I suspect it's hardening around the skin of the potato (hence can't eat the skin) and essentially letting the potato steam itself?
Kind of like if you deep-fried it submerged in oil, but with even less ingress, and much less than boiling.
I commented this elsewhere and someone suggested sous vide ought to be very similar then, of course. (Added benefit perhaps: you can eat the skin!) Only difference might be that the rosin can get hotter, more like oil, than boiling water circa 100C.
If you've never done it, deep fried whole potatoes are amazing. It gets creamy in a way that good mashed potatoes do. Just be careful as they can explode like anything else with a lot of water. You want to poke holes in it so the steam can escape.
I will not allow myself to purchase a deep fat fryer because it's not healthy to live exclusively off of fried potatoes and associated toppings.
I guess a similar effect of “steaming potato within its hardened skin” can be reached wrapping potatoes in aluminum and then putting them in a fireplace
I've done that on barbecues & beach bonfire/barbecues - I wouldn't say it's different from an oven baked potato particularly. Not that I know that rosin-cooked is markedly different (to me) either of course.
The potato itself will not go above 100C until the entirety of its water has boiled off, so that’s unlikely to have a meaningful effect. I imagine deep-frying, or a slow confit will have the exact same effect with a lot less danger.
Why? Boiling water won't go above boiling point because it quickly looses energy with freely departing vapor. In a viscous environment energy removal may not be as quick, and thus temperature may rise above 100⁰C.
I have the impression there isn’t anything you can mix with water that would raise the boiling point more than a few degrees. You need pressure to go beyond, which a potato most definitely does not provide. Since the evaporating water becomes the easiest way for energy to escape, the internal temperature is bound by it.
There seems to be a lack of literature regarding the internal temperature of potatoes, to the point that this thread itself is now one of the search results, so unfortunately I could not find further references!
I suspect it's hardening around the skin of the potato (hence can't eat the skin) and essentially letting the potato steam itself?
Kind of like if you deep-fried it submerged in oil, but with even less ingress, and much less than boiling.
I commented this elsewhere and someone suggested sous vide ought to be very similar then, of course. (Added benefit perhaps: you can eat the skin!) Only difference might be that the rosin can get hotter, more like oil, than boiling water circa 100C.