Here is my experience with kefir as a component used in sourdough bread making.
Summary: When adding kefir milk/curds/whey to my typical slow-ferment (no-knead) bread dough recipe, I find the quality of the gluten to be degraded: the dough tears more than stretches compared to if I use plain water instead. I suspect that proteases present in the kefir are cleaving the gluten strands.
Background: I have been making bread dough using the "no-knead" method and the "5-minutes-a-day (refidgerated)" method, employing regular dry yeast (with proofing), instant yeast (without proofing), and sourdough starters (including my own local wild yeast starter and Carl Griffith's Oregon Trail Sourdough Starter). I thought that adding kefir (instead of water) to my various doughs might add more flavor.
Method: Using a 80% hydration ratio: 100g whole-wheat, 400g bread flour (13% gluten), I compared a loaf using 400g of water to another loaf using 400g of kefir milk/curds/whey, plus 50g of water (to account for the solids in the milk). To these, I added 1/4 cup of my sourdough starter. Primary fermantation of the dough (first rise) was done in my cool Michigan basement for 12 to 18 hours, covered in a plastic bag. For baking, I used the preheated dutch oven method at 450deg for 30 min, then uncovered at 375 for 20 min.
Results: After the 12 hour rise, the kefir bread dough did not seem "over-risen" compared to the control (water) dough. However, using kefir instead of water seemed to degrade the gluten: the resulting kefir dough was much more prone to tear, and the resulting baked kefir loaf did not have the elastic crumb compared to the non-kefir (water-only) control.
Comment: As far I know, there is no well-established historical cultural tradition of using milk kefir to leaven bread. Although kefir might add more flavour than water, the resulting dough and loaf seem inferior to using traditional sourdough starters with plain water in the method described above. There may indeed be an adventage in using kefir in fermenting/levening other types of bread (using different flours), or varying the water/kefir ratio, or using younger kefir or older kefir. Such variables may be seen as either as a headache, or an opportunity to explore. Because these 2 loafs were prepared and baked on different days, I plan to repeat this experiment under better identical conditions. If there is enough interest, maybe I should post photos at each stage.
Until then, your kefir-levening experience comments/advice are greatly appreciated,