Is it possible that the liquid nitrogen is entraining and liquefying
oxygen from the air, carrying it into the slush during mixing. When the slush
reaches its stable temperature of 160C it is above the temperature at which
oxygen changes back to gas. Thus the oxygen turns to gas and escapes.
This type of phenomenon has caused explosions in vacuum lines with
liquid nitrogen cold traps. If the line breaks and air circulates through the
cold trap, of a mercury diffusion pump for example, oxygen liquefies in the cold
trap. If the system is then sealed and the liquid nitrogen removed the oxygen
trapped in the system can over pressure it.
Martin M. Cassidy