Chris Young, co-author of Modernist Cuisine: The Art and Science of Cooking, headed The Fat Duck Experimental Kitchen for five years.
Clothe your food
For delicate foods that aren’t too large, there is another approach to deep-frying in a single step without overcooking: clothe them with a wet batter. As the water in the batter steadily boils, it will keep the effective cooking temperature beneath the surface cooler. Depending on the batter used, this can be a slight cooling effect (thin tempura-like batters) or it can be substantial (thick beer-batters).
If greater protection of a delicate food is necessary, but a thin crust is still desired, then a good strategy is to foam the batter slightly. As the foam dries and hardens, it further insulates the food from the intense heat of the frying oil. Foaming a batter can be as simple as incorporating a carbonated beverage into it. For greater insulation and a crisper crust, include a leavener into the recipe. Baking powder does an excellent job of aeration; when heated, the acids and alkaline ingredients it contains react to fill the batter with bubbles of carbon dioxide. Finally, for a more direct approach, you can go high-tech with a whipping siphon. This approach has the advantage that the batter is foamed to order and thus can be held throughout a service without losing its bubbles.
Another useful trick is to replace some of the water in a batter with alcohol. Neutral spirits like vodka work well and can replace as much as 40 percent of the water in a typical batter recipe. Although the alcohol will destabilize a foamy batter, it also effectively dries out the batter so that it cooks quickly. And because alcohol boils at a lower temperature than water, it has a stronger cooling effect. This means that the food inside the batter is exposed to a lower effective cooking temperature and the batter itself becomes dry and crisp sooner. Thus, alcohol-imbued batters are particularly useful for the most delicate foods, such as fish, that are easily overcooked in a deep fryer.
Sometimes a delicate, crisp coating isn’t ideal, and a hefty, crunchy coating is more suitable. Breading is the solution. It involves taking solid particles, such as bread crumbs, panko, even flaked or puffed cereals, and affixing them to the food using a binder like eggs, dissolved gelatin, or other modernist hydrocolloids. As with batters, breading insulates the food it clothes, but because breading recipes tend to be fairly dry, they provide much less of a cooling effect. Thus, we usually use breading for the texture it imparts. Breading can also become sturdy enough to form a crunchy shell that holds its shape, even as delicate centers become molten and soft. The cromesqui, or croquette, is a particularly excellent example of this technique in action.
A crispy batter or crunchy breading is the hallmark of great deep-frying. But when poorly executed, deep-fried foods clothed with a crust become unpleasantly greasy. The surprising fact, however, is that greasiness is mostly the result of what a chef does after the food has been fried.
For the complete article on deep-frying, including a discussion of the most important ingredient in deep-frying (oil), please click here.