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As I understand it, tangzhong in relation to bread making is an Asian technique in which a portion of the flour in the recipe is pre-cooked with water to make a roux, before being cooled and added to the rest of the ingredients. It is supposed to make the resulting bread very soft, often for days longer than non-tangzhong breads. I thought maybe it works by binding the proteins so gluten development is restricted, but I'd have thought that would inhibit rise, which seems not to be an issue.

Disclaimer: I have not yet made a tangzhong bread, but I believe I have eaten a few... My first attempt is currently doing a bulk rise.

How does tangzhong work exactly? How does it actually change the dough to achieve the distinct softness for which it is so famous?

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The tangzhong is composed mainly of gelated starches. According to Harold McGee's "On Food and Cooking"

During the baking of bread and cakes, the starch granules absorb water, swell, and set to form the rigid bulk of the walls that surround the bubbles of carbon dioxide. At the same time their swollen rigidity stops the expansion of the bubbles and so forces the water vapor inside to pop the bubbles and escape, turning the foam of separate bubbles into a continuous spongy network of connected holes. If this didn’t happen, then at the end of baking the cool- ing water vapor would contract and cause the bread or cake to collapse.

In the Tangzhong, the starches are already swollen before mixing begins. This means that they do not need to compete with the proteins to absorb water as the completed dough warms up in the oven. As you mix the ingredients the plump starches perforate the gluten-water sheets. As McGee mentioned, these starch granules' rigidity accounts for the spongy, close structure of the tangzhong crumb, dough with less perforated gluten mesh forms larger bubbles and a more open crumb.

The gluten in the flour used in the tangzhong is denatured by the process before it can form into an extensive network. However, since the tangzhong accounts for only a small portion of the total flour used, there is plenty of gluten left to maintain structure during rise and oven spring until the starch structure sets.

Other resources:

Tangzhong Water Roux Pain au Lait: Soft, Springy Sandwich Bread

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I think I understand the basics of how tangzhong works now, but how does the 'spongy, close' crumb structure make the bread more soft (as opposed to just more moist)? Or is that partially the same thing? –  ccsdg Jan 5 at 18:20
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Moistness is integral to softness, as starch granules lose water they become hard. This is effectively what staling is, the water evaporates from the starch granules, which then contract, and the crumb becomes a web of chewy gluten matrix with hard starch granules interspersed within it. –  Didgeridrew Jan 6 at 0:11
    
Keep in mind that other ingredients like sugar, milk, and fat that are typically included in tangzhong bread recipes also limit gluten formation (yielding less chewiness) and prevent evaporation (slowing staling). –  Didgeridrew Jan 6 at 0:20

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