With the world's largest population and a native cuisine noted for its use of oil as an ingredient, China consumes about 25 million tons of cooking oil each year, about one fifth of the global total.

Yet what to do with used oil and how to tap its commercial value have long posed challenges to inventors and researchers.

A recent KLM Royal Dutch Airlines' search for cooperation with Chinese companies to collect used cooking oil and make aviation fuel has again triggered calls for China to have similar technologies to make better use of the waste.

Chinese scientists began researching how to transform used oil into something valuable nearly 10 years ago.

The first patent filing for using waste oil came in 2003 when applicants created a process to make fuel for industry that has more latent energy than diesel oil.

To date, 94 applications for invention patents and 22 for utility patents to use waste cooking oil have been filed in China, and some mature technologies have been patented, according to the State Intellectual Property Office (SIPO) patent database.

"Though research in the field started late in China, some breakthroughs have already been made, especially in processing the oil into biofuel," said Wang Qunhui, a professor in the School of Civil and Environmental Engineering, Beijing Science and Technology University.

Yet "few scientists and scholars now want to research it because this kind of technology is hard to commercialize as it can barely create a profit", she noted.

Biofuel made from waste cooking oil sells for about 5,000 yuan ($775) a ton, nearly the same as the cost of processing it, which is 4,000 to 5,000 yuan a ton, said Wang.

"And there is no effective and convenient channel to collect used oil for recycling, which has deepened the difficulty in commercialization," she added.

One technology that can be commercialized is equipment to detect unsafe used oil.

According to industry watchdogs, about 10 percent of the cooking oil consumed in China each year is an unsanitary recycled product made by shady businesspeople that is mostly used in small restaurants due to its low cost.

"The oil looks clean, but poses a serious health risk and can even cause cancer," said Zhao Xiaolian, a food safety expert and general manager of Jin Kun Biology Co Ltd.

"Authorities have cracked down hard on the illegal product and imposed harsh penalties, but there are still people taking the risk because of the lure of money."

Yet he noted "research on detection of unclean oil is on the rise".

Nine patent applications on used oil detection technologies can now be found in the SIPO database, all filed in 2010.

Zhao said the Jin Kun company "created a detection instrument last year that can tell whether the oil is processed from used oil or not".

In addition to more advanced technologies and measuring instruments, experts have also called on various levels of government to strengthen management and regulation of recycled kitchen waste to control the source of oil used in recycling.