Mix the flours, cornmeal, baking powder, baking soda, and salt in a large bowl.
Add a mixture of the buttermilk, molasses, and raisins; stir only enough to moisten the flour.
Pour an equal amount of batter into 3 well greased cans (18 to 20 ounces each).
Cover cans tightly with aluminum foil or baking parchment tied securely with string.
Steam 3 hours.
Remove cans from kettle; run a knife around inside of cans to loosen loaves and unmold onto wire rack.
Serve warm, or store the cooled loaves (wrapped in moisture-vaporproof material) in a cool place.