Grease two 28-oz (796 mL) tomato cans. Grease two 8-inch (20 cm) squares of parchment paper or foil; set aside.

In large bowl, whisk together whole wheat flour, rye flour, cornmeal, baking soda and salt. In separate bowl, whisk together buttermilk, molasses and apple sauce; pour over flour mixture and stir just to blend. Divide between prepared cans. Cover tops with prepared paper, greased side down and leaving 1-inch (2.5 cm) space for bread to rise; tie securely with string.

Place cans on steamer rack or heatproof trivet in deep pot or Dutch oven just large enough to accommodate width of cans. Pour in enough boiling water to come halfway up sides of cans. Cover and return to boil over medium heat; reduce heat and simmer until cake tester inserted in centre comes out clean, about 2-1/2 hours. Remove from cans; let cool on racks.