Passage
When bread dough is kneaded and then left in a warm place, it slowly swells to nearly twice its size and becomes light and full of small holes. Two students agreed that the dough expands because gas is trapped in bubbles inside it, but they disagreed about where that gas comes from.
Student 1: The rising is caused mainly by yeast, a living microorganism mixed into the dough. As the yeast feeds on sugars in the flour, it releases carbon dioxide gas. Kneading forms a stretchy network that traps this gas in tiny bubbles, and as more gas is produced the bubbles grow and the dough expands. Dough made without yeast, or dough kept cold enough to slow the yeast, should rise very little even if it is left for hours.
Student 2: The rising is mainly a physical effect and does not require a living organism. Kneading beats many small air bubbles into the dough, and as the dough warms, that trapped air expands and some of the water in the dough turns to vapor, puffing the bubbles up. On this view, temperature and the amount of trapped air matter most, and even dough made without yeast should rise noticeably if it is kneaded thoroughly and warmed enough.
