If you’ve ever sat down at a pub to a plate of really good fish and chips—the kind in which the fish stays tender and juicy but the crust is supercrisp—odds are that the cook used beer as the main liquid when making the batter. Beer makes such a great base for batter because it simultaneously adds three ingredients—carbon dioxide, foaming agents and alcohol—each of which brings to bear different aspects of physics and chemistry to make the crust light and crisp.
Beer is saturated with CO2. Unlike most solids, like salt and sugar, which dissolve better in hot liquids than they do in cold, gases dissolve more readily at low temperatures. Put beer into a batter mix, and when the batter hits the hot oil, the solubility of the CO2 plummets, and bubbles froth up, expanding the batter mix and lending it a lacy, crisp texture.
That wouldn’t work, of course, if the bubbles burst as soon as they appeared, as happens in a glass of champagne. Instead beer forms a head when poured because it contains foaming agents. Some of these agents are proteins that occur naturally in the beer, and some are ingredients that brewers add to produce a creamy, long-lasting head. These compounds form thin films that surround the bubbles and slow the rate at which they burst.
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Foams also make good thermal insulators. When you dunk a piece of beer-battered fish into a deep fryer, most of the heat goes into the batter rather than into the delicate food it encloses. The bubbly batter can heat up to well over 130 degrees Fahrenheit—the point at which so-called Maillard reactions create golden-brown colors and yummy fried flavors—while the fish gently simmers inside.*
The alcohol in the beer also plays an important role in moderating the internal temperature and crisping the crust. Alcohol evaporates faster than water, so a beer batter doesn’t have to cook as long as one made only with water or milk. The faster the batter dries, the lower the risk of overcooking the food. If the chef works fast enough, he can create a beautiful lacework in the coating that yields that classic beer-batter crunch.
*Erratum (2/10/11): The temperature is incorrectly stated as Fahrenheit. It is 130 degrees Celsius.