The bread is sour-sweet. At a mashing stage (saccharification) amylase
enzymes convert starches to sugars, mostly maltose. When starter added,
maltose is happily lowered by yeast enzymes to sucrose and eaten.
Subsequently, at the dough stage, molasses and sugar are added. As
bread is sour-sweet (and should be like this according to the
description), the yeasts leave us some sugar... Maltose substantially
improves rising and baking qualities of bread. With only a sourdough
starter (no bakers yeast) and 85% rye flour the bread is fairly light:
about 50% porosity.

For the bread I use Carl's starter: thank you, Carl's friends!
The recipe has two standard versions: 4 stage (with sponge) and 3 stage
(w/o sponge). Storage starter (usually 1 week old) is very thick: about
60% hydration.

And here follows the 4-stage recipe.

The mashing-cooling takes 4-5 hours and to prepare the working starter
3-4 hours: could be launched about the same time.

The recipe is for 2 loafs ~900g each.

Working starter: 70g storage starter, 60g whole rye flour, 40g water.
Ferments 180-240 min at 25-28C.
The mash: to 150g whole rye flour in a thermoresistant bowl (plastic
or other convenient) add about half of 500ml nearly boiling (96-98C)
water, mix well, add the rest of the water, mix. Add to the mixture 50g
of ground rye malt and 3g freshly ground coriander.
Rye malt is sold as a grain at homebrew stores. Ask not to crush: it is
better to coarsely grind it before adding to flour-water mix. Put the
cover on the bowl and get the bowl into the oven preheated to about
70-80C. I use alarm type digital thermometer to look after the
temperature of the mixture ("the mash") . It is important not to surf
out of the 63-65C range. After 90-120 min take the bowl out. Don't
forget to smell the porridge-like mash but bewa it is addicting! Let
it cool down to about 30C for 2-3 hours.

Sponge: add 170g of the working starter and 350g of rye flour to the
cool mash and mix well. Almost forgotten, you mix with, for example,
wide and short wooden spoon. After mixing you start knead. How to make
kneading rye dough easy and pleasant? Knead in the bowl with your wet
hands. When the hands wet the dough doesn't stick. Kneading this way is
quite a pleasant experience: somehow reminds happy childhood. Knead
about 10 min. I knead 5 minutes but I'm lazy.
Important note: In any case, don't improvise with water and flour
quantities!
Don't add flour or water arbitrary just because it looks like it will
help to add water or flour. You will manage with the quantities of the
recipe just wet the hands. I measure ingredients with electronic weight
Leave the sponge to ferment for 210-240min at 28-30C.
Dough: mix 200g rye flour and 150g wheat bread flour. Dilute 10g of
salt and 60g of sugar in 100g water (hard work!). Add the flour, the
sugar-salt solution, 40g of black molasses and about half of 2g more
ground coriander to the sponge. Mix and knead (wet hands down!) for
10-15min (me 5min).
Leave the dough rise for 60-90 min in the bowl at 28-30C.
For the last rise divide the dough between two bread pans (I use
silicon ones), spread the rest of coriander on it and let the dough
rise for more 50 min at 32-34C. To provide humidity sprinkle water on
the top of the dough and inside polyethylene bags, and put the pans
into the bags (or any other more appropriate technique). Preheat the
oven to ~250C. Sprinkle and bake about 50 min at 200-220C. Watch the
crust; if necessary shield by aluminum foil. I used to bake up to 205F
inner temperature.
After the bread out of the oven, sprinkle water on it immediately or
smear 3% potato starch solution.
That is Borodinsky bread.