There are hurdles to overcome for a crispy edible batter. The problems are that the batter itself has to dry-out at just the right time, and it must not absorb either oil or water, despite being cooked in oil from the outside, and blasted with water (steam escaping from the food) from the inside. To make matters worse, the gluten in wheat-flour is both water and oil-soluble.
Like a lot of cooking it becomes a compromise — a delicate balancing act between opposing and incompatible outcomes.
Ten percent or so of wheat-flour is the protein gluten. This complex molecule has both water and oil-soluble regions to its structure. Like most proteins, in its natural state it is bundled in on itself like a little ball of string.
When mixed with water and stirred, the gluten unfolds and becomes straighter. The longer strands entangle with themselves, trapping water and in doing so thickening the batter (like a gel).
The down side is that because these unfolded proteins are attracted to both oil and to water in different parts of their structure, they do the exact opposite of what we want a batter to do — they absorb wetness and oiliness and so promote greasiness.
It’s not helpful to remove the gluten altogether, as the batter would be too thin to coat. The Japanese tempura tries to keep unraveling to the barest minimum by light and gentle mixing with near-freezing water (battered food was introduced into Japanese cuisine by Portuguese missionaries, and, in typical style, the method was then refined to be something that transcended the original).
Another approach is to replace some of the wheat-flour with a gluten-free substitute such as tapioca flour or cornflour. A 50:50 mixture turns out to be a convenient ratio, and halves total gluten content. Or use a low-gluten flour, such as rice flour, instead of wheat-flour. This produces a lighter batter that is reminiscent of a tempura.
What can be done about the water? It’s too wet of course. The batter can only crisp when it dries out (and steam escaping from the food drops off). One way to regulate this process is to replace some (or all) of the water with alcohol. The alcohol boils at a lower temperature than water, so it is evaporated off more quickly. It’s no coincidence that most batters call for some amount of alcohol.
What else can be done? Generally, light and crispy batters are preferred, and one modernist technique is to put the batter in a whipping siphon (the sort used to make whipped cream, which is charged with nitrous-oxide cartridges and the cream dispensed through a nozzle). This greatly aerates the batter and makes a big improvement — whipped ‘cream’ batter. This is an example of a modernist use of an ordinary device.
Here are two batter mixes that I have found successful, and that serve different purposes:
Crisp. Firm. Airy.
Wheat-flour: 125g, potato starch: 125g, powdered yeast: 2g, egg: 1 (50g), lager: 330ml, Salt: 6g
Rice four: 225g, corn starch: 45g, cold water: 225g, egg: 1 (45g)
In each case, mix to a smooth paste and place in a 1L-whipping siphon with two charges of N2O. I strongly recommend this step, although the batters will function without it.
Let rest in the refrigerator for ~30mins, or better, use an ice-water bath.
Dispense in small batches to coat food as required.
Be sure oil temperature is at least 190C. Fry.
Drain the fried food quickly, and then place on paper towels to wick off the remaining oil. As the food cools, it will absorb oil clinging to its surface and become greasy, so the more oil that can be physically removed the better. Draining is not enough because of oil’s high surface tension.