A, wash the oil pan with detergent, and the detergent forms emulsion, so the detergent has emulsifying effect, which is correct but does not meet the meaning of the question, so the option is wrong;

B, gas combustion releases heat, so cooking with a gas stove is to use internal energy to heat objects correctly but does not meet the meaning of the question, so the option is wrong;

C. When cooking, the "white gas" emitted by the pot cover is small droplets of liquefied water vapor in the pot, not vaporized water vapor. Water vapor is a colorless gas and cannot be seen. The original question is wrong, so the option is right;

D. Boiling beef in pressure cooker is easy to boil, because the boiling point of liquid increases with the increase of air pressure, so the temperature increases, and the cooked beef is more likely to rot, which is correct but does not meet the meaning of the question, so the option is wrong;

So choose C.