Here is a report entitled "Change in sodium content of potato, pasta and rice with different cooking methods".
So the "scientific answer" is:
- The sodium content in pasta cooked with different levels of salt increased approximately linearly with the amount of salt added to the cooking water. Pasta cooked in 4g salt/100g raw took up on average 28 mg Na/100g whereas when cooked in 40g/100g raw this increased approximately 10 fold to 230 mg Na/100g.
HTH!
PS: This may help you forget the Na intake temporally
SUMMARY: Glass containers are perfectly fine for fermentation. It's usually other design aspects of the container that create fermentation problems.
Do you have any sources that actually say glass isn't a good container for fermentation? I've never heard or read that anywhere. The only negative thing I can say about glass is that it's usually transparent, which means you should store it in a dark place to avoid growth of microorganisms that can sometimes be fueled by light.
I assume the reason your linked question mentions that glass jars are "high maintenance" is because most people who ferment in, say, your standard Mason jar may not use airlocks or weights to keep food submerged. If food stays on or above the surface of the liquid, it is much more likely to mold, especially if the container allows outside air to get in. Some people therefore tend to stir or skim their fermented foods periodically, and this agitation will prevent some surface molds from forming, sometimes even with food that floats. With a standard Mason jar, though, your only other option is to keep outside air from contaminating the container by tightening the lid, but then you have to "burp" the jar periodically to avoid pressure build-up.
"Burping" or stirring/skimming on a daily basis (or even more often) in standard jars would probably be "high-maintenance." But these would be problems with any container material (not just glass).
Of course, neither of these situations is ideal -- what you really want to do is (1) use some sort of weight to keep the food submerged (weights in a big container or a baggie full of water in a small container are typical) and (2) have an airlock of some sort on the container. If you try to ferment in glass jars -- or any container -- without those two things, you need to use "high-maintenance" techniques to try to prevent mold. And even if you do the "high-maintenance" things, you may still encounter growth of bad things, so I wouldn't recommend such a setup.
I have no idea why Alton Brown uses plastic in his recipe other than the fact that it's probably the cheapest option for someone to find a container that holds five pounds of cabbage and likely comes with a good lid. There's nothing wrong with plastic as long as it's non-reactive and doesn't leach anything into the ferment. Most food grade plastic containers should be fine (though many people avoid them anyway in favor of more traditional glass or ceramic).
The two containers you link to both include an airlock of sorts, and both have common methods to keep food below the surface. (The ceramic crock usually is used with weights, while the plastic container has an inner plastic lid that can be moved down to keep food down.) The first is similar to the container recommended in your linked question, and it's those aspects which make it more desirable and requiring less maintenance.
Anyhow, if you want to read more about these issues, you might find this link interesting, which included a microscope study to look for growth of nasty things after a 28-day ferment in 18 different fermentation container setups. Bottom line, as I said: the container material is basically irrelevant as long as it's sterile and non-reactive. The more important things are keeping outside air out and keeping food below the surface.
Best Answer
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.