Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
S. Gardner asks via twitter, "Kitchen mystery: if you salt soaked, dried beans as you begin cooking them, will they toughen b/c of the NaCl? Corriher says no"
I have to say right off that it is not generally my way to disagree with Shirley Corriher about the science of food. While I am certainly on my journey to Complete Food Knowledge, it is a rare leg of that journey that I make that Shirley and others haven't made before me.
In this particular case, I'm going to have to agree with Shirley. Beans will not be tough at the end of cooking if you salt the cooking water. Well, not necessarily.
Okay, so beans have this starchy coating around them that slows the cooking, and when the beans are dried, the starchy coating is even more protective of the bean. The goal of cooking the bean is to infuse the bean with water and heat. The heat and water break down cell walls and allow an enzyme called lipoxygenase to convert the fatty acids into flavor molecules.
The cell walls contain, among other things, magnesium. Magnesium reinforces the cell walls, protecting the starches inside from being properly cooked. If you cook in hard water, which contains heavy amounts of magnesium and calcium, you will add extra reinforcement to the cell walls, and thus keep them from cooking quickly. Sometimes, it will keep the beans from cooking completely, even if you cook them longer. Magnesium and calcium are tenacious.
Salt performs molecularly much the same way as the magnesium and the calcium, in that it fits quite nicely in the same places of the cell walls that the calcium and magnesium do. However, salt does not do as good of a job of reinforcing the walls. This means that adding salt to water will initially slow down the cooking of the beans, but will eventually speed the process up somewhat.
Your best bet to speed the cooking time of the beans is to pre-soak them. There's an open point of the bean's shell called the hilum. This is the place at which the bean was connected to the rest of the plant. Water starts infusing into the bean through that point, and eventually saturates the bean. After that, cooking is easy. Why?
Beans contain starch, and the way people most like eating starch is when it's had a chance to expand. In order for starches to expand, they have to be heated in water. This is the secret to thickening agents like flour and corn starch: mix them into water, heat them up, and poof! They expand like crazy and enhance the texture of the sauce, soup, or whatever it is that you're thickening. If you're missing either the heat or the water, then the starch doesn't cook properly.
If you've already infused the bean with water, then the heat just finishes the job that the water started. Cell walls are broken down easily and more or less uniformly, starches expend, and everyone's happy.
If you haven't infused the bean with the water, then you are basically using the heat to speed the process of infusing by getting water in a certain amount, damaging the surrounding cell walls with the cooking, which lets more water in, until you're done.
It's kind of like defrosting your freezer: you can chip away at the ice, which lets the speeds up the warming of the next layer down, cleaning up the water as you go. Or you can wait for all the ice to melt, then clean it up quickly and easily. It takes longer to just let the ice melt, but the part that you actually have to do work for is significantly faster.