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To the dismay of teenage girls everywhere, researchers who investigated a 2009 outbreak of E. coli that sprang from tubes of Nestlé’s Toll House raw cookie dough are advising people to bake their cookies before eating them.

The 2009 outbreak was the first to link E. coli O157:H7 with ready-to-bake prepackaged cookie dough (it was again linked to E. coli in January 2010). From March to July 2009, 77 people in 30 states were sickened in the outbreak; 55 were hospitalized. Two-thirds of those who fell ill were under 19, and 71% were female. Many said they’d bought cookie dough with no intention of ever cooking it.

Reporting [PDF] on Friday in the journal Clinical Infectious Diseases, the researchers, led by Centers for Disease Control and Prevention epidemiologist Dr. Karen Neil, came to two conclusions:

Manufacturers should consider formulating ready-to-bake commercial prepackaged cookie dough to be as safe as a ready-to-eat product. More effective consumer education about the risks of eating unbaked cookie dough is needed.

Investigators launched an extensive traceback effort at the time of the outbreak, including laboratory and environmental analyses, to identify its origin. Although it led to the recall of 3.6 million packages of cookie dough, researchers were unable to pinpoint any single ingredient, vehicle or production process as the source of the contamination.

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The eggs used in ready-to-bake cookie dough are pasteurized, which rendered them an unlikely culprit. Other ingredients like molasses, sugar, baking soda and margarine all undergo “kill steps” during processing, which eliminates pathogens. Chocolate chips — which have been implicated in outbreaks of food-borne illness in the past — were another suspect ingredient, but in this case it turned out that chocolate chip cookie dough was less strongly associated with E. coli illness than other cookie-dough flavors.

The investigators ultimately settled on flour as the likely offender. Flour doesn’t usually undergo a kill step. Also, because it is frequently purchased in large quantities by food manufacturers for use in products, a single purchase of contaminated flour could have been used in multiple lots and varieties of cookie dough over a period of time.

The authors conclude that foods containing raw flour should be considered possible vehicles for E. coli outbreaks in the future, and encourage manufacturers to consider using heat-treated or pasteurized flour in any ready-to-cook products that consumers may choose to eat raw, despite package label instructions warning against it.

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But seeing as how raw-cookie-dough junkies aren’t likely to give up their habit anytime soon, the authors suggest that all commercial manufacturers of ready-to-bake prepackaged cookie dough just make stuff safe to eat raw.

Nestlé and several other cookie-dough makers have already switched to heat-treated flour.

Previous E. coli-related food-borne illnesses have been associated with ground beef, leafy green vegetables, sprouts, melons, salami, and unpasteurized apple cider.

Sora Song is the editor of TIME Healthland. Find her on Twitter at @sora_song. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.