Anyone who’s ever made a pudding cake has, for all intents and purposes, employed the tangzhong method. It’s the same basic idea: you add a pre-prepared starch gel to your batter/dough and what you get in return is a finished product that’s higher and lighter than it would otherwise be, that retains more moisture and that has a very tight and even crumb. The big difference of course that in a tangzhong (essentially “soup starter” in Chinese) there’s no sugar or flavorings in the mix — just flour and water combined at a ratio of 1-5 and cooked to roughly 150 degrees Fahrenheit.
But then what does the tangzhong gel do in the bread dough? It’s a very good question since baked bread is already a starch gel to some extent. But let’s back up a bit. Flour (white flour) as you’ll recall is nothing more than the finely ground endosperm of the wheat berry. Think of the endosperm as a dense pack of very long and stringy starch molecules all packed in together. Grind it and you get endosperm granules, which I think of as tightly bound bundles of sticks.
What happens when you get those bundles wet and hot? They start to come apart. Water molecules start working their way between the starches in the bundle. The bundle swells. As the process continues individual starch molecules start to break away from the bundles. If there’s a whole lot of water around the starches will actually float off and get tangled up with other stringy breakaway starches. The end result is a mesh of much smaller starch bundles and tangled starch molecules which traps and holds the water molecules around them. In other words, a gel.
Notice that the extent to which the process goes on is directly related to how much water there is in the mixture. If there’s some water but not a whole lot you won’t have a proper gel but something that’s often called “gelated” starch: a mass of starch granules that are dissolved only slightly, just enough so that a few starch molecules come loose and more or less tie all the granules together.
That works pretty well for the purposes of bread making. However this sort of structure is highly prone to what you might call a gelling reversal, which starts to happen as soon as the bread cools. The large granules start to contract and when they do they squeeze out the water molecules that had initially worked their way into them. The water then evaporates, the matrix hardens and you’re left with stale bread.
But what if you were to take steps to undermine the staling process by oh say, adding a very wet pre-made gel to the dough in the mixing step? You’d be introducing a whole bunch of small starch bundles and free starch molecules with lots and lots of water molecules trapped in between them — molecules that won’t be easily forced out of the matrix because they’re only loosely held by free starches, not crammed inside big granules that will eventually squeeze them out.
So you see that adding a tangzhong to a bread dough is very different than simply adding more water to a bread dough. When you add a tangzhong to a dough you’re not simply adding moisture, you’re adding a moisture-retaining structure that does double duty as a mechanical leavener, since the thick gel surrounds air bubbles in the rising dough, giving them more resilient walls and preventing them from popping. So you get a higher loaf in the bargain. And because the all the water in the gel undermines gluten development the individual bubbles never get very big, which means a very fine crumb. Also, the loaf is very tender and again because of all the moisture there is almost no crust.
A very, very neat trick, all from a little hot water and flour. Cool.