As tempting as it may be to eat raw cookie dough, scientists who investigated the 2009 outbreak of E.coli in the United States recommend baking the cookie dough before eating it.

A new study that looked into the May 2009 E.coli outbreak that sickened 77 people in 30 states after eating tubes of Nestle's Toll House raw cookie dough found that the source may be raw flour.

"My recommendation, the general recommendation, is that you should not consume raw cookie dough, regardless of who makes it, whether it's made at home or as a commercial product," Dr. Karen Neil, a medical epidemiologist at the Centers for Disease Control and Prevention, said in an interview with the Associated Press."That is the safest thing to avoid illness."

The report published Friday in the journal Clinical Infectious Diseases details the investigation from the first outbreaks of E.coli infection in 13 states to interviewing the patients to trying to determine which ingredient caused the illnesses. The patients were then asked what foods they had recently eaten. Among the foods frequently eaten were strawberries, ground beef, poultry, apples, leafy greens. For a time the most likely suspects were ground beef and leafy greens but evidence seemed to point in a different direction.

"Because of that, we kind of scratched our heads and tried to think about what other things could be causing this outbreak, and so we ended up doing these open-ended interviews," Neil said. Five patients pointed to raw uncooked cookie dough.

"And so we asked more cases about cookie dough and started focusing on cookie dough and, lo and behold, a lot of people ate raw, pre-packaged commercial cookie dough, and a lot of them were naming a specific company," she said.

After some testing, the cookie dough products turned up positive for E.coli.

Despite finding out the culprit for the outbreak, researchers were still unable to find out why cookie dough caused the illnesses. The researchers ruled out some factors such as food handling, safety violations and intentional contamination and even looked into specific ingredients like unpasteurized eggs, sugar, margarine, chocolate chips and baking soda. All of them were exonerated except for flour.

Although there is no smoking gun that flour was the main cause of the outbreak, the researchers made the case that flour was the only ingredient that could have been distributed to a number of lots. And flour isn't processed to kill pathogens.

So to reiterate don't eat raw cookie dough. The researchers also suggested that companies should look into using heat-treated flour because people are still going to be eating raw cookie dough anyway.