Getting there but not quite - need help!!

that is 85% hydration. I find I have much better luck with a lower hydration such as 74 to 78%. Try cutting back on the water and see what happens. 

The rest of your procedure sounds fine except for the really long retarding. I don’t have any luck if I go past 9-10 hours in the fridge. That seems to be the sweet point for me. 

ETA: Okay, I used a calculator instead of my head and I didn’t do my math right. Too early in the morning for me. ? It really is 77% hydration which should be fine. Try baking earlier and see if you can get improvement. 

But I also agree with you that it looks under-fermented.  However, it's possible the final fermentation was too long and it overproofed and collapsed.... I dunno.   Next time your schedule limits your options, perhaps just leave the bulk fermentation to go at room temperature and let it go long and slow.  And shorten the final rise in the fridge.

Quick question: what did you bake your loaves in?  like a cloche? Dutch oven?  did you have steam? 

How's your scoring?  It's taken me 4 years to get the scoring right.  Sometimes if I don't score properly, the loaves don't get enough oven spring and are denser than I want.  (I'm thinking aloud here).

hester

High C,

Sorry, I can't resist playing with words.

I see progress, good progress. That said, I have some comments. 

My math, assuming your sourdough leaven is 100% hydrated is:

Total flour = 900g white flour + 100g whole wheat flour + 100g leaven flour = 1100g total flour 

Total water = 700g water + 50g saltwater + 100g leaven water = 850g total water

yielding 850/1100 times 100 = 77.2 (rounding to the nearest integer) = 77% Final dough Hydration

That's a very wet dough. It appears from your photographs you handled it quite well. I bake similar dry ingredients at 67% to 70% hydration; my go-to sourdough is 90% White flours and 10% course rye meal.  I'm not criticizing, merely curious: What do you hope to gain using that high a level of hydration? Well developed doughs of similar ingredients at 77% hydration produce loaves with large holes--much like the ones in your cut loaf, but more of them: open crumb loaves suited to soaking up soups and sauces. My go-to faccocia is all-white flour at 72% hydration. I've planned to bake a ciabatta today: all white flour, 75% hydration.

Fermentation begins the moment you mix your leaven with water and flour. it ends when the internal temperature reaches 140°F (60°C) when the yeast cells die. All the time between mixing and baking fermentation is ongoing. Ambient and internal dough temperature speeds it up, or slows it down, fats and sugars influence the yeast's metabolic efficiency, but as long as the water present isn't frozen or otherwise bond fermentation goes on. 84.2°F (28°C) is the sweet-spot for both commercial and natural yeast as far as I've been able to research. You may have done yourself a disservice fermenting the dough at the elevated temperature while you manipulated the dough. Common practice is to manipulate dough at room temperature or cooler between manipulations. Furthermore, plunging the shaped loaves into the refrigerator would have reduced the yeast's activity quite slowly. 

On the other hand, normally your  chilling the dough for 17 hours in the refrigerator would have a positive effect on the doughs gluten web, and developed flavors. I chill all my lean doughs, excepting rye doughs, in the refrigerator during the usual time spent manipulating the dough, and then leave it overnight in a wine cooler at 54°F in bulk form. In the early morning I turn up my home-made proof box to the sweet-spot temperature, and warm the dough for an hour before degassing--vigorously--dividing and shaping loaves. Subsequently, the loaves take 2 to 2-1/4 hours to proof, this hasn't change since I created my long-followed process.

Helpful tools: If you know about these two tools already, consider this a testimony that the are indeed helpful. I've scanned other comments and haven't seen them mentioned, but I didn't read them all thoroughly.

The good news is you don't have to buy anything, you have the tools for both already: your fingers.

Window Pane test: Before doing S&F # X take a small piece of S&F # X-1 flatten it and gently stretch it out using both hands. Do this in front of a window in daylight or a lamp. One of two things will happen. 1: the dough will rip, the light source will be clearly recognized. 2: As you continue to stretch it out the dough will continue to stretch and the light source will begin to show through the thinned dough. Continue to stretch it carefully until you estimate you could read at least the headlines of a newspaper through the thinned area. If result #1 happens S&F #X is necessary, the dough isn't ready. If result #2 happens S&F #X ain't needed--go directly to bulk fermentation.

Poke test: this ultra-scientific test helps you judge the progress of proofing the shaped loaves. Choose a finger--anyone will do, but thumbs are clumsy. Partially uncover a convenient part of a loaf's surface. Using the selected finger, with high confidence, and a gentle touch poke the loaf and remove your finger. Make a worthy poke, don't be timid. (a half-inch deep poke is probably good) One of three things will happen. 1: The loaf's surface will spring back and the poke hole will be barely noticeable, or not at all, in a few seconds. 2: The poke hole will attempt to spring back but long before the surface is once again pokiness the spring will quit. More the half of the poke hole's depth will remain. 3: The poke hole will silently stare back at you no attempt to spring back will be seen, but it might seem to appear guilty.

If result #1 happens continue to proof the loaf, it ain't ready. If result 2 happens the loaf is ready proceed to score and bake. If number 3. happens bake score and bake the loaf, but don't have high-high expectations.

Don't worry about the remaining poke hole(s); it/they will disappear in the baking,

Some recommendations:

Unless you have a good reason to continue with with 77% hydration I recommend trying the same ingredients with 67%. Don't exceed 70% until you've built-up your confidence, then try a soup soaking foccocia or ciabatta.

Use a probe thermometer to check the loaves done-ness. For lean doughs 96°C to 100°C at the center of the loaf is good. Note: internal temperature will not exceed 100°C until all the internal water vapor is gone, but you can burn the crust: don't push it.

I also second the the comment one of our other fellow TFLers offered. Chose a recipe and stick to it until you've learned all there is to learn. If you must fiddle, fiddle only with one thing at a time. (This was the hardest discipline for me to stick to.)

If you haven't been using the two tests, give them a try. they'll do no harm and will likely help.

A guess. I think the photographed loaves in your post are slightly over-proofed for the reasons stated above. I also think the temperature swings and over-proofing weakened the gluten network. I suggest had the gluten net been only a bit weaker it would have broken resulting in a line of gluten stalactites in the middle of the loaf much like the loaf in your first post.

I recommend mixing at room temperature, manipulating at room temperature or chilling the dough mass between manipulations. Furthermore, bulk ferment at room temperature or chilling temperature if you are going to ferment overnight .Shape after completing bulk fermentation especially if you are chilling overnight. I do not recommend chilling shaped loaves. Final Proof at the sweet spot. Bake lean doughs to  96°C or higher.

Lastly, here's a link 

You'll have to use the Search function on TFL home page to access it. I've forgotten how to post active links herein.

I wrote this shortly before I quit frequently  visiting and posting on The FreshLoaf, in part, as a thank-you to the many men and women that helped me learn to bake "better" breads. There is always a host of knowledgeable bakers active on this site. There is also an ultra-large collection of facts, opinions and even fictions in its archives. I highly recommend using the search function when you are looking for almost anything baking related. For example try searching for Window Pane Tests; I bet you can find photographs of what I tried to explain only with words.

Again, I see good progress.

Happy baking,

David G