What's the role of bread in pickling cucumbers? In the summer, my mother always made Hungarian-style pickled cucumbers (kovászos uborka) by putting cucumbers, spices, salt, and water (no vinegar!) in a jar, putting a piece of bread on top, and letting the jar stand in the sun. Many many recipes say that the bread is meant to aid fermentation, as do the question and the top answer in this thread, for instance.

Wikipedia says:


  Additionally, a slice or two of bread are placed at the top and bottom of the solution, and the container is left to sit in the sun for a few days so the yeast in the bread can help cause a fermentation process.


My question is, how does this work biochemically? Is the bread just traditional voodoo? I'm a bit sceptical for two reasons:


Many breads are leavened with baker's yeast, not with sourdough. Yeast performs alcoholic fermentation, so it should not have an influence on the lactic acid fermentation in the pickles, right?
Yeasts and bacteria are temperature-sensitive, so after baking the bread in the oven, everything in it should be dead. How can dead yeasts or bacteria affect the fermentation process?


My best guess is that since sourdough bread is slightly acidic, the acidity helps a little bit with creating a favorable environment for the desired bacteria. However, I imagine that the bacteria do not come from the bread. But I would like to understand this better.
There's enormous amount of yeast and lactobacteria on the old bread (mostly sporulated, captured from the air). There's also the same tremendous amount of them readily available on fresh cucumbers.

While the yeast don't thrive much in the brine, lactobacteria are tolerant to salt -- similar process happens in sauerkraut. The difference from sauerkraut here is that getting into cucumbers is a bit harder, and cucumbers don't contain nearly as much nutrients as cabbage. Bread provides lots of starting sugar (and much other tasty stuff) for the bacteria/yeast to survive before they get into the cucumbers and adapt to feeding on something new.

Putting the jar into direct sunlight can get the temperature to 30-35 degrees, which is ideal for growing lactobacteria.

Specifically to your two questions:


Alcohol easily oxygenates into acetic acid (vinegar). This is further supported by presence of various other bacteria (esp. acetobacters).
Bread is an ideal catcher of bacteria&amp;yeast from the air, so even if baking kills everything, a 2-3days old loaf will do. Even by touching an otherwise sterile bread you provide enough microflora to start visible fermentation in a few days. You just filter out the "bad" bugs by putting it into the brine, where only few mentioned species survive.