Baked Beans

If you buy dry beans in bulk, they must be soaked in water overnight in a covered container before they are ready to be baked. To bake them without soaking would require an enormous amount of cooking time. An alternative preparation is to “parboil” them in a cooking pot that is, simmer them. Simmer means “to be on the verge of boiling.”

How does one know that the beans have simmered enough? The test involves good physics. Take up a few beans in a spoon and, after making sure that no liquid is in the spoon, blow a stream of air gently with pursed lips against the beans. If the bean skin cracks, the beans are ready for baking. Why must the lips be pursed, and why do the bean skins then crack open?

Answer

The hot beans have taken up water and swelled so that the skins are under great tension. Blowing cool fast-moving air across the beans from pursed lips (instead of an open mouth) lowers their surface temperature and reduces the ambient air pressure. The inside is still hot, so the larger pressure difference results in the hot, high-pressure vapor under the skin pushing outward just a bit more. If the pressure gradient is great enough, the skin will rupture. The slight cooling of the skin material increases the skin tension to reduce the time to rupture.

We can simplify the physics to applying Newton’s second law perpendicular to the skin surface. Three forces acting on the skin are important: (1) the inward force of the ambient air pressure, (2) the inward force of the tension in the skin, and (3) the outward force produced by the hot, high-pressure gas within. Blowing the air reduces the ambient air pressure enough to create a net force outward, and skin rupture occurs when the skin tension has exceeded its elastic limit. During the rupture process, bean skin molecules have been separated because the electrical force holding one molecule to the next has been exceeded.