Natural occurring wild yeast and bacteria to create the leaven for your baked goods. Sourdough. Some studies find that sourdough bread acts as a prebiotic where the fiber and plant compounds, called polyphenols, become more bio-available because of the longer rise and fermentation process. And the fermentation process improves the bioavailability of fiber and minerals. During the rise of bread, the naturally occurring compound found in grains, called phytic acid, is broken down which enables people to access the grain’s nutrients more readily.