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.
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.