I’ve found sites that say it’s ok to start a brine for a partially frozen turkey. My question is, when you thaw a turkey using the cold water method, they say to change the water every 30 minutes. If you’re brining a partially frozen turkey, do you have to change the brine (including all the herbs and whatnot) every 30 minutes as well?
I believe the reasoning for changing the water every 30 minutes is to speed up defrosting. I imagine the defrost time would be reduced drastically. However, that seems like a mess and a lot of work. If you're brining a partially frozen turkey, from a safety perspective, I would not change the water out. I'd just leave it in the water. The main thing you are looking for is that the water temp never goes above 4C/40F. You can do this by doing in the fridge, or cooler, or outside if you live in the right place.
See these related questions for more info:
You cannot brine frozen or partially frozen food. Brining is in essence the process of osmosis. By adding the meat to a saline solution, you repeatedly dehydrate and hydrate the meat. The salt molecules draw moisture out of the protein structures which is then replaced by the water. This is done because this process breaks down the cell walls in the protein, which aids in moisture retention, which just means juicier meat. Something which works well with lean cuts of meat like chicken and turkey.
But the saline solution cannot do this when the cell walls are frozen solid. You are also probably risking some sort of pathogen growing in the meat. You can dredge the frozen meat in luke warm water while still in its plastic to aid in a quicker defrost. Also it is worth noting that cold water and cold meat leads to a retardation in the osmosis process, you really want the brine and the meat to be at room temperature. If you use the correct amount of salt with defrosted meat the salt will have a anti-microbial effect on the water and the meat, which will keep things safe.