14. What is the difference between 'Classical' and 'Modern' sourdough?

In days of yore, all bread was sourdough.  So, it wasn't called
sourdough unless it was real sour.  The way to make it real sour was
to let a sponge sit for many extra hours, preferably warm.

Many people do that today when making sourdough bread.  You can call
it "souring the sponge".  The process favors the acid forming
bacilli, and lowers yeast activity.  If you goof and the yeast
activity gets too low, you can always throw in some bakers' yeast for
the final rise.  You also can get some rise by blowing the loaves up
on a hot stone, in spite that a real sour sponge may not have much
leavening activity.

Denizens of yore had no access to bakers' yeast, nor did they have
modern bread flours.

Today's bread flours, as well as having uniformly high gluten content
(typically 13%), also contain diastatic enzymes and dough
conditioners.  The enzymes liberate sugars from starch allowing the
rise to go on much longer than otherwise would be expected. Dough
conditioners can have profound effects towards helping the gluten to
hang together long enough to support a phenomenal rise.

The result is that a kind of modern sourdough bread is now possible
that the yore people could not have anticipated.

A long rise allows that sourdough bread may be very light, and may be
baked quite effectively in bread pans in an ordinary oven (without a
stone).  For this bread, a "sweet" (high yeast activity) (sourdough
yeast, that is) sponge is used.  Acidity and flavor which typify
sourdough bread develop during the rise, not primarily in the sponge,
as is the the case in the alternative classical method.  More and
more people are doing it this modern way.

But it is not clear to most people that two strategies are under
discussion here.  People on track B should learn how to avoid advice
from track A people, and conversely.  (Advice givers cannot be
controlled, since a new bunch is born each week.)

How to recognize:

Type A: "Let the sponge proof in a warm place for a long time". "A
sour (tangy) starter is needed." "Use King Arthur (no additives)
flour." "Use all purpose flour." "Punch it down (N) times, let it
double, slash, and toss it on a hot stone." "May be necessary to add
some dry yeast."  (Per most bake books, FAQs here, and sourdough
packet instruction sheets.)

Type B:  Starter is kept frothy, or activated to the frothy stage
before seeding the sponge.  Sponge is developed to the frothy stage,
no longer.  Use bread flour (malted, bromated or whatever).  Bakers'
yeast is never used.  Very little or no punching down.  Slash (coupe)
before the rise.  Volume quadruples, maybe quintuples, before the
bake.  Special attention needed to avoid deflation if transferred to
a hot stone (but easy in tins). Special attention is needed to avoid
drying out during the long rise (which might be 12 hours in a cool
room).

-Dick