Refrigerative cooling employed during long distance transportation of food stuffs is normally done by

ANS:B - liquid nitrogen

Liquid nitrogen is the liquid form of nitrogen gas, which is obtained by cooling nitrogen gas to extremely low temperatures. Here's an explanation of liquid nitrogen:

1. Production: Liquid nitrogen is typically produced by the liquefaction of nitrogen gas through a process called cryogenic distillation. In this process, nitrogen gas is compressed and cooled until it reaches temperatures below its boiling point (-195.79°C or -320.3°F), at which point it condenses into a liquid.
2. Properties: Liquid nitrogen is extremely cold, with a boiling point close to absolute zero. It is colorless, odorless, and non-toxic. At atmospheric pressure, liquid nitrogen boils and evaporates rapidly, producing a large volume of cold nitrogen gas.
3. Applications:
   • Cryopreservation: Liquid nitrogen is commonly used in cryopreservation to store biological samples, tissues, cells, and embryos at very low temperatures to maintain their viability for long periods.
   • Cryotherapy: Liquid nitrogen is used in cryotherapy treatments to freeze and destroy abnormal or diseased tissue, such as warts, skin lesions, and certain types of skin cancer.
   • Food Freezing: Liquid nitrogen is used in the food industry for flash freezing and quick chilling of food products to preserve freshness, texture, and flavor.
   • Laboratory Research: Liquid nitrogen is used in various laboratory applications, including cooling and freezing samples, maintaining low temperatures in experimental setups, and as a coolant for certain types of equipment.
   • Coolant: Liquid nitrogen is sometimes used as a coolant in industrial processes and machinery where low temperatures are required.
4. Safety: While liquid nitrogen is generally considered safe when handled properly, it can cause frostbite and severe cold burns upon contact with skin or other materials. Additionally, rapid evaporation of liquid nitrogen in enclosed spaces can displace oxygen and pose a risk of asphyxiation.

Overall, liquid nitrogen is a versatile and widely used cryogenic fluid with applications in various industries, including healthcare, food processing, research, and manufacturing. Its extremely low temperature and rapid evaporative properties make it valuable for a range of cooling and preservation applications.