A new study ahs revealed that raw cookie dough was behind the E. coli outbreak in 2009, which affected people from several states.

In 2009, 77 patients were identified with the illness while 35 people were hospitalized, according to Dr. Karen Neil and colleagues at the Centers for Disease Control and Prevention and state health departments.

The researchers included extensive traceback, laboratory and environmental analysis. The study also resulted in the recall of 3.6 million packages of the prepackaged commercial cookie dough. The team was not able to identify a particular source, vehicle or production process linked to the dough that could be blamed for the infection.

Researchers suspect that flour could be the ingredient that caused the infection but their research ahs not comprehensively backed the judgment. The study authors concluded "foods containing raw flour should be considered as possible vehicles of infection of future outbreaks of Shiga toxin-producing Escherichia coli."

They also said that producers must use heat-treated or pasteurized flour in ready to cook materials to minimise the risk of infections.

The study was published in Clinical Infectious Diseases.