Soften the gelatin in 1/2 cup of the apple juice. Boil remaining juice with the sugar for 15 minutes to concentrate it. Mix the lemon and lime juice, and add all but 1/4 cup cornstarch to dissolve it in the mixture. Add both gelatin mixture and cornstarch mixture to the boiled juice and boil again rapidly for 10 minutes until very thick, stirring constantly. Taste for sweet and sour and add more lemon juice if wanted.

Mix in the walnuts and pour mixture into a 9-by-9-by-2-inch baking pan that has been dipped in cold water. Let paste harden 12 hours or overnight, then cut with a sharp knife into squares. Mix together the reserved 1/4 cup cornstarch and powdered sugar. Remove squares with a spatula and roll each in the powdered sugar mixture (the cornstarch helps keep the sugar dry).