China said Tuesday it had arrested 32 people over the sale of cooking oil made from leftovers taken from gutters, in the latest food safety scandal to hit the country. The sting operation comes more than a year after state media revealed that up to one-tenth of cooking oil used in China was made from waste oil recycled from restaurants, which contains a carcinogenic substance. The Ministry of Public Security said police first received reports in March that a group of people were buying waste oil from restaurants and turning it into cooking oil. After a four-month-long investigation, police in the eastern provinces of Zhejiang and Shandong and the central province of Henan busted six places that sold the illegally made cooking oil, and detained 32 people. They also found more than 100 tonnes of the recycled oil made from leftovers taken from gutters, the ministry said in a statement. "Under the pretence of processing biodiesel, the suspects had been buying waste oil from Zhejiang, Sichuan and Guizhou (both in the southwest) since 2009, turning it into cooking oil and selling it on the market," it added. When the cooking oil scandal emerged last year, experts estimated that people in China consumed about two to three million tonnes of illegal oil every year. The revelation forced the food safety watchdog to step up its inspections, but experts said the business was extremely profitable because the cost of buying food waste and refining it was low. China's food industry is notorious for safety problems, despite regular government crackdowns. One of the biggest safety scandals emerged in 2008 when huge amounts of the industrial chemical melamine were found to have been illegally added to dairy products to give the appearance of higher protein content. The scandal was blamed for the deaths of at least six infants and for making 300,000 others ill in China. Two people were executed for their role in the incident. In September last year, authorities in China -- including the Supreme People's Court and the Ministry of Public Security -- called for tougher penalties including the death sentence in serious food safety cases.