In a study published this week, researchers at the University of Wisconsin-Madison’s (UWM) Food Research Institute have concluded that caramel apples most likely become contaminated with Listeria from one source--the inserted dipping sticks upon which they stand. Additionally, they found that apples stored at room temperature pose higher health risks.

Last year, 35 illnesses, seven deaths and one miscarriage are all believed to have been the result of a multi-state Listeria outbreak stemming from pre-packaged caramel apples. The recalled Granny Smith and Gala apples were traced to Bakersfield, CA-based Bidart Bros. Testing had revealed the presence of Listeria at the company’s apple plant.

This new research by UWM was prompted because experts couldn’t figure out why or how Listeria could grow or flourish among hot caramel or raw apples because Listeria is known to thrive in moist, less acidic environments.

For this study, researchers began by deliberately covering the plain apples with Listeria to analyze its growth in a number of different environments. Using clean, well-washed apples with no bacteria would not have yielded the outcomes researchers were looking for. It is believed that when inserting the stick into the apple, inner juice leaks to the surface and creates an ideal microscopic environment between the apple and the caramel where bacteria can grow. According to research findings, the flourishing of such bacteria appeared to accelerate when apples were left unrefrigerated.

Although researchers agree that pre-packaged caramel apples should remain refrigerated until ready to eat, they just be warmed up to keep the hardened caramel from possibly chipping a tooth.