A local media report has raised concerns that some small restaurants and catering companies are using old rice, which has been stored for more than a year, in order to cut costs.
Though sales of such rice is banned in Shanghai, the Youth Daily reported that some businessmen in wet markets are still selling old rice to small restaurants.
The price of fresh rice is 3.4 (41 US cents) to 3.6 yuan per kilogram, while old rice sells for only 2.6 yuan per kilogram.
According to Youth Daily, a rice vendor in the Dagu Road Wet Market said she sells old rice to small restaurants to make box lunch and some of the rice looks good after it is cooked.
The owner of a small restaurant on Zhongxing Road in Zhabei District also told Youth Daily he bought old rice from a local wet market, one of the four to five stable suppliers in the city where such rice is available.
Officials from the Shanghai Industrial and Commercial Administrative Bureau said authorities have take efforts to rid markets of old rice and welcome local residents' tips.
"Inspections for old rice should be the responsibility of local health facilities," said Zhou Shuguang, a bureau official.
However, officials from the Shanghai Health Supervision Agency said they only check to see if rice has gone moldy or bad, and pay no attention to how long it has been in storage.
"The agency pays a great deal of attention to producers of local students' box lunch and big licensed box lunch companies, while most violators using poor-quality rice are small and unlicensed stores," said Zhang Wei, an official with the agency's inspection team.
 "Such stalls are like mushrooms hidden in alleys. It is difficult to detect and eradicate them."
Medical experts said that the old rice can be harmful.
"Old rice contains fewer vitamins, which can flow away with age," said Li Mingyue, a nutritionist at Baosteel Hospital.
Li added that stored rice can contain aflatoxins - toxic compounds caused by certain molds - which can be dangerous if being eaten in large quantities.
Since box lunch is popular among office clerks and workers, old rice can be potential danger to people's health if being eaten for a long time, experts said.