Tuesday, March 6, 2007

Latent Heat

I just found out the coolest thing, no pun intended. Hans showed me an article today that contained a hypothesis of why steamy ovens make for better bread crusts. This is a well-known fact in the baking world, and few professional bakers have never heard the phrase, "steam-injected oven".

The key, according to the hypothesis in the article, is something called latent heat. This refers to the amount of energy it takes for a substance, such as water, to undergo a phase change, such as from liquid to gas. This is different from that substance's specific heat, which is just the amount of energy required to raise its tempurature within a state (such as bringing water to a boil).

As we all know, water can only be heated to 212F at sea level pressure. When one goes above sea level, water boils at a lower heat, with the boiling point dropping about 2F for every 1000 feet above sea level. When one applies pressure, such as with a pressure cooker, they can force water to boil at a higher tempurature.

Unfortunately, we are taught that since water cannot go above 212F under normal circumstances, that means that steam (which is composed primarily of water) is also no hotter than 212F. I've questioned this fact personally, because it didn't make sense to me. If steam was only 212F, then wouldn't it still be just boiling water?

As the article points out, the idea behind latent heat describes why a pot of water doesn't just turn into steam as soon as it reaches a boil. While it takes a lot of energy to cause water to boil, it's nothing compared to the amount of energy it takes to change that water's state from a liquid to a gas. The really important thing here is that when it changes back from a gas to a liquid, it has to release that energy. When you bake a loaf of bread in a steamy oven, the water condensing on the side of the loaf is releasing energy into the loaf, which forms that crust that has artisan bread lovers world-wide going crazy.

Now that I've just regurgitated the content of that article (you should read it anyway), here's the part that I found important. I've recently seen mention of water crocks, and how their porous clay surface somehow manages to cool down the water inside. Everything I've read talks about how it's due to the moisture evaporating from the surface, and then just leaves it at that, as if that's enough of an explanation. I think you already know that it's not enough for me.

When I read how latent heat works, I realized that the energy required for that moisture to evaporate from the sides of the crock is causing a cooling effect to the rest of the vessel. In case you haven't figured it out already based on everything I've already said, this is because energy is being sucked from the vessel into the moisture that is now evaporating. Ancient civilizations have supposedly managed to make ice this way, even when the tempurature outside is well above freezing.

I'm pretty happy to have found out about the concept from Hans and from the article, and even happier to have worked out the water crock thing myself based on that information. Hans may be using this to make his bread crust better, but for me, it's time for me to look into buying or building a water crock to play with this.

1 comment:

  1. Have a look at Stefani Ceramic Water Crocks. These natural drinking water purifiers use the same evaporative techniques to cool water. The site link is http://www.stefaniterracotta.com

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