Sourdough starter is a mix of yeast and lactic acid bacteria. Those bacteria will, as a side effect of their activity, break down proteins. That study looked

Sourdough starter is a mix of yeast and lactic acid bacteria. Those bacteria will, as a side effect of their activity, break down proteins. That study looked at amplifying this action with additional proteases (which also break down protein) to eliminate enough wheat proteins to make the dough non-toxic to celiacs. Since it is driven by bacteria, this action is unique to sourdough breads.