with cement so buyers do not know they have bought the real thing until they try to eat them.

In Henan province, walnut sellers are filling up their nuts with cement then gluing the shells back together. This allows the crooks to sell the nuts at market price, while the kernels can be sold seaprately.

Counterfeiting is a huge industry in China, from electrical goods to designer fashion, and there has been a surge in fake foodstuffs in recent years. In 2008, six babies died and thousands were admitted to hospital after consuming formula milk powder laced with melamine, a component of plastics.

Walnut fraud has become such a problem that a film has been produced telling consumers how to tell if their nuts are filled with cement.

Fake nuts big business.

In a step-by-step presentation, the film shows how the food fraudsters fill their nuts. One trick is that the cement is wrapped up in paper so it does not rattle when shaken by a buyer.

In 2011, reports emerged that rice was being covertly replaced by potatoes, sweet potatoes and even plastic industrial resin. Consumers started to realise something was wrong when they discovered that no amount of cooking would soften the resin "rice".

"Eating three bowls of this fake rice would be like eating one plastic bag," an official from the Chinese Restaurant Association said.

There has also been a problem with fake eggs, which were made in laboratories from alginic acid, potassium alum, gelatin and calcium chloride.

Walnut prices have increased in recent years.

The chemical white and yolk of the egg were injected into fake shells made from chalk. One clue for consumers is that after cooking the yolks often become bouncy.

China has also experienced meat scandals with marinated pork being passed off as much more expensive beef.

Meanwhile, walnuts have become a hot commodity in China and soaring prices have encouraged fraud.

Walnuts were once health toys in China's imperial court used to help blood circulation by being rotated in the palm of the hand. Today, some pairs of nuts sell for up to 30,000 yuan (£3,000), Reuters reported.

Walnut trader Hu Zhenyuan said: "Walnut investments go up every year. A pair of walnuts at 350 yuan 10 years ago can sell for 3,500 yuan or even 20,000 or 30,000 yuan."