Peeling Vegetables

A friend of ours peels ripe tomatoes by impaling the tomato on a fork, then holding it over a gas flame and rotating gently. If you try this procedure, use appropriate safety procedures to protect your eyes and body. Peeling fresh beets is also a messy chore. Their colored liquid stains everything, including your fingers. Another friend of ours peels fresh beets by first boiling them, then immediately holding them under cold water with a fork. What is the physics in both of these methods used for preparing vegetables for peeling?

Answer

When a tomato is held carefully over a flame and rotated, some of the thermal energy gained by the tomato vaporizes the water just under the skin to locally rupture it. A paring knife can remove the skin easily after the tomato has cooled. Often, just pulling on the ruptured skin is enough. Very hot water can be used instead of a flame, but the effects are not as dramatic, and the peeling is a bit more difficult.

The boiling raises the temperature of the beets to cook them and, simultaneously, a small amount of hot water enters them, so they are slightly swollen. Cold water on the outer surface of the hot beet causes the skin to shrink, but the innards remain hot and swollen. So the stretching skin bursts in several places and becomes easier to remove with a paring knife without being so messy.

The procedure is exactly opposite to placing an ice cube in hot water. Now the outside of the cube tries to expand, but the inside is still cold. One can hear the thermal stresses crack the ice cube.