Max
I don't remember where I got this information, I've known it for a long time
but the source remains forgotten. Anyway, the characteristic harsh yeasty
smell is produced by DEAD yeast cells being autolysed and, if you've produced
bread from a biga started with minimal yeast (I use 1 GRANULE of dried yeast
to 200 gm starter) then nearly all the yeast in the final dough has been
produced by multiplication from a very small start population hence the yeast
has not run into food shortage or waste product accumulation so there are
relatively few dead yeast cells.
If you use large amounts of yeast at the start then (particularly with long
fermentations) a significant proportion of the cells in the yeast do not
revive and the living cells autolyse the dead ones for food, producing a
"yeasty" smell.
John Wright