During its formation bread, becomes porous due to release of CO2 by the action of:

The Baker’s yeast or the Saccharomyces cerevisiae is the common strain of yeast used in the bakery industry for baking bread and other bakery products. Baker’s yeast serves as a leavening agent. The yeast ferments sugars present in the dough into carbon dioxide (CO2) and ethanol (C2H5OH). The released carbon dioxide helps in the rising of bread and the bread becomes softer and lighter.

The Baker’s yeast or the Saccharomyces cerevisiae is a unicellular fungus. They are  round/ovoid in shape and about 5–10 μm in diameter. Yeast reproduces by budding. The optimum temperature requires for the growth of Baker’s yeast is 30–35 °C or 86–95 °F.