Batter is used to help protect delicate foods such as fish from the intense heat of deep oil. A crisp coating forms which stops oil penetrating the food, so it stays soft and tender inside. The batter used for fish is usually a savory coating batter, using beer instead of milk to add flavor as well as air, or a more robust yeast-raised batter.
Yeast batter is preferred for the coating of fish because although it is strong enough to protect the fish, it has a particularly light texture. To make a yeast batter that will coat 350g fish, you will need 1 teaspoon dried yeast, 150ml milk or water, 125g plain flour and a pinch of salt. The yeast is dissolved in the liquid and then added to the flour and salt.
Unlike an ordinary batter, yeast-raised batter is beaten vigorously to develop gluten in the flour. Leave it to stand in a cool place for up to an hour, until risen and bubbly before using.
** Asian Recipes