Preheat oven to 350°F. Lightly coat six 8-ounce individual soufflé dishes with cooking spray; place dishes on a baking sheet. Refrigerate until ready to fill. In a small saucepan, stir together granulated sugar, flour and salt. Whisk in milk and lemon juice until combined. Cook and stir over medium-low heat for 8 to 10 minutes or until mixture is thickened and bubbly. Stir in lemon zest. Remove from heat.
In a large bowl, beat egg yolks with an electric mixer on medium to high speed for 3 to 5 minutes or until thick and lemon-colored. Stir milk mixture into yolks to combine.Thoroughly wash beaters with warm, soapy water. In another large bowl, beat egg whites with an electric mixer on medium to high speed until foamy. Add cream of tartar. Beat on high speed until stiff peaks form (tips stand straight).
Gently fold egg whites into milk mixture. Evenly divide batter among the prepared soufflé dishes. Bake about 20 minutes or until a knife inserted near the centers comes out clean. If desired, lightly sift powdered sugar over. Serve immediately.