Of course milk can smell sour if it goes bad, but in the situation you
mention, it is likely that it is the breakdown of fat to free fatty
acids which gives the sour smell. It means the lipase in the milk has
been activated. This does not make the milk "bad", just unpleasant.
What on earth are daycare workers doing feeding a bottle to an 8 month
old? Don't they believe in development? An 8 month old should be
learning or have already learned to drink from a cup. But this is
another one of my pet peeves, and was not part of the question.
Jack Newman, MD, FRCPC