The Shanghai Quality and Technical Supervision Bureau said Thursday it is further investigating a food safety issue involving Shanghai-based Bright Dairy & Food Co, after the company announced Wednesday to recall alkaline-tainted milk.

Alkaline water used to clean equipment was mixed into Bright Dairy's Ubest milk during equipment maintenance because an automatic valve failed to switch on Monday, the bureau said.

About 300 cartons of 950-milliliter Ubest milk available on the market Tuesday were affected, Bright Dairy said on its website, noting that it has already pulled all these cartons from shelves.

"We have made door-to-door apologies and provided compensation to around 20 consumers who bought the tainted milk," Gong Yanqi, a spokesperson of Bright Dairy, was quoted by news portal Netease as saying Wednesday, noting the milk purchased by these consumers are basically the only tainted products that were sold.

The dairy firm said it has conducted tests on equipment stability to ensure the quality of future products.

The Shanghai watchdog warned consumers not to drink tainted milk and asked Bright Dairy to offer reports including recall results, consumer complaints and food safety hazards. The bureau also sent some tainted milk samples for examination.

The incident will hurt the sales of Bright Dairy, Chen Lianfang, a dairy industry analyst at Beijing Orient Agribusiness Consultant, told the Global Times Thursday.

The recall comes just weeks after China's leading dairy producer Inner Mongolia Yili Industrial Group recalled baby formula milk due to mercury contamination. And last December, some milk products of China Mengniu Dairy Company were found to contain excessive amount of a cancer-causing substance.

Different from the melamine scandal that hit the dairy sector in 2008, the latest problems were caused by mismanagement rather than intentional wrongdoing, Chen said. "The companies need to improve their management and consumers should treat these companies with reason and give them time to improve."

In another incident, some drinks of US beverage giant Coca-Cola produced in Shanxi Province were reported to contain chlorine. The company made recalls half a month after the reports, triggering consumer dissatisfaction.

Mismanagement is forgivable, but companies should take responsibilities rather than covering up their mistakes, said Song Liang, a researcher at Beijing-based Distribution Productivity Promotion Center of China Commerce.

Bright Dairy's shares closed up 1.19 percent to 9.35 yuan on the Shanghai Stock Exchange Thursday after slumping about 4.7 percent in the morning.

"It is a good trend that consumers, through Weibo, have strengthened supervision on food safety and companies need to take the initiative to admit mistakes before the problems are revealed by consumers and media, so as to win consumers' trust," Song said.