This is a typical Keralan snack with raw green bananas, which have a taste more like potato than banana.
In a deep, heavy-based saucepan, pour enough vegetable oil to fill the pan about a third of the way. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 360 degrees F, or until a bread crumb sizzles, and turns golden brown when dropped in it.
Caution: Hot oil can be dangerous - do not leave unattended.
Add the slices of banana to the pan and, and using a slotted spoon, try to separate the chips from each other as they fry, otherwise they will stick together.
Mix together the water, turmeric powder, and salt, in a small bowl.
When the banana chips are beginning to become crisp and the edges are golden brown, carefully add the turmeric-salt powder to the pan. Take care as the oil will sizzle when the water is added. Stir gently to combine. When the banana chips are golden brown and cooked through, remove them with a slotted spoon, and drain on a tray lined with paper towels.
Sprinkle the chile powder, to taste, over the chips, and serve while the chips are hot.