EAM: Sourdough starters (especially those made with whole grain flours) contain minerals including potassium, phosphate, magnesium, and zinc. However, phytase and other plant compounds in flour bind these nutrients, preventing us from digesting them. The microbial digestive enzymes and low pH in sourdough starters both work to degrade phytic acid, releasing those micronutrients and making them available. In addition, the microbes produce folate (a B vitamin). 

https://www.healthline.com/nutrition/sourdough-bread#section3https://www.ncbi.nlm.nih.gov/pubmed/19747602