This may seem a silly question, but it has always stricken me as odd that chicken should dry out when I boil it in water. Intuitively it just seems weird, but thinking about this a bit while cooking just now, it also makes little sense to me from a basic physics point of view. I googled this first of course, but I can't seem to find an answer as to why this could be.
Assuming I don't put any salt in the water in which I boil it, the concentration gradient caused by ions and other molecules in the meat should cause the water to diffuse/osmose into the chicken. Furthermore any minor temperature gradient should also be pushing water into the chicken. The most plausible explanation I came up with is that the chicken is over-saturated with water to begin with -either naturally or introduced during processing of the meat- and boiling it then somehow reduces the capacity of the tissue to retain the liquid.
Best Answer
I may be wrong but I'll give it a shot.
When you heat chicken (or any other muscle for that matter) it tenses up. As it tenses up it is essentially squeezing juices out, because all the fibres are closer together. Just think what happens when you slice a rare steak without letting it rest, there will be blood everywhere.
In general I believe boiling anything is bad practice. A slow simmer maximum is required to retain all the desired qualities. Hence the saying "low and slow" which I take quite literally band apply to just about everything culinary wise. From cooking stew to proving bread, the longer the better.
Edit:
I wrote that then went in the shower and remembered pasta & potatoes, so maybe low and slow unless it's a starchy carbohydrate. Or if you are searing, but often a blazing hot seat is accompanied by a nice rest.