When I was a kid, there were always two loaves in our pantry: a whole wheat sandwich loaf and a milk bread. I went through a phase of taking plain slices of milk bread to school as a snack. During recess I reveled in the bread’s softness, pressing my face into each pillowy slice before eating it, ignoring the stares from the kids who just didn’t get it.
Part of what makes milk bread so satisfying to eat is that feathery softness, which many bakers attribute to tangzhong, a roux that’s incorporated into the dough. Tangzhong begins with a mixture of flour and liquid—usually water, milk or both—that is whisked together in a small saucepan over low heat until a thick, stretchy slurry forms.
Bakers often cite Yvonne Chen—the author of 65ºC Bread Doctor, which was published in Chinese in 2005—as the first person to popularize this technique. In her book Chen explains that heating flour and water at the high temperature of 65º C (that’s 149º F) helps to gelatinize the starches in flour, which improves water absorption and results in a bread that’s soft and elastic. Milk bread recipes frequently mention tangzhong’s magical ability to produce ultra-tender bread with a prolonged shelf life, but it’s rare to see an explanation of the actual science behind this phenomenon. What is it about starch gelatinization that makes tangzhong a better method for bread?
First, it’s important to understand the science of staling. It’s easy to assume that when a piece of bread goes stale, it’s because it’s dried out—moisture evaporates from the soft and pillowy bread, rendering it hard and dehydrated. But staling is actually a more complex chemical process than that.
In 1852 the French agricultural chemist Jean-Baptiste Boussingault demonstrated that a perfectly sealed loaf of bread could go stale even without evaporation. More than a hundred years later, the chemist Dr. G.H. Elton offered an explanation: The main staling process in bread isn’t actually dryness from water evaporation—it’s starch crystallization. As bread cools after baking, the starch molecules firm up. This can be a good thing, as it helps the crumb set and makes it possible to cut into a loaf without tearing it. But this crystallization also spells the beginning of the end. In his book On Food and Cooking, food scientist Harold McGee notes that as the starches cool down and crystallize, they “slowly rebond to each other in tighter, more organized associations, and the granule becomes firmer and harder, a process called retrogradation.” And as the starches crystallize, they can no longer hold onto water molecules. The water molecules move to surrounding gluten strands and turn a once supple loaf into a dry one.
Tangzhong helps to delay this process by pre-gelatinizing the starches: Starch granules are stiff when dry, and begin to swell when mixed with water. The temperature of the water matters greatly: McGee explains that starch granules only absorb a limited amount of water when the water they’re mixed with is cold. When the water is hot, however, the starches “absorb more water and swell up…the granules suddenly lose their organized structure, absorb a great deal of water, and become amorphous networks of starch and water intermingled.” This allows the starches to hold onto moisture for longer—even after baking—which makes for a longer lasting loaf. But does it contribute anything to its soft, airy texture?
Pastry Love by Joanne Chang
According to the authors of Modernist Bread, it does not:
