Is it true that Pasteurisation is the process of destroying germs in milk by rapidly heating and cooling? 

Pasteurization is the process of heating milk or other liquids to a specific temperature for a certain amount of time, followed by rapid cooling to destroy any harmful microorganisms while preserving its nutritional value.

In the case of milk, pasteurization is typically done by heating it to 72°C (162°F) for at least 15 seconds, and then rapidly cooling it to 4°C (39°F). This process kills most of the harmful bacteria and extends the shelf life of milk. However, it does not make milk sterile, so it must still be refrigerated and consumed within a certain period of time to prevent spoilage.

Therefore, the statement "Pasteurization is the process of preservation of milk by rapidly heating and cooling" is partially true, as pasteurization does involve heating and cooling, but its primary purpose is to eliminate harmful bacteria rather than simply preserving the milk.