I think your best bet is to change your recipe. While trying to figure out the exact process that sometimes makes your soup work does indeed sound like an interesting science project, it doesn't sound like its going to yield a reliable recipe. Especially since you've got to deal with supermarket chicken from suppliers that may change their processing procedures whenever it suites them—maybe even from package to package, depending on which plant it came from, or the specifics of the chickens the plant processed that day.
There are easy, reliable ways to get salt into chicken. The following two will get you salty chicken, every time you do it:
- Put your chicken (chopped up or whole) in the fridge submerged in a 13% brine for a day, and you'd at that point have chicken which would be (once cooked) inedibly salty (among other problems).
- You could chop your chicken fairly thin, and pack it (again in the fridge) in kosher salt. Then it'd become dry, and also very salty.
Of course, that'll be far more salty than you want. So you'll want to scale back—use a 5–6% brine, put it in only for a few hours, etc. But that will get flavorful chicken every time.
Then, to keep your soup base from being salty:
- rinse the brined or salted chicken before adding to the soup (to remove any salt resting on the surface)
- keep salting of the soup to a minimum.
- don't overcook the chicken, that'll force more liquid from it.
- make sure to use low-sodium chicken broth. Normal store-bought broth/stock is pretty salty.
- if too much salt leaches from the chicken, cook the chicken separately and drain it.
</rant>
edit: random suggestions since the above apparently doesn't work
- Commercially, many things are quick-frozen (e.g., fish) to prevent ice crystal damage. It would seem to follow then, that since you're trying to cause ice crystal damage, you want to freeze as slowly as possible. Easy way to do this would be (assuming your chicken is already under 40°F e.g., in the fridge) to insulate it before throwing it in the freezer. So, put it in a freezer bag, but then wrap the freezer bag in some kitchen towels, then toss that in the freezer.
- In previously-frozen (commercially) chicken, there may be some anti-ice-crystal additives, I have no idea. Previously-frozen isn't always sold frozen. Check the package, it should say (probably in tiny print).
- You could try a second thaw/freeze cycle (just make sure to thaw in the fridge, or in cold water, not the microwave, for food safety reasons—keep it under 40°F). This will certainly increase the effects of freezing (and would normally be avoided for flavor and texture reasons)
- This isn't freezing, but may accomplish the same goal: you could try one of the 40+-blade meat tenderizers.
Also, as a final note, it turns out that a lo of how we (humans) perceive flavor has nothing to do with the food. The ambiance, how we're feeling that day, etc. affect perceived flavor substantially. Keep this in mind, since you're not using e.g., a salinity meter, its possible you're chasing down differences that aren't due to the food itself.
I suspect that the biggest problem here is that your brine isn't anywhere close to being strong enough. Cooks Illustrated has a good guide to the entire process but in a nutshell:
Sea salt is expensive and inefficient for brining; the impurities actually make it more difficult to dissolve and disperse properly. Kosher salt is generally recommended, although table salt is also fine.
A typical brine is 1/4 cup table salt and 1/2 cup sugar per quart, which translates to about 70 g and 140 g respectively per L. For very high-heat methods (grilling/broiling), you halve the amounts. Also, for kosher salt you need to double the volume (no change if measuring by weight). Even the lower, high-heat cooking concentration is almost twice as concentrated as what you're doing.
You also need to scale the amount of brine with the weight of the bird itself. The rule of thumb is 1 quart or L per pound (2.2 kg) of meat. For a whole chicken, which is generally around 6 or 7 pounds, 1.5 L of brine is nowhere near enough, especially if you're brining in a pot as opposed to a bag (does your 1.5 L even cover the chicken?).
It doesn't really matter if you butcher the chicken first (although most people don't). You're exposing slightly more surface area that way but not really enough to matter.
Make sure you are actually dissolving all the crystals! From what you're describing, you're getting high concentrations of salt in some areas and none in others. That means you didn't get proper dispersion. You really need to make sure that all of the salt (and sugar, if you're using any) is completely dissolved, otherwise you don't have a "brine", you have water with a bunch of little piles of salt. Some people will suggest heating or even boiling your brine to ensure proper dissolution; just make sure you let it cool off afterward if you do this, before submerging the bird.
In answer to your specific questions:
The container should be well-sealed to prevent evaporation, not to mention off-odours in your fridge. However, I've used pots with loose-fitting lids and had no problems. It doesn't make a huge difference as far as the efficacy of the brine.
Fridge temperature is ideal. Do not even think about using room-temperature water, that is highly unsafe for storing raw meat for 6-8 hours at a time.
As long as you don't overcrowd the vessel and do disperse the crystals properly, the actual amount of space is not a major issue. If it's exposed, it's exposed.
Longer than 12 hours is not recommended. Actually, according to CI, longer than 8 hours is not recommended. Don't overdo it - you're brining, not marinating.
No matter how you cook any piece of meat, it will give up a certain amount of water and therefore a certain amount of salt (from the brine). Left unstated is why you would even consider boiling a brined chicken; brining is primarily a technique for dry-heat cooking (roasting/grilling), and if you want to boil/poach/braise/whatever then you should be focusing more on flavouring the cooking liquid than the meat itself. I wouldn't bother brining if you're making chicken soup, there are better ways to flavour that.
Best Answer
Myth busters beware, are you ready? Go!
No brining nor seasoning will make your chicken too salty. 5%, 7%, 10%, The only sodium saturating process known to men is the injection, the needle injection, the so called "added solution".
If you're only brining, you're miles away from needle injection consequences, yet another misunderstood process. Brining is not to be feared, injections is.
Edit (after all these comments/questions showed up):
Brining is a complex subject which I cannot address here, and a lot of myths are floating around, mostly related to nuances, while people do not understand the enormous impact of:
You can debate this thing for days and years, but what matters here is the question. The assumption is "I marinated (5%) a chicken and now that my marination is over, I'm thinking of seasoning (salt) before cooking. Am I running a risk to saturate the end product with sodium?"
The clear answer is NO, as long as your brine didn't last weeks, or your meat was not previously injected with a sodium solution, e.g 12% or 15%. The only time you have to worry about salt seasoning is when your meat has already been salted with injection, or if you just managed to cook a peace of meat with a higher than 5% salt solution for more than a week, which won't happen unless we are talking about dried cured meat, which is an aberration as I've never seen it.
Please be careful with references to articles, books, so called scientific published papers. You really need to read them slowly and carefully, interpretation is key, understand where they came from, who wrote them, who paid for it, who's getting paid for it, which is the source, ... etc, even if the almighty gods of molecular gastronomy are named on it.
FYI: I've been fighting a few USDA/FDA strict rules for years (e.g. caning, meat temp), while believing their overall work is awesome.