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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:

  1. 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?
  2. 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.

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    The only thing I can imagine bread doing directly, is adding some complex carbohydrates to the brine, which would be some food for the bacteria. Also, if the bread was made and kept in a place where a bread baking is common, dust in the air will have a heavy bioload of lactic bacteria. The bread will have the dust on it and will seed the pickles. – Kevin Nowaczyk Aug 11 '17 at 11:18
  • I would suppose that the best answer would be one which explains the species of bacteria — as it seems to be, and not a yeast — that performs the fermentation, and therefore continue to advise as to selecting a preferable specie of bread which produces a tastier batch of kovászos uborka. – can-ned_food Aug 24 '17 at 3:20
  • Get an old copy of the Settlement Cookbook. It will tell you how to make both summer and winter pickles, sweet or dill. No bread is involved. – Wayfaring Stranger Jun 16 '18 at 0:11
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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:

  1. Alcohol easily oxygenates into acetic acid (vinegar). This is further supported by presence of various other bacteria (esp. acetobacters).
  2. Bread is an ideal catcher of bacteria&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.
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"Kovász" is the hungarian word for "sourdough". So, "kovászos uborka" means "sourdoughed cucumber" (it may be a "gherkin", instead of a cucumbre, though). And that,exactly, is the role of the bread. This is also the reason, why using a non sourdoughed bread will usually result in failure to create that very distinct flavour, this dish has.

On a sidenote: the juice of this dish itself is a refreshing and tasty summer drink. It helps to replenish the salt we lose by sweating.

EDIT: Often people add garlic. Personally I don't like it, that way. Of course, tastes are different, but especially if you want to also drink the juice, it is more refreshing without it.

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