The Science Behind... scalded flour

After accidentally using hot water instead of warm water during the autolyse period of Preston Yancey's white bread, I was surprised to learn that intentionally scalding the flour is a technique used in Japan for super-soft bread.

I wanted to learn more.  Why does scalding the flour lead to soft bread?

In this post, I will answer the following questions:

1. What is an autolyse?
2. Why would you add boiling hot water?  Why can't you do this for regular bread recipes?
3. What is starch gelatinization?
4. What is tangzhong?
5. How can I use scalded flour in my bread baking?

1. First off, what is an autolyse?  It's a technique used by some bakers to boost the gluten formation in their bread. By mixing water and flour before adding any other ingredients, you allow the protein to absorb all the water without having to share it with the starch and without being stressed by the carbon dioxide bubbles from the yeast pushing against the gluten net before it's had a chance to firm up.  After an autolyse of just a few minutes, the gluten is very well formed and is ready to stretch with the expansion of yeast bubbles.  Salt and yeast are added after the autolyse period is completed.

2. What makes scalded bread different from the autolyse?  Why would you add boiling hot water to flour?  With an autolyse, you add warm water.  This is because protein can absorb more warm water than cold water. The autolyse is purely to develop the gluten.

Scalding flour with boiling water, in contrast, activates another chemical reaction entirely-- starch gelatinization.  Instead of being absorbed by the protein, the hot water is absorbed into the starch molecules.  That leaves the gliadin and glutenin (the proteins in flour responsible for creating gluten) bereft of the water they need for strong gluten formation.

Bread with high gluten formation tastes chewy.   This is why you don't want to overmix doughs like muffins and cakes- they can become rubbery.  By taking water away from the gluten and jump-starting starch gelatinization instead, you create a dough that is super soft.

Why can't you do this for regular bread recipes? Simply put, you cannot substitute hot water for the usual warm water because it kills the yeast.  That is why it really only works when you add hot water to the flour and let it cool before adding the yeast and salt.

3. What is starch gelatinization? As you may recall from my The Science Behind... series on the baking process, starch gelatinization occurs in the high heat of the oven.  This graphic might clarify things:

Here is what happens when dough is in the oven.  First, the yeast go crazy and produce lots of bubbles before the heat is too high and they die off.

Next, the gluten strands release water to solidify around the carbon dioxide bubbles created by the yeast.

With all the free water suddenly available (released by the gluten), the starches absorb it until they can't absorb any more; they then burst, releasing the water and sugars in a process called gelatinization.

So instead of waiting for starch gelatinization to occur in the oven, scalded flour introduces it much sooner in the bread baking process.

But what exactly IS it?  This article explains it best:

This formation of gummy networks gives polysaccharides (heated starches) the ability to replace eggs in many baked goods with relative success. The tangled networks can entrap water and other molecules in the batter, locking in moisture and keeping ingredients evenly dispersed. Polysaccharides also interrupt the springy gluten networks formed by flour to create a more tender, delicate texture often desired in muffins and quick breads. And just as a network of egg proteins can hold things together, polysaccharide gels help keep baked goods from falling apart. Who would have thought that such different molecules could function so similarly in the kitchen!

Here's a graphic.  As you can see, the starch molecules absorb water under heat until they burst, forming a tight net of gel.

https://scienceandfooducla.wordpress.com/2013/04/30/baking-without-eggs/

Here is the problem, one that I ran into and one that has been solved by the Japanese technique of tangzhong (more on that below): you don't want to scald the entire flour content of your dough.

You do still want some gluten in your dough to help with structure.  This is where tangzhong comes in.  It can be understood as something of a roux.  Like a roux you might use in a gumbo, adding just a little bit of scalded flour will help soften your dough, while maintaining the gluten structure.

4. What is tangzhong?  Tangzhong is a technique I learned about recently.  According to this post,

 The Tangzhong roux technique was developed in Asia around 2000. The technique was first mentioned by Yvonne Chen in her book, “Bread Doctor”, published in Taiwan in about 2003. Tangzhong means "soup" in Chinese.  

A Tangzhong Roux (also called a Tangzhong Water Roux or Water Roux) is a flour and water roux that is added to yeast bread recipes. This is done in order to make a loaf of bread that is lighter, has a more tender crumb and a longer shelf life.

The flour and water are mixed and heated to 149-F (65C). This gelatinizes the flour and forms an unflavored translucent pudding-like roux.

http://yireservation.com/recipes/soft-asian-milk-bread/

If you plan to use a water roux in a regular recipe such as my 2-3 white bread, use only about 5% of the total flour weight to calculate your roux flour weight.  The flour in a water roux absorbs 5x the flour weight in water.

So if you are planning on making a dough with 375g of flour, your roux should be about 19g of roux flour (375 * .05) and 95g of roux water (19 * 5).  The post on Chowhound suggests just adding it to your recipe without doing any recalculations for hydration.

Just add your paste to the amount of water you would regularly use (in the case of my 2-3 dough, 250g of water) and then, when it is lukewarm, adding it to the normal 375g of flour. 
 
5. How can I use scalded flour in my bread?  Besides the Japanese soft breads, I learned that scalded flour is a technique practiced in Scandinavia as well.  Virtuous Bread has some tips for using scalded flour (in sources below).  I learned through my research that scalded flour is most often used in non-wheat doughs like ryes, which have very little gluten content and do not hold their structure well.  By scalding the flour, bakers can achieve a soft dough that has a cake-like consistency, less bitter flavor, and holds together without the help of gluten.

For a rye bread recipe with scalded flour, check out this post:  rye bread with scalded flour.

I hope you enjoyed this tutorial on scalded flour and starch gelatinization.  It's something I was unfamiliar with, but I'm thinking it could be a great way to add softness to breads without fats like eggs or milk.

Have you ever used scalded flour or a water roux in baking?  Let me know in the comments!

Sources:

Sourdough Library

Science and Food

Virtuous Bread

Chowhound Tangzhong Roux FAQ