A number of foods, especially seafood, poultry, and vegetables, are sometimes coated with a layer of flour batter before deep-frying (or sometimes baking). A good batter adheres well to the food, and fries into a crust that has a long-lasting crunchiness and that readily breaks apart in the mouth without an oily residue. Problematic batters fall away during frying or produce a crust that’s greasy, chewy, and tough, or soft and mushy.
Batters include some kind of flour, a liquid that might be water, milk, or beer, sometimes a chemical leavening to provide gas bubbles and lightness, and often eggs, whose proteins promote adherence to the food and allow the use of less flour. Of all the ingredients, the flour has the largest influence on batter quality. Too much can produce a tough, bready coating; too little and it will be fragile. The gluten proteins in ordinary wheat flour are valuable for the clinginess they provide, but they form elastic gluten and absorb both moisture and fat, and so are responsible for both chewiness and oiliness in the fried crust. For these reasons, moderate-protein flours make better batters than bread flour, and some batters are made from other grains, or from a mixture of wheat flour and other flours or starches. Rice proteins don’t form gluten and absorb less moisture and fat, so batters that contain a substantial proportion of rice flour fry crisper and drier. Similarly, corn flour improves crispness because its relatively large particles are less absorbant, and its proteins dilute wheat gluten and reduce the chewiness of the crust. Adding some pure corn starch also reduces the proportion and influence of wheat gluten proteins. Root flours and starches don’t work well in batters because their starch granules gelate and disintegrate at relatively low temperatures, do so early in frying, and produce a soft crust that gets soggy quickly.