Water boils at 212 F.
The Merck index lists borates and boric acid being quite soluable in glycerol. It states that 1 gram of Sodium Borate is soluable in 1 ml of water at 20 deg C. That is what I quoted above regarding borates in glycerine which is what you asked me.
A temperature of 250 F is above the BP of water at normal atmospheric pressure. A roast is nominally at 350 deg F coming out of the oven and will burn the pan unless you have kept liquid in the bottom of some sort. Grilling temperatures for meat are above 400 F. This too must be moistened somehow to prevent excess burning of the meat. The internal temperatures either way should reach 160 F or higher depending on the degree of doneness you wish. Most organic chemists love to cook.
I don't discount the formation of an ester, but it would have to take place at higher temperatures, I think, or would require a catalyst. What you observe is entirely in line with Kirk's explanation and not with the formation of an ester.