According to Forbes magazine's 2007 survey of billionaires, Theo Albrecht is the 22nd richest person on the planet. Other than that, not much else is known about the recluse.

Even his birthday is subject to speculation, and biographers argue about whether it really fell on March 13, 1922. They are also not sure where he was born. It was potentially in the German town of Essen, where Theo and his brother Karl took over their mother's grocery store after World War II.

In 1962, the brothers opened the first Aldi (Albrecht Discounter) store. Now, there are more than 7,800 stores worldwide -- from America, where the Albrechts also own Trader Joe's, to Australia. Aldi is one of the world's biggest retail firms, with an estimated yearly turnover of 38.1 billion euros ($50.2 billion).

No knickknacks

The Albrecht brothers adopted a strict no-frills approach to their stores. They did away with in-store decorations and sell only a limited the number of products (often own-brand), which are stacked in cardboard boxes on shelves and pallets.

The secret of Aldi's success is "actually quite easy" according to Dieter Brandes, a former Aldi manager who has written several books about the company.

"When you can go in to any store, and get the best quality and you're guaranteed the lowest prices, there's no reason not to go there," Brandes said.

Brandes explained that the Aldi formula functions on keeping the stores and business structure simple.

"Others think that they have to build up an enormous administrative apparatus and that they need a controlling department and management consultants," he said. "Aldi doesn't have all of this."

Although some people might baulk at shopping in an industrial looking store, the cheap prices have proven a draw-card -- 86.5 percent of all Germans regularly shop at the discounter.

In the 1990s, Aldi introduced special offers for items such computers and mobile phones. They have now expanded to offering a pay-as-you-go rate for mobiles, and even package tours.

Brandes said he is a "bit skeptical" about the changes.

"But Aldi's principle is to have a limited range for daily needs, and travel has become a daily need," he said.

Secretive family

Both Albrecht brothers are publicity shy, and the Aldi company has been remarkably free from scandals and corruption rumors. The only time Theo Albrecht made the headlines was when he was kidnapped in 1971. He was freed 17 days later after paying 7 million marks ($3 million) in ransom.

Theo's older brother Karl, aged 87, has already retired from the Aldi business, and even Theo is giving up more and more control.

Despite an estimated personal income of 17.5 billion euros, Theo Albrecht is said to live modestly on the German island of Föhr in the North Sea where he reportedly plays golf and collects typewriters.

Brandes said he believes the billionaire will be around for some time yet, "I always thought -- that's a man that could live to one hundred."