Now how long does it take to cool from 160 to 180 down to 140 in a styrofoam cup? How about one of those cardboard cups that burn your hands if you try to hold them?
I'd think that would kind of cut back on the enjoyment of drinking it then with the 3rd degree mouth burns?
Ever watched fire walkers. They walk on fire hot enough to cause significant burn damage to the feet. Yet, when done properly, they do not get burned. It's due to heat transference and other factors that keep the feet from getting burned.
It's quite likely that the act of drinking coffee, at about the 135-140 degree mark, doesn't result in burns because of the saliva in the mouth absorbing heat and the short amount of time the liquid is in the mouth. The mucus lining in the esophagus probably helps as well. But I'm just guessing.
I'm not going to state either way about the MacDonalds case because I don't know the exact temperature it was served to her and we do know that she spilled the coffee before being able to add in either cream or sugar which cools the coffee significantly.
I'm just presenting facts that contradict what many people making statements seem to believe about the brewing and serving temperatures of hot drinks. Which is that they are brewed and served at temperatures that will damage the skin and can do so severely. This is nothing new.
There is no need to heat a beverage to the point where it would injure your mouth when it is served.
Millions of coffee and tea drinkers would disagree. That's also why, traditionally, the act of drinking these beverages is preceded by a process that helps cool it before consumption. Such as adding in other products and stirring. But all of them are indeed served at scalding temperatures.
The hotter the liquid the faster it cools. However, adding in cold cream or milk lowers the overall temperature of the system so that while it will be cooler than a cup of black coffee it will lose temperature at a slower rate. I believe the same is true for adding sugar but I haven't seen the numbers.
Fun stuff.