Place the banana slices in a bowl and sprinkle with lemon juice, sugar and rum. Toss lightly. Cover and marinate for 40 or 50 minutes. Batter: In a bowl combine the flour, salt and sugar. Add the beer and incorporate it into the flour. Do not overbeat, just blend it in. Add just enough water to give the batter the consistency of heavy cream. Add the melted butter and let the batter stand for 40 minutes to 1 hour. Heat the oil (to the depth of 3 inches) in a large skillet. You do not need a deep fryer. While the oil is getting hot, beat the egg whites and add them to the batter. Drain the banana slices and dip them into the batter. Drop 4 to 6 at a time into the hot fat and fry them until they are nicely browned. Drain the beignets on paper towels and sprinkle with lots of confectioners' sugar. Serve immediately. Variation: apple slices can be substituted for bananas. From "Seasonal Kitchen" by Perla Meyers.