What does the beer in a beer batter coating do? Can something else be substituted?
Beer batter—made by combining beer (usually a lighter style such as a lager), egg, and flour—is often used to coat fish, onion rings, and other types of pub-style fare before deep-frying. Though we’ve found that including hard liquor in the batter can lead to more-tender results in tempura, the alcohol in most lagers and pilsners is so low (about 5 percent by volume) that its effect would be minimal at best. Far more important is the fact that beer is carbonated, which affects the batter in two ways. First, the bubbles provide lift as they escape from the batter during frying. Second, the carbonation makes the batter slightly more acidic, which limits how much gluten can form when the beer and flour mix, preventing the batter from turning tough. This is because gluten forms most readily in a pH of 5 to 6, while most carbonated beverages share a similar pH of 4 (unless they contain a strongly acidic ingredient). In theory any bubbly drink with a neutral or appropriate flavor profile could serve as a substitute. To prove this point, we fried fish in batters made with beer, nonalcoholic beer, seltzer, and water and found that all the batches with a carbonated beverage did indeed lead to noticeably lighter, lacier crusts than the batter made with plain water. In sum, carbonation and pH are the biggest factors in delivering a better batter-fried crust, so feel free to use bubbly substitutes such as nonalcoholic beer or seltzer water.