Sunken loaves can also be caused by a dough which has risen too much before
being placed in the oven. The dough rises some more before all the yeast
are "killed". This may cause the collapse.
Make sure that
- the dough rises only about a third more from the container
- the oven temperature is hot enough to stop the yeast activity
( about 150 deg. Fahrenheit). Normally the baking temperature
is higher than this so check that the oven is pre-heated.
I agree with Jan Edler that salt affects the activity of the yeast.
Aside from this, salt adds flavour though some breads do not have any
salt added in them e.g. several southern Italian breads. This was to
prevent early bread spoilage because salt attracts moisture. Also, their
prosciutto hams normally are salty enough.
Jan Edler made a very good comment how machines do not detect the write
amount of moisture. Doing it by hand, one can really feel how much
moisture is required. I suppose technology one day would be able to
do it but it is fun to make bread by hand. It is a good stress reliever!