This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The amount of liquid used in popovers is large, and thus the hydration of the gluten particles is too great for them to adhere tenaciously to each other. Therefore, the popover batter may be beaten without increasing the compactness and without toughening the baked popovers to any appreciable extent. Egg is added to popovers. It is necessary to furnish additional, extensible protein to hold the steam formed from the liquid and to coagulate during baking to form the popover walls. If baked with no added egg, popovers do not increase in volume during baking and the texture is very soggy and compact. Except during the period of the year when the baking quality of the eggs is greatest, better popovers are often produced if the number of eggs in the recipe is increased to 3. If the eggs are not weighed and are small in size, better volume is always obtained by using 3 eggs.
The popover batter may be put into hot or cold containers for baking. There seems to be little difference in the finished popovers, whichever way started.
Popovers are probably best baked at rather high temperatures, 220° to 240°C, to form steam rapidly. They need to be baked until they feel firm while still in the oven, for they soften after being taken from the oven, and unless they are quite firm will collapse. The heat may be turned off towards the last of the baking period to prevent excessive browning and the popovers left in the oven to become crisp, for the inside of the popover is hollow and because of the thin batter rather moist.