Photo by Rick Geary

Doubling disrupts the proper liquid balance in the batter.

Matthew Alice: If you want to double the amount of product from a reaction, you have to double the amount of reactants. In other words, if you want twice as much green paint, mix twice as much blue and yellow paint. But if I need a double batch of my famous brownies, they are certain to fail if I simply double the ingredients and put them in a pan with double the area. In fact, I noticed on a box of brownie mix that you are supposed to use the same amount of oil in a double batch as in a single batch, less than double the amount of water, and double the amount of eggs. Why? — R.S., Mira Mesa

A: If blue paint + yellow paint = green paint is Chemistry 101, then brownie baking is some rarefied graduate-level seminar. Look at it this way. Flour + shortening + liquid + leavening = every kind of baked product from Saltines to angel food cake. It’s how you balance them that makes most of the difference.

Virtually every cookbook will warn that you shouldn’t even consider doubling a recipe for most baked goods because the chemical interaction among the ingredients is so touchy that something is bound to go wrong. Professional bakers create large-volume recipes using formulas based on the more accurate method of weights, not the volumes of the various ingredients, but they still have to tinker with the formulas to account for the types of fats, liquids, and flours used. Standard, homemade-type brownies are fairly sturdy and certainly more forgiving of errors than most cakes. You actually might get away with doubling your favorite recipe if you didn’t carry the logic through by baking them in a pan with twice the area. Batter meant for an 8-inch-square pan isn’t going to cook properly in a 9-by-13-inch pan. To rise properly, brownies depend on chemical action aided by heat. In the big pan, the edges will dry out before the center rises, and the whole thing will probably fall flat once it cools because the heat penetrates the batter unevenly. Divide the batter into two of the smaller-sized pans instead. And if they still don’t work out, call them cookies or fudge and eat them anyway. Or send ’em to me, and I’ll be glad to eat ’em.

But back to batter chemistry. Not knowing the recipe for R.S.’s Neverfail (if I Follow the Recipe) Brownies, we’ll have to work with the boxed mix instructions. Two boxes of brownie mix will contain twice the sugar (the main ingredient), flour, cocoa, baking soda, and commercially treated emulsified shortening. This kind of shortening helps counteract the sugar’s tendency to make the brownies heavy. And it also helps distribute the fats through the batter and make the flour retain more water. When you add twice the number of eggs to the boxed ingredients, you’re not only doubling the egg protein that holds the cooked batter together, you’re also doubling the emulsified fat and water. (Eggs are full of fat and water.) This disrupts the proper liquid balance in the batter, so you compensate by lowering the amount of water and vegetable oil in the two-batch recipe so the batter doesn’t turn gummy and thick. Fat is the only ingredient in your brownies that doesn’t change chemically when it’s mixed into the batter. It just distributes itself in smaller and smaller clumps throughout the mix. A little fat goes a very long way. All things considered, it’s a wonder any baked product comes out of the oven in some edible form. Batter’s a very touchy chemical soup. Actually, it might be easier to lose half your friends than to try to double your brownie recipe.