Preheat oven to 325°.
Cook broccoli in boiling water 4 minutes or until tender. Drain. Cool to room temperature on paper towels.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and milk in a saucepan; stir well. Bring to a boil over medium heat; cook 1 minute, stirring constantly. Reduce heat to medium-low. Stir in sour cream, rind, salt, yolks, and garlic; cook for 1 minute or until thick, stirring constantly. Pour mixture into a large bowl, and stir in broccoli.
Beat the egg whites with a mixer at high speed until stiff peaks form. Gently stir one-third of egg whites into broccoli mixture; fold remaining egg whites into broccoli mixture. Spoon mixture into a 2-quart souffle dish coated with cooking spray. Place on middle rack of oven. Bake at 325° for 40 minutes. Serve immediately.