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I'm trying to stay low on sodium so I limit salt intake to minimum wherever possible (most of it comes from bread, fermented vegetables or a tiny dash here and there for seasoning)

For vegetables I use 2% salt to water ratio brine as most fermentation recipes call for

  1. Why the 2% and why at all? Is the salt just for seasoning or does it have anything to do with safety? If I get it right, the whole point of fermentation is for the good bacteria to grow, out-crowd the bad bacteria, and act as a natural preservative; So what purpose does salt serve in the ferment other than flavor?

  2. How much of the salt/sodium is soaked into the vegetable during fermentation and storage? (7-10 days at room temp and then in the fridge for consumption in about a month)

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The salt is very important from a safety perspective, yes. The good, desirable, lactobacillus bacterial cultures that will produce a safely fermented product don't always or naturally overwhelm "bad" or undesirable cultures. You have essentially no control over the mix of bacteria in and on whatever you're fermenting; all you can control is the starting environment. The purpose of the salt is to make that environment more favorable to lactobacillus, and less favorable to other stuff - mold, spores, potentially harmful bacteria that is inhibited by salt.

Lactobacillus can tolerate a saline environment; another place it lives is in your own gut (it's a major part of the human digestive microbiota) which has a fair amount of dissolved sodium. Adding salt to your ferment helps create an initial environment in which primarily lactobacillus thrives, giving it time to rapidly take over, crowd out other bacteria, and eventually produce enough lactic acid to prevent other salt-tolerant cultures from growing.

Why 2%? Much less won't inhibit other bacteria enough; too much will also impede the desirable lactobacilli. The good cultures simply "like" that salt concentration.

As far as absorption, I would expect that through osmosis the salt balance would roughly equalize inside and outside the vegetables. There will be some initial water weight inside the vegetables not included in the 2% ratio of your brine; how much depends on the vegetable and quantity. The water ratio in the brine may also increase slightly due to evaporation; it's probably conservative to say that this effect will be no larger than the water drawn out of the vegetables, and that the maximum salt content of the final fermented vegetables will be no higher than 2% by weight.

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Purposes of salt include

  1. To draw liquids out of cells via osmosis (beating the vegetables up first also helps with this) which creates a brine. You want a liquid as the competitors to lactic acid bacteria like oxygen, so you minimize that by keeping the fermented material submerged in the brine the salt helped create.
  2. Salt hardens pectin, which alters the texture of the vegetables (more crunch).
  3. Lactic acid bacteria also like a slightly salty environment, and competitors do not.
  4. Adding salt avoids putrefaction (preservative).
  5. Adding salt limits booze-causing yeasts; if you do want alcohol production, that's a different discussion.

The salt percent can be as low as 0.8% but is more typically higher, perhaps 1.5%; too high a percentage of salt can inhibit lactic acid bacteria (a goldilocks problem). So in your case it may help to try lower percentages, perhaps 1% or 1.5%, depending on how accurate your measurements are.

High salt ferments (preserved lemons, etc) probably should be avoided.

A soak and rinse of the fermented materials may help reduce sodium, but could also wash away nutrients.

You could also try salt-free ferments, though these may or may not work out as the lactic acid bacteria will have more competitors and there will be more putrefaction (softer mushy texture).

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