Vaporisation is the right lead - I believe the fuel flash point is the most critical factor here - pure paraffin oil's flash point is as low as 38°C - while the JET-A flash point should be well above this maybe the 49°C ensure a certain margin also with regard to JET-B or other alternate fuels with lower flash points. (Oh, and as to the specific value of 49°C - I recon thats a typical Boeing conversion of US customary units - 49°C=120°F)