Kenneth wrote:
> Hi Sam,
>
> (I don't know that I have ever made such a "mixed leavening"
> bread, but...)
>
> In the situation above, why would those sourdough benefits
> not be available from the finished loaf?
>
Rising a loaf requires energy to overcome gravity and internal dough
cohesion, amongst other obstacles.
Baker's yeast fermentation differs from sourdough fermentation in
several ways, one aspect is that it uses selected yeast strains
optimized to create gas for rising.

Sourdough fermentation uses "wild" yeast strains and lactic acid
bacteria not optimized for gas production.

To create somewhat similar rising power as baker's yeast with sourdough,
more fermentation is required.

If sourdough fermentation is considered beneficial and baker's yeast is
used in addition, not so much sourdough fermentation is required to rise
a loaf and the beneficial component is reduced.

In the extreme example with the link I posted - a sterile "sourdough"
additive - no sourdough fermentation at all is going on with the dough,
just what's in the additive creates the impression of a sourdough. Using
an overripe, non-performing starter and yeast to make up for the rising
deficiency is somewhat similar.

Sam