The whistle blowing in a pressure cooker indicates that the food is fully cooked, not that it will cook during the subsequent thaw out of the pressure. Allow me to explain the process in detail:
When you cook food in a pressure cooker, you seal the lid tightly to create a high-pressure environment inside the pot. As the pot heats up and the water or liquid inside boils, steam is generated. The steam increases the pressure inside the cooker, which raises the boiling point of the water and allows the food to cook faster than in a regular open pot. This is because the higher temperature under pressure helps break down the food's fibers and tenderizes it more effectively.
The whistle on the pressure cooker is a safety valve that releases excess steam when the pressure inside the cooker reaches a certain level. It prevents the pressure from becoming too high, which could be dangerous. The whistle acts as an indicator that the pressure inside the cooker is at the desired level for efficient cooking.
Once the food is fully cooked and you turn off the heat, the pressure inside the cooker will naturally decrease as it cools down. The pressure does not contribute to cooking the food further during this cooling or thawing out phase. The cooking process stops as soon as the heat is turned off, and the pressure gradually reduces until the lid can be safely opened.
Thawing, on the other hand, refers to the process of defrosting frozen food before cooking. It is a separate process from pressure cooking. If you were to cook frozen food in a pressure cooker, it would take longer to come up to pressure and reach the desired cooking temperature, and the overall cooking time would be extended. It is generally recommended to thaw food before pressure cooking for more efficient and even cooking.