Preheat oven to 400°F (200°C) and coat a 6-cup (1.5 L) souffle dish with cooking spray. In a large bowl, beat egg yolks, sweet potatoes, yogurt, brown sugar, nutmeg, cloves, orange zest, orange juice, salt and pepper. In a separate bowl, beat egg whites and a pinch of salt until soft peaks form; fold into sweet potato mixture. Transfer to prepared souffle dish and bake for 35 minutes, until puffed and brown. Serve immediately.