Bread becomes porous due to the release of CO2 by the action of
yeast
Baker's yeast or saccharomyces cerevisiae is used to leaven breads or to raise them and give them the airy and fluffy texture. Yeast releases CO2 as a byproduct of aerobic respiration and this gas causes the dough to double in size by increasing its porosity over time to get it ready for baking.