To significantly improve tempura batter’s lightness, replace part of the flour with cornstarch, which does not develop gluten. Cornstarch also contributes to great crispness by increasing the batter’s starch content.
Switching from tap water to effervescent seltzer water lightens the batter further, because its bubbles provide lift as they escape during frying, making the coating even lacier. And the carbonation makes the batter slightly more acidic, limiting how much gluten can form.
But the ace in the hole for ethereally light fried food can be found in the liquor cabinet. The test kitchen has used vodka to minimize the effects of gluten in pie dough, since the alcohol in vodka inhibits gluten development by limiting the amount of water that’s available to the flour.
Substituting vodka for a portion of the water solves the problem of batter that toughens as it sits in an unconventional but very effective way. Because vodka is about 60 percent water and 40 percent alcohol, it makes the batter fluid and keeps gluten formation in check no matter how much you stir the batter or allow it to sit.