If you’re making a half batch of muffins or cupcakes and thus don’t fill all the cups in the pan with batter, many sources suggest filling the empty cups with water. Is this really necessary?
Proponents of this practice contend that filling empty cups with water serves two functions: preventing the pan from warping and acting as a “heat sink” to ensure that muffins next to empty cups heat evenly (avoiding stunted growth or spotty browning).
We tested this theory by baking one muffin pan completely filled with batter, one pan in which only half of the 12 cups were filled with batter and the remaining six with water, and one pan in which six of the cups were filled with batter and the other six left empty. The results? All muffins had the same height, texture, and color, and none of the tins warped.
On reflection, the results made sense: In a full 12-cup muffin pan, all but the two center muffins are directly exposed to the oven’s heat on at least one side to no ill effect. Furthermore, if your muffin pan warps, that’s a sign that you need to find a better quality pan.