What property does yeast extract lack that a true starter culture like sourdough possesses?

Answer

Leavening power or active culture

Yeast extract is a processed byproduct used for flavor enhancement and does not contain live, active cultures capable of consuming sugars and producing gas to make dough rise.

What property does yeast extract lack that a true starter culture like sourdough possesses?
culturefermentationbreadmicrobeyeast