Michigan State University Extension
Preserving Food Safely - 01600822
Green tomatoes will usually ripen if they are removed from the vine and wrapped individually in newspaper. (Wrapping the tomatoes helps to prevent rotting and provides the proper atmosphere for ripening to occur). Allow tomatoes to ripen at room temperature or slightly cooler. It may take awhile for totally green tomatoes to ripen.
If at all possible, green tomatoes should be harvested before the first frost. Green tomatoes hit by a light frost should ripen if treated as outlined above. However, if they are hit be a heavy frost, they will not ripen and should be used as soon as possible in the green state.