Where does the phrase "piping hot" come from? I see it on cooking instructions for food all the time.

In Scotland, ceremonial dishes of food are often brought to the table to the accompaniment of bagpipes, i.e. they are 'piped in'. This could easily be imagined to be the origin of 'piping hot'. It isn't though. The derivation of this little phrase is the sizzling, whistling sound made by steam escaping from very hot food, which is similar to the sound of high-pitched musical pipes.

An early citation of the phrase is given in Philemon Holland's translation of Pliny's Historie of the world, 1601:

"Beanes... fried all whole as they be, and so cast piping hot into sharp vineger."

http://www.phrases.org.uk/meanings/283850.html

Very hot, as in These biscuits are piping hot. This idiom alludes to something so hot that it makes a piping or hissing sound. [Late 1300s]

http://www.answers.com/topic/piping-hot

The sense of piping that’s relevant here is the one for making a musical sound, as by playing the pipes. The idea is that a dish that’s piping hot is one so hot it makes a sizzling or hissing noise, perhaps not closely similar to the sound of the pipes, but at least audible. It’s first recorded near the end of the fourteenth century, in Chaucer’s Canterbury Tales. In the Miller’s Tale it says (in modernised spelling): “Wafers piping hot out of the gleed”, where a wafer is a kind of thin cake, baked between wafer-irons, and gleed is the hot coals of a fire.

http://www.worldwidewords.org/qa/qa-pip1.htm

I found the phrase on a few phrase meaning sites, they all said something similar to "the sizzling, whistling sound made by steam escaping from very hot food, is similar to the sound of high-pitched musical pipes." To which I say, as an explanation it aint so hot. I can almost see, or rather hear, the sound of steam escaping a pie as being the sound of pipes, but then again, we usually say a tea pot whistles and not that it pipes. A couple of sites mentioned the mistaken beleif that piping hot refers to food being presented at gala feasts in Scotland, accompanied by the sound of real pipes.
So as unsatisfying as it is, to me at least, its 'piping hot' cause it sounds like the 42nd Blackwatch baked in a pie.