Thousands of dead pigs in a Shanghai river have cast a spotlight on China's poorly regulated farm production, with the country's favourite meat joining a long list of food scares.

As of Friday, the number of carcasses recovered in recent days from the Huangpu river -- which cuts through the commercial hub and supplies over 20 percent of its drinking water -- had reached more than 7,500.

Shanghai has blamed the farmers of Jiaxing in neighbouring Zhejiang province for casting pigs thought to have died of disease into the river upstream, but officials from the area have admitted to only a single producer doing so.

The city has stepped up inspections of markets to stop meat from the dead animals from reaching dining tables of its 23 million people.

From recycled cooking oil to dangerous chemicals in baby milk powder, a series of food scandals in China has caused huge public concern.

Pork is king in China, accounting for 64 percent of total meat output last year, and urban residents with growing wallets and waistlines ate 20.63 kilograms (45 pounds) of the meat per person in 2011.

Images of Shanghai's dead pigs have hit the nation's collective gut, but in Zhulin village, a major hog-raising centre in Jiaxing, the farmers claim their innocence in the scandal.

"The government is very strict. We give our pigs vaccinations. If they are sick, they can't be sold," said Pan Juying, 57, as she hoisted two baskets of freshly cut grass to feed her eight pigs.

But a bloated piglet lying by the roadside a hundred metres (yards) away from a stream showed that not all dead animals are properly disposed of.

Wang Wei, a veterinarian for the Hengyuan Company which produces medicines for farm animals, said a large number of pigs died from unknown causes in Zhejiang just before and after the Chinese Lunar New Year in February.

"It must have been a big pig farm" that was responsible for events in Shanghai, he said in Zhulin's main street, which is lined with animal medicine and feed stores. "They can't control an outbreak of infectious disease."

Despite laws against the practice, animals that die from disease in China can end up in the food supply chain or improperly disposed of.

In Wenling, also in Zhejiang, authorities announced this week that 46 people had been jailed for up to six-and-a-half years for processing and selling pork from more than 1,000 diseased pigs.

China was rocked by one of its biggest-ever food safety scandals in 2008 when the industrial chemical melamine was found to have been illegally added to dairy products, killing at least six babies and making 300,000 people ill.

Across China, cheap recycled cooking oil is available made illegally from leftovers scooped out of restaurant drains. Amid public disgust authorities arrested more than 30 people over its sale, but it remains commonplace.

In another recent incident, US fast food giant KFC was hit by controversy after revealing some Chinese suppliers provided chicken with high levels of antibiotics, in what appeared to be an industry-wide practice.

Zhu Yi, a professor at China Agricultural University, said that the country's vast number of small-scale farmers were "hard to supervise and regulate".

"Food safety is an issue that requires continuous efforts, you simply cannot put everything right once and for all," she said. "The current livestock breeding model is too crude, and the standards too low."

But large-scale production also carried risks of its own, she added.

"Every country has its own problems," she said. "The highly industrialised European Union was caught up by the horse meat scandal."