Just a note on Sohla's Rules for Chicken Soup video
Wonderful video. As are the rest on Food52's YouTube channel.
Just a note regarding the resulting scum when cooking chicken ( or other meat protein ) to create the broth.
A food scientist would be able to provide the protein(s) involved and the temperature when the transformation occurs. When the chicken is boiled at greater than 200F, one or more proteins change their structure, resulting in that scum forming on the top of the broth.
One solution that works is the following:
Bring a pot of water to a boil.
Turn heat down to almost low, and add the chicken pieces. Initially, one might have to use a candy thermometer to keep the temperature between 180F - 190F. Once the process is done a couple or five times, you will be able to not use the thermometer.
This takes a bit more time to poach the chicken pieces, but ends up with more goodness in the soup ( ie, nothing to skim off ). And also, ending up with moist chicken....yes, chicken that has been boiled too long can taste "dry" in soup.
Once the meat is cook, remove and let cool down in order to remove skin and bones.
While the meat is cooling, toss in the root vegetable first, then the softer ones. Season to "almost there," and in the chicken pieces ( chopped, shredded, pulled, etc ), and do the final seasoning.