It happened around 10am at the Denton Food Mart on Denton Street at Laura Koppe Road.
The store was not open at the time, so no one was inside when the fire started.
Crews arrived to find heavy smoke showing.
One firefighter hurt a shoulder while gaining entry to the building and was transported to an area hospital, officials said.
The building sustained smoke damage and firefighters cut a hole in the roof to ventilate the shop.
They later determined the fire started near a cooler, but the exact cause is under investigation.