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In this sourdough recipe, it suggests basting the bread with a cornstarch slurry. I would assume this is meant to promote crust development, but how does that work? I usually see such a slurry used to thicken liquids.

Other recipes that use this method:

It appears to be a presentation thing (makes it look "professional") and possibly a Jewish tradition.

Edit: Do note that the question is "Why cornstarch? How does that work?". I understand the desire for a good crust, and I understand that the slurry is meant to promote crust development, but I fail to understand what it is about cornstarch that mimics good crust development.

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    I can honestly say I've never heard of that before. – rfusca Feb 23 '12 at 21:26
  • @rfusca It's the weirdest thing I've come across, that's for sure. – Yamikuronue Feb 23 '12 at 21:34
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    kitchencookingrecipes.com/forum/bread-recipe-forum/… amused me while searching for links: it appears to be talking about this technique, then some bot or something comes along and posts an answer about thickening sauces – Yamikuronue Feb 23 '12 at 21:40
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    I have used this slurry and it makes an amazing shiny crusty bread...first used this nearly 25 years ago and still use it – user34583 Mar 30 '15 at 15:30
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I've seen bread recipes like the one that you described.

When bread is baked in an oven with steam- the starch in the crust is able to gelatinize before it all dries out and becomes crispy. This is what makes the crust crisp, shiny, and delicious- characteristic of "artisan" breads.

Most people don't have steam enhanced ovens (or the ability to hack their oven to add steam: How can I create steam in a normal oven to promote bread oven spring?)

The recipe you linked has water added for steam but then takes out extra insurance (they cheat) by adding the cornstarch glaze to mimic the effect. By adding extra, pure, starch on the surface of the loaf more gelatinization occurs. Additionally, cornstarch gelatinizes at a lower temperature that wheat starch. Conceivably, you should be able to use any starch and see similar results but, in the US at least, cornstarch is by far the most common.

It shouldn't be necessary if you are able to produce enough steam in your oven.

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Cornstarch slurries are used because they make the crust of the bread shine. This happens because Cornstarch mixes are translucent; whereas flour mixes are opaque.

  • Why cornstarch and not plain water, then? Water is translucent. – Yamikuronue Apr 9 '12 at 16:11
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The wash also can be used to allow seeds or oatmeal, for example, to adhere to the crust of the bread for aesthetic purposes.

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This is something I'm still learning to execute beyond "theory". So, take it for what it is. My understanding from a chemical point of view is that the cornstarch heats, and on a molecular level breaks into a gel like substance on the surface of the bread due to the contact with the hot steam, and that gel like substance solidifies in/on the surface of the bread, making the crust, well, crusty. As noted before, wheat starches and other starches do this in the steam of the oven. For lack of much steam in a regular oven (though you can do some tricks to create it without a steam injection oven-- google it) the water part of the slurry will become the steam that's in contact with the cornstarch part of the slurry. So, steam via slurry rather than steam injection oven, and cornstarch gelatinization and solidification instead of wheat starch gelatinization and solidification to yield... dun dun dah dah... CHEWY CRUSTY CRUST without a steam injection oven! Cheers.

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