One is when you have an essentially sourdough bread, but need to make sure that it will rise on schedule or always to the same volume. It is important in bakeries, especially in large bakeries to bake and to deliver bread on time. Then you add a bit of yeast.
Another is a normal yeasted bread where you add a bit of sourdough as a bread improver, for aroma, taste and keeping qualities.
The third one is probably 'just because'. You combine them in one dough just because the recipe says so. If it works, regardless of why, then just follow the recipe.
Homemade and commercial sourdough starters vary a lot in their gassing power. Some rise fast and others are slow risers. Commercially sold yeast is very reliable in that sense, which makes programmable bread machines possible, for example, so adding it to the sourdough recipes might help sharing good recipes for good bread among bakers.
Often call for a pinch of yeast in his sourdough breads. I always miss the yeast out, there's no big difference in timing and they always turn out very nice. As Mariana says they often do this in bakeries and Hamleman's book "Bread" does have the commercial baker in mind too. It's just for assurance and more predictability when you're baking many loaves.
Another reason might be for enriched doughs. The enrichments can slow the fermentation of the dough, adding pinch of commercial yeast can help speed fermentation up. As well, for enriched doughs you might not want too much sour tang in the bread, again adding commercial yeast will speed up fermentation so the LAB don't have as much time to produce as much lactic and acetic acid.
I know at least two reasons.
One is when you have an essentially sourdough bread, but need to make sure that it will rise on schedule or always to the same volume. It is important in bakeries, especially in large bakeries to bake and to deliver bread on time. Then you add a bit of yeast.
Another is a normal yeasted bread where you add a bit of sourdough as a bread improver, for aroma, taste and keeping qualities.
The third one is probably 'just because'. You combine them in one dough just because the recipe says so. If it works, regardless of why, then just follow the recipe.
Homemade and commercial sourdough starters vary a lot in their gassing power. Some rise fast and others are slow risers. Commercially sold yeast is very reliable in that sense, which makes programmable bread machines possible, for example, so adding it to the sourdough recipes might help sharing good recipes for good bread among bakers.
Often call for a pinch of yeast in his sourdough breads. I always miss the yeast out, there's no big difference in timing and they always turn out very nice. As Mariana says they often do this in bakeries and Hamleman's book "Bread" does have the commercial baker in mind too. It's just for assurance and more predictability when you're baking many loaves.
Another reason might be for enriched doughs. The enrichments can slow the fermentation of the dough, adding pinch of commercial yeast can help speed fermentation up. As well, for enriched doughs you might not want too much sour tang in the bread, again adding commercial yeast will speed up fermentation so the LAB don't have as much time to produce as much lactic and acetic acid.
Thank you all