Directions for making a buttermilk substitute by adding lemon juice to milk always call for letting the mixture sit for a while. Can you skip this rest if you're short on time, or is it important?
“Clabbered” milk is widely recommended as a substitute for buttermilk in baked goods. The usual approach is to stir lemon juice into milk (1 tablespoon per cup) and let the mixture sit for 10 minutes to “clabber” (or thicken) before proceeding with the recipe. But after following this method and closely observing what transpired, we discovered that clabbering milk doesn’t give it the smooth, thick consistency of buttermilk. Small curds formed almost instantly, but after a 10-minute rest, most of the milk had not thickened at all. And more waiting still didn’t give clabbered milk the consistency of buttermilk.
It turns out that when lemon juice is added to milk, the citric acid changes the electrical charge on the dairy’s casein proteins, causing them to coagulate tightly into clumps. On the other hand, the Lactobacillus bacteria added to milk to produce commercial buttermilk remove some of the sugar molecules bonded to the proteins, allowing them to form a gel that gradually becomes thicker over time.
So, does waiting after treating milk with lemon juice impact its baking properties? To find out, we made multiple batches of biscuits and buttermilk pancakes: one set with clabbered milk that had rested for 10 minutes and one set in which we mixed the milk into the batter immediately after adding the lemon juice. All of the biscuits and pancakes were virtually identical in appearance, flavor, and texture.
Our conclusion: Adding lemon juice to milk simply acidifies it, allowing the leavening in the batter to do its job—the same role played by buttermilk. Since this change happens immediately, you can safely skip the resting time.