MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Bread dough before (right) and after (left) rising. Dough is a thick mixture of flour, baker's yeast and other ingredients. Baker's yeast (Saccharomyces cerevisiae), a fungus, is added to make the dough rise (also called leavening). The yeast reacts (ferments) with sugar that is present, producing carbon dioxide. This carbon dioxide gas produces air pockets in the dough, giving it a spongy appearance and causing it to expand.
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