I've come across this particular recommendation many times in various recipes and cookbooks and probably even given it out myself once or twice. Poultry is sufficiently cooked when the juices run clear, not red or pink.
In recent months and years, as I've grown more interested in and knowledgeable about the science of cooking, I've learned to be skeptical of such simplistic claims. This one is repeated everywhere - even the Ontario Ministry of Health says it - but I've also run across various claims that it is dangerous advice.
What I'd like to know is this:
What does it actually mean - chemically or biologically speaking - when poultry juices run clear? Is it actually a reliable indicator that the food is safe to eat?
P.S. I am quite well aware that the way one is supposed to test for doneness is to use a thermometer and ensure that the internal temperature has reached 165° F / 74° C. I always do this, but thermometers can break, run out of battery, etc., so I think it is still helpful to know if the juice test is ever a viable alternative. But most importantly I am interested in understanding what is happening to the meat that causes the juices to change colour, and under what other conditions this can happen.