combine one banana, some orange juice concentrate, and sugar-water mixture. Blend until smooth and transfer into a very large mixing bowl or large pot. Repeat this step until all the bananas, orange juice concentrate, and sugar-water mixture are blended. Add the pineapple juice to the banana-orange mixture. Stir until blended. At this point, you may freeze or refrigerate until ready to serve. When ready to serve, add equal amount of 7-Up to the banana-orange-pineapple mixture (50:50 ratio). This is very attractive in a punch bowl because there is a layer of foam on the top. Pureed strawberries or other pureed fresh fruits may be added to this punch.