The day before you plan to serve the pudding, line a 2 quart mixing bowl with plastic wrap. Line the plastic wrap with the white bread slices, trimming to fit so that the pieces fit together neatly. Save enough bread to cover the top of the pudding.
In a saucepan, combine the apple juice concentrate and all the berries. Bring the mixture to a boil and cook until the strawberries begin to soften, about 1 minute. Turn off the heat immediately.
Stirring, dissolve the gelatin in the water, then add the mixture to the berries and stir to combine. Spoon into the bread-lined bowl. Cover with remaining bread slices, cover the bowl with plastic wrap, and slightly weight it down. Chill overnight.
When ready to serve, whip the cream with an electric mixer at high speed until it forms soft peaks.
Remove the pudding from the refrigerator and remove the plastic wrap. Carefully turn the pudding out onto a platter, remove the bowl and the plastic wrap. Serve in slices with whipped cream on top.
Recipe courtesy of Gale Gand, from "American Brasserie" by Gale Gand, Rick Tramonto, and Julia Moskin: Wiley Publications Inc., 1997